upon the nervous system, interferes seriously with the regulation of the
body temperature. By dilating the capillaries, it increases the
circulation in the skin and leads to an undue loss of heat. At the same
time the excess of blood in the skin causes a _feeling of warmth_ which
has led to the erroneous belief that alcohol is a heat producer. If taken
on a cold day, it deceives one about his true condition and leads to a
wasting of heat when it should be carefully economized. Not only is
alcohol of no value in maintaining the body temperature, but if taken
during severe exposure to cold, it becomes a menace to life itself.
Arctic, explorers and others exposed to severe cold have found that they
withstand cold far better when no alcohol at all is used.(92)
HYGIENE OF THE SKIN
Much of the hygiene of the skin is included in the problems of keeping it
warm and clean. It is kept warm by clothing; bathing is the method of
keeping it clean.
*Clothing* should be warm and loose-fitting. Woolen fabrics are to be
preferred in winter to cotton because, being poorer conductors of heat,
they afford better protection from the cold. But wool fails to absorb the
perspiration rapidly from the skin and to pass it to the outside where it
is evaporated. This, together with its tendency to irritate, makes woolen
clothing somewhat objectionable for wearing next to the skin. This
objection, however, is obviated by woolen underwear which is lined by a
thin weaving of cotton.
*Bathing.*--The solid material from the perspiration, which is left on the
skin, together with the oil from the oil glands and the dirt from the
outside, tends to close up the pores and develop offensive odors. Keeping
the skin clean is, for these reasons, necessary from both a health and a
social standpoint. While one should always keep clean, the frequency of
the bath will depend upon the season, the occupation of the individual,
and the nature and amount of the perspiration. As to the kind of bath to
be taken and the precautions to be observed, no specific rules can be laid
down. These must be determined by the facilities at hand and by the health
and natural vigor of the bather. Severe chilling of the body should be
avoided, especially by those in delicate health. If a hot bath is taken,
one should dash cold water over the body on finishing. One should then
quickly dry and rub the body with a coarse towel. The dash of cold water
closes the pores of the skin and lessens the liability of taking cold.
*The Tonic Bath.*--The cold bath has been found to have a beneficial effect
upon the general health beyond its effect upon the skin. When taken with
care as to the length of time and the degree of cold, decided tonic
effects are observed on the circulation and on the nervous system. The
rapid changes of temperature vigorously exercise the non-striated muscles
of the blood vessels (page 57) and the nerves controlling them. The
irritability of the nervous system in general is also lessened. For this
reason the cold bath is one of the best means of keeping both mind and
body in good condition during the warm months. Sponging off the body with
cold or tepid water before retiring is also an excellent aid in securing
sound sleep during the hot summer nights.
Danger from the cold bath arises through the shock to the nervous system
and the loss of heat from the body. It is avoided by using water whose
temperature is not too low and by limiting the time spent in the bath. A
brisk rubbing with a coarse towel should always follow the cold bath.
People past middle age are, as a rule, not benefited by the cold bath; and
those in delicate health, especially if inclined toward rheumatism, are
likely to be affected injuriously by it.
*Care of the Complexion.*--A good complexion is a natural accompaniment of
good health and depends primarily upon two conditions--a clear skin and an
active circulation of the blood through it. Clearness of the skin depends
largely upon the elimination of waste material from the body, and where
the solid wastes are not effectively removed through the natural channels
(the liver, kidneys, and bowels), blotches, sallowness of the skin, and
skin eruptions are likely to result. In seeking to clear the complexion,
attention must be given to all those agencies that favor the elimination
of waste, and especially should there be a free and thorough evacuation of
the bowels each day. The general health should also be looked after,
attention being given to exercise, fresh air, proper food,(93) sufficient
sleep, etc.
Bathing is the chief means employed for increasing the circulation in the
skin, although exercise which is sufficiently vigorous to cause one to
perspire freely is a valuable aid. A daily bath of warm or hot water,
finished off with a dash of cold, followed by a thorough rubbing of the
entire surface, and this by a kneading of the skin with the thumbs and
fingers, will in most cases bring about the desired results. A little
olive oil, thoroughly worked into the skin during the kneading process, is
beneficial where one lacks flesh or where the skin is dry and thin. The
olive oil is also beneficial where the baths are exhausting or render one
susceptible to cold. In rubbing and kneading, the skin should not be
bruised or irritated.
The much advertised "complexion beautifiers" which are applied directly to
the face frequently have the effect of clogging the pores and of causing
eruptions of the skin. On the other hand, certain authorities state that
the cold cream preparations may be of advantage in giving the skin a
desired softness, and that when judiciously used (the face being cleansed
after each application) they do no harm. Of the different kinds of face
powder those prepared from rice are considered the least injurious.
*Treatment of Skin Wounds.*--Skin wounds which may not be serious in
themselves frequently become so through getting infected with germs. Blood
poisoning often results from such infections, one of the worst forms being
_tetanus_, or lockjaw. A wound should be kept clean, and if it shows signs
of infection, it should be washed with some antiseptic solution. Or, it
may be cleansed with pure warm water and then covered with some antiseptic
ointment,(94) of which there are a number on the market. A weak solution
of carbolic acid (one part acid to twenty-five parts of water) makes an
excellent antiseptic wash. It may be used not only for cleansing wounds,
but also in counteracting the poisonous effects that follow the bites of
insects.
A wound resulting from the bite of an animal (cat or dog), even though
slight, should receive more serious attention, and as soon as possible
after the occurrence. Such wounds should be cauterized, and for this
purpose pure carbolic, acid (undiluted with water) may be used. A wooden
toothpick is dipped into the acid and this is worked about in the wound.
The acid is then washed out with warm water. A deep wound from a rusty
nail or a thorn should be treated in the same manner and should be kept
open, not being allowed to heal at the surface first. If one has reason to
believe he has been bitten by a mad dog, the wound should be cauterized as
above, and a physician should be summoned at once. Deep wounds from
explosives, or other causes, should also receive the attention of the
physician. Many cases of lockjaw result every year from wounds inflicted
by the toy pistols, firecrackers, etc., used in our Fourth of July
celebrations. These are due to the embedding in the skin or flesh of small
solid particles on which are lockjaw germs. Wounds of this nature should,
of course, receive the attention of the physician.
*Care of the Nails.*--Relief from a blood blister under the nail is secured
by boring a small hole through the nail with the sharp point of a
sterilized penknife (page 38). This simple bit of surgery not only
relieves the pain, but is frequently the only means of saving the nail.
Ingrown toe nails are relieved by scraping a broad strip in the middle of
the nail until very thin. This relieves the pressure, preventing the sides
of the nail from being forced into the toe. While the finger nails should
be trimmed in a curve, corresponding to the end of the finger, it is
recommended that the toe nails be cut straight across (Fig. 124), as this
method diminishes the pressure from the shoe and keeps the nails from
ingrowing. Shoes that pinch the toes should, of course, not be worn (page
238).
[Fig. 124]
Fig. 124--Proper method of trimming nails of toes.
*Care of the Hair.*--Occasional washing of the hair is beneficial, but too
much wetting causes decay of the hair roots, which leads to its falling
out. The worst enemy of the hair is dandruff. A method of removing
dandruff which is highly recommended is that of rubbing olive oil into the
scalp and later of removing this with a cleansing shampoo. The olive oil
is placed on the scalp with a medicine dropper and thoroughly rubbed in
with the fingers. After three or four hours the hair is washed with soap
and water (any good toilet soap will do) and rinsed with pure water. The
hair is then dried, the surplus water being removed with a coarse towel.
Where the dandruff is very troublesome, this treatment may be given once
or twice a week; but in mild cases once a month is sufficient. Massage of
the scalp, by increasing the circulation at the hair roots, is beneficial,
but irritation by a fine-tooth comb, a stiff hair brush, or by other means
should be avoided. Frequent brushing and combing, however, are necessary
both for the good appearance of the hair and for spreading the oil
secreted by the glands at the hair roots.
*Summary.*--The skin forms the external covering of the body and also
serves additional purposes. It is a most important agency in adapting the
body to its physical surroundings, as shown by the part which it plays in
the regulation of the body temperature. The skin should be kept clean and
active, and skin wounds, even though small, should be guarded against
infection.
*Exercises.*--1. Name an example of each of the protective coverings of the
body.
2. Compare the dermis and the epidermis with reference to thickness,
composition, and function.
3. To what is the color of the skin due? How is the color of the skin
affected by the sunlight?
4. What modifications of the epidermis are found on our bodies? What are
found on the body of a chicken?
5. What different kinds of protection are provided by the skin?
6. How does the perspiration cool the body?
7. What change occurs in the circulation in the skin when the body is
becoming too cold? When becoming too warm? What is the purpose of these
changes?
8. How does alcohol cause one to _feel_ warm when he may be losing too
much of his heat?
9. What precaution should be observed by one in poor health, in taking a
bath?
10. How may the cold bath be a means of improving the general health?
PRACTICAL WORK
*Observations on the Skin and its Appendages.*--Examine the palm of the
hand with a lens. Note the small ridges which correspond to the rows of
papillae beneath the cuticle. In these find small pits, which are the
openings of the sweat glands.
2. Examine the epidermis on the back of the hand and palm. At which place
is it thickest and most resisting? Is it of uniform thickness over the
palm? Try picking it with a pin at the thickest place, noting if pain is
felt. Inference?
3. Examine a finger nail. Is the free edge or the root the thickest? Trim
closely the thumb nail and the nail of the middle finger of one hand and
try to pick up a pin, or other minute object, from a smooth, hard surface.
The result indicates what use of the nails? Suggest other uses.
4. Examine with a microscope under a low power hairs from a variety of
animals, as the horse, dog, cat, etc., noting peculiarities of form and
surface.
*To illustrate Cooling Effects of Evaporation.*--1. Wet the back of the
hand and move it through the air to hasten evaporation. Observe that, as
the hand dries, a sensation of cold is felt. Repeat the experiment, using
ether, alcohol, or gasolene instead of the water, noting the differences
in results. These liquids evaporate faster than water.
2. Wet the bulb of a thermometer with alcohol or water. Move it through
the air to hasten evaporation. Note and account for the fall of the
mercury.
CHAPTER XVII - STRUCTURE OF THE NERVOUS SYSTEM
*Cooerdination and Adjustment.*--If we consider for a moment the movements
of the body, we cannot fail to note the cooeperation of organs, one with
another. In the simple act of whittling a stick one hand holds the stick
and the other the knife, while the movements of each hand are such as to
aid in the whittling process. Examples of cooeperation are also found in
the taking of food, in walking, and in the performance of different kinds
of work. Not only is cooeperation found among the external organs, but our
study of the vital processes has shown that the principle of cooeperation
is carried out by the internal organs as well. The fact that all the
activities of the body are directed toward a common purpose makes the
cooeperation of its parts a necessity. The term "cooerdination" is employed
to express this cooeperation, or working together, of the different parts
of the body.
A further study of the movements of the body shows that many of them have
particular reference to things outside of it. In going about one naturally
avoids obstructions, and if anything is in the way he walks around or
steps over it. Somewhat as a delicate instrument (the microscope for
example) is altered or adjusted, in order to adapt it to its work, the
parts of the body, and the body as a whole, have to be _adjusted_ to their
surroundings. This is seen in the attitude assumed in sitting and in
standing, in the position of the hands for different kinds of work, in the
variations of the circulation of the blood in the skin, and in the
movements for protecting the body.(95)
*Work of the Nervous System.*--How are the different activities of the body
controlled and cooerdinated? How is the body adjusted to its surroundings?
The answer is found in the study of the nervous system. Briefly speaking,
the nervous system controls, cooerdinates, and adjusts the different parts
of the body by fulfilling two conditions:
1. It provides a complete system of connections throughout the body,
thereby bringing all parts into communication.
2. It supplies a means of controlling action (the so-called impulse) which
it passes along the nervous connections from one part of the body to
another.
The present chapter deals with the first of these conditions; the chapter
following, with the second.
*The Nerve Skeleton.*--If all the other tissues are removed, leaving only
the nervous tissue, a complete skeleton outline of the body still remains.
This nerve skeleton, as it has been called, has the general form of the
framework of bones, but differs from it greatly in the fineness of its
structures and the extent to which it represents every portion of the
body. An examination of a nerve skeleton, or a diagram of one (Fig. 125),
shows the main structures of the nervous system and their connection with
the different parts of the body.
Corresponding to the skull and the spinal column is a central nervous
axis, made up of two parts, the _brain_ and the _spinal cord_. From this
central axis white, cord-like bodies emerge and pass to different parts of
the body. These are called _nerve trunks_, and the smaller branches into
which they divide are called _nerves_. The nerves also undergo division
until they terminate as fine thread-like structures in all parts of the
body. The distribution of nerve terminations, however, is not uniform, as
might be supposed, but the skin and important organs like the heart,
stomach, and muscles are the more abundantly supplied. On many of the
nerves are small rounded masses, called _ganglia_, and from many of these
small nerves also emerge. At certain places the nerves and ganglia are so
numerous as to form a kind of network, known as a _plexus_.
[Fig. 125]
Fig. 125--*Diagram of nerve skeleton.* The illustration shows the extent
and general arrangement of the nervous tissue. _A._ Brain. _B._ Spinal
cord. _N._ Nerve trunks and nerves. _G._ Ganglia.
It is through these structures--brain and spinal cord, nerve trunks and
nerves, ganglia and nerve terminations--that connections are established
between all parts of the body, but more especially between the surface of
the body and the organs within.
*The Neurons, or Nerve Cells.*--While a hasty examination of the nerve
skeleton is sufficient to show the connection of the nervous system with
all parts of the body, no amount of study of its gross structures reveals
the nature of its connections or suggests its method of operation. Insight
into the real nature of the nervous system is obtained only through a
study of its minute structural elements. These, instead of being called
cells, as in the case of the other tissues, are called _neurons_. The use
of this term, instead of the simpler one of nerve cell, is the result of
recent advances in our knowledge of the nervous system.(96)
[Fig. 126]
Fig. 126--*Diagram of a mon-axonic neuron* (greatly enlarged except as to
length). The central thread in the axon is the axis cylinder.
The neurons are in all respects cells. They differ widely, however, from
all the other cells of the body and are, in some respects, the most
remarkable of all cells. They are characterized by minute extensions, or
prolongations, which in some instances extend to great distances. Though
the neurons in certain parts of the body differ greatly in form and size
from those in other parts of the body, most of them may be included in one
or the other of two classes, known as _mon-axonic_ neurons and _di-axonic_
neurons.
*Mon-axonic Neurons.*--Neurons of this class consist of three distinct
parts, known as the cell-body, the dendrites, and the axon (Fig. 126).
The _cell-body_ has in itself the form of a complete cell and was at one
time so described. It consists of a rounded mass of protoplasm, containing
a well-defined nucleus. The protoplasm is similar to that of other cells,
but is characterized by the presence of many small granules and has a
slightly grayish color.
The _dendrites_ are short extensions from the cell-body. They branch
somewhat as the roots of a tree and form in many instances a complex
network of tiny rootlets. Their protoplasm, like that of the cell-body, is
more or less granular. The dendrites increase greatly the surface of the
cell-body, to which they are related in function.
The _axon_, or nerve fiber, is a long, slender extension from the
cell-body, which connects with some organ or tissue. It was at one time
described as a distinct nervous element, but later study has shown it to
be an outgrowth from the cell-body. The mon-axonic neurons are so called
from their having but a single axon.
*Di-axonic Neurons.*--Neurons belonging to this class have each a
well-defined cell-body and two axons, but no parts just like the dendrites
of mon-axonic neurons. The cell-body is smooth and rounded, and its axons
extend from it in opposite directions (Fig. 127).
[Fig. 127]
Fig. 127--*Diagram of a di-axonic neuron.* The diagram shows only the
conducting portion of the axon, or axis cylinder.
*Structure of the Axon.*--The axon, or nerve fiber, has practically the
same structure in both classes of neurons, being composed in most cases of
three distinct parts. In the center, and running the entire length of the
axon, is a thread-like body, called the _axis cylinder_ (Fig. 126). The
axis cylinder is present in all axons and is the part essential to their
work. It may be considered as an extension of the protoplasm from the
cell-body. Surrounding the axis cylinder is a thick, whitish-looking
layer, known as the _medullary sheath_, and around this is a thin
covering, called the _primitive sheath_, or neurilemma. The medullary
sheath and the primitive sheath are not, strictly speaking, parts of the
nerve cell, but appear to be growths that have formed around it. Certain
of the axons have no primitive sheath and others are without a medullary
sheath.(97)
*Form and Length of Axons.*--Where the axons terminate they usually
separate into a number of small divisions, thereby increasing the number
of their connections. Certain axons are also observed to give off branches
before the place of termination is reached (Fig. 131). These collateral
branches, by distributing themselves in a manner similar to the main
fiber, greatly extend the influence of a single neuron.
In the matter of length, great variation is found among the axons in
different parts of the body. In certain parts of the brain, for example,
are fibers not more than one one-hundredth of an inch in length, while the
axons that pass all the way from the spinal cord to the toes have a length
of more than three feet. Between these extremes practically all variations
in length are found.
*Arrangements of the Neurons.*--Nowhere in the body do the neurons exist
singly, but they are everywhere connected with each other to form the
different structures observed in the nerve skeleton. Two general plans of
connection are to be observed, known as the anatomical and the
physiological, or, more simply speaking, as the "side-by-side" and
"end-to-end" plans. The side-by-side plan is seen in that disposition of
the neurons which enables them to form the nerves and the ganglia, as well
as the brain and spinal cord. The end-to-end connections are necessary to
the work which the neurons do.
*Side-by-side Connections.*--On separating the ganglia and nerves into
their finest divisions, it is found that the nerves consist of axons,
while the ganglia are made up mainly of cell-bodies and dendrites. The
axons lie side by side in the nerve, being surrounded by the same
protective coverings, while the cell-bodies form a rounded mass or
cluster, which is the ganglion (Fig. 128). But the axons, in order to
connect with the cell-bodies, must terminate within the ganglion, so that
they too form a part of it. To some extent, also, axons pass through
ganglia with which they make no connection. The neurons in the brain and
spinal cord also lie side by side, but their arrangement is more complex
than that in the nerves and ganglia.
[Fig. 128]
Fig. 128--*Diagrams illustrating arrangement of neurons.* _A, B._ Ganglia
and short segments of nerves. 1. Ganglion. 2. Nerve. In the ganglion of
_A_ are end-to-end connections of different neurons; in the ganglion of
_B_ are the cell-bodies of di-axonic neurons. _C._ Section of a nerve
trunk. 1. Epineurium consisting chiefly of connective tissue. 2. Bundles
of nerve fibers. 3. Covering of fiber bundle, or perineurium. 4. Small
artery and vein.
The side-by-side arrangement of the neurons shows clearly the structure of
the ganglia and nerves. The nerve is seen to be a bundle of axons, or
nerve fibers, held together by connective tissue, while the ganglion is
little more than a cluster of cell-bodies. Their connection is necessarily
very close, for the same group of neurons will form, with their axons, the
nerve, and, with their cell-bodies, the ganglion (Fig. 128).
*End-to-end Connections.*--These consist of loose end-to-end unions of the
fiber branches of certain neurons with the dendrites of other neurons. The
purpose of such connections is to provide the means of communication
between different parts of the body. There appears to be no actual uniting
of the fiber branches with the dendrites, but they come into relations
sufficiently close to establish _conduction pathways_, and these extend
throughout the body (Fig. 129). They connect all parts of the body with
the brain and spinal cord, while connections within the brain and cord
bring the parts into communication with each other.
[Fig. 129]
Fig. 129--*Diagram of a nerve path* starting at the skin, extending through
the spinal cord, and passing out to muscles. A division of this path also
reaches the brain.
*Nature of the Nervous System.*--The nervous system represents the sum
total of the neurons in the body. In some respects it may be compared to
the modern telephone system. The neurons, like the electric wires, connect
different places with a central station (the brain and spinal cord), and
through the central station connections are established between the
different places in the system. As the separate wires are massed together
to form cables, the neurons are massed to form the gross structures of the
nervous system. The nervous system, however, is so radically different
from anything found outside of the animal body that no comparison can give
an adequate idea of it. We now pass to a study of the gross structures
observed in the nerve skeleton.
*Divisions of the Nervous System.*--While all of the nervous structures are
very closely blended, forming one complete system for the entire body,
this system presents different divisions which may, for convenience, be
studied separately. As physiologists have become better acquainted with
the human nervous system, different schemes of classification have been
proposed. The following outline, based upon the location of the different
parts, presents perhaps the simplest view of the entire group of nervous
structures:
[Table]
*The Central Division.*--This division of the nervous system lies within
the cranial and spinal cavities, and consists of the brain and the spinal
cord. The brain occupying the cranial cavity and the spinal cord in the
spinal cavity connect with each other through the large opening at the
base of the skull to form one continuous structure. The brain and cord are
the most complicated portions of the nervous system, and the ones most
difficult to understand.
[Fig. 130]
Fig. 130--*Diagram of divisions of brain.*
*The Brain.*--The brain, which is the largest mass of nervous tissue in the
body, weighs in the average sized man about 50 ounces, and in the average
sized woman about 44 ounces.(98) It may be roughly divided into three
parts, which are named from their positions (in lower animals) the
forebrain, the midbrain, and the hindbrain (Fig. 130). The forebrain
consists almost entirely of a single part, known as
*The Cerebrum.*--The cerebrum comprises about seven eighths of the entire
brain, and occupies all the front, middle, back, and upper portions of the
cranial cavity, spreading over and concealing, to a large extent, the
parts beneath. The surface layer of the cerebrum is called the _cortex_.
This is made up largely of cell-bodies, and has a grayish appearance.(99)
The cortex is greatly increased in area by the presence everywhere of
ridge-like _convolutions_, between which are deep but narrow depressions,
called _fissures_. The interior of the cerebrum consists mainly of nerve
fibers, or axons, which give it a whitish appearance. These fibers connect
with the cell-bodies in the cortex (Fig. 131).
The cerebrum is a double organ, consisting of two similar divisions,
called the _cerebral hemispheres_. These are separated by a deep groove,
extending from the front to the back of the brain, known as the _median
fissure_. The hemispheres, however, are closely connected by a great band
of underlying nerve fibers, called the _corpus callosum_.
[Fig. 131]
Fig. 131--*Microscope drawing* of a neuron from cerebral cortex. _a._ Short
segment of the axis cylinder with collateral branches.
At the base of the cerebrum three large masses of cell-bodies are to be
found. One of these, a double mass, occupies a central position between
the hemispheres, and is called the _optic thalami_. The other two occupy
front central positions at the base of either hemisphere, and are known as
the _corpora striata_, or the striate bodies.
*The Midbrain* is a short, rounded, and compact body that lies immediately
beneath the cerebrum, and connects it with the hindbrain. On account of
the great size of the cerebrum, the midbrain is entirely concealed from
view when the other parts occupy their normal positions. However, if the
cerebrum is pulled away from the hindbrain, it is brought into view
somewhat as in Fig. 130.
The midbrain carries upon its back and upper surface four small rounded
masses of cell-bodies, called the _corpora quadrigemina_. The upper two of
these bodies are connected with the eyes; the lower two appear to have
some connection with the organs of hearing. On the front and under
surface, the midbrain separates slightly as if to form two pillars, which
are called the _crura cerebri_, or cerebral peduncles. These contain the
great bundles of nerve fibers that connect the cerebrum with the parts of
the nervous system below.
*The Hindbrain* lies beneath the back portion of the cerebrum, and
occupies the enlargement at the base of the skull. It forms about one
eighth of the entire brain, and is composed of three parts--the cerebellum,
the pons, and the bulb.
*The Cerebellum* is a flat and somewhat triangular structure with its
upper surface fitting into the triangular under surface of the back of the
cerebrum. It is divided into three lobes--a central lobe and two lateral
lobes--and weighs about two and one half ounces. In its general form and
appearance, as well as in the arrangement of its cell-bodies and axons,
the cerebellum resembles the cerebrum. It differs from the cerebrum,
however, in being more compact, and in having its surface covered with
narrow, transverse ridges instead of the irregular and broader
convolutions (Fig. 132).
*The Pons*, or pons Varolii, named from its supposed resemblance to a
bridge, is situated in front of the cerebellum, and is readily recognized
as a circular expansion which extends forward from that body. It consists
largely of bands of nerve fibers, between which are several small masses
of cell-bodies. The fibers connect with different parts of the cerebellum
and with parts above.
[Fig. 132]
Fig. 132--*Human brain* viewed from below. _C._ Cerebrum. _Cb._ Cerebellum.
_M._ Midbrain. _P._ Pons. _B._ Bulb. I-XII. Cranial nerves.
*The Bulb*, or medulla oblongata, is, properly speaking, an enlargement of
the spinal cord within the cranial cavity. It is somewhat triangular in
shape, and lies immediately below the cerebellum. It contains important
clusters of cell-bodies, as well as the nerve fibers that pass from the
spinal cord to the brain.
*The Spinal Cord.*--This division of the central nervous system is about
seventeen inches in length and two thirds of an inch in diameter. It does
not extend the entire length of the spinal cavity, as might be supposed,
but terminates at the lower margin of the first lumbar vertebra.(100) It
connects at the upper end with the bulb, and terminates at the lower
extremity in a number of large nerve roots, which are continuous with the
nerves of the hips and legs (Fig. 133). Two deep fissures, one in front
and the other at the back, extend the entire length of the cord, and
separate it into two similar divisions. These are connected, however,
along their entire length by a central band consisting of both gray and
white matter.
[Fig. 133]
Fig. 133--*Spinal cord*, showing on one side the nerves and ganglia with
which it is closely related in function. _A._ Bulb. _B._ Cervical
enlargement. _C._ Lumbar enlargement. _D._ Termination of cord. _E._ Nerve
roots that occupy the spinal cavity below the cord. _P._ Pons. _D.G._
Dorsal root ganglia. _S.G._ Sympathetic ganglia. _N._ Nerve trunks to
upper and lower extremities.
The arrangement of the neurons of the spinal cord is just the reverse of
that in the cerebrum--the center being occupied by a double column of
cell-bodies, which give it a grayish appearance, while the fibers occupy
the outer portion of the cord, giving it a whitish appearance.
The spinal cord is not uniform in thickness, but tapers slightly, though
not uniformly, from the upper toward the lower end. At the places where
the nerves from the arms and legs enter the cord two enlargements are to
be found, the upper being called the _cervical_ and the lower the _lumbar
enlargement_. These, on account of the difference in length between the
cord and the spinal cavity, are above--the lower one considerably above--the
places where the limbs which they supply join the trunk (Fig. 133).
*Arrangement of the Neurons of the Brain and Cord.*--The cell-bodies in the
brain and spinal cord are collected into groups, and their fibers extend
from these groups to places that may be near or remote. Guided by the
white and gray colors of the nervous tissue, and also by the structures
revealed by the microscope, physiologists have made out three general
schemes in the grouping of cell-bodies, as follows:
1. _That of surface distribution_, the cell-bodies forming a thin but
continuous layer over a given surface. This is the plan in the cerebrum
and cerebellum, and here are found devices for increasing the surface: the
cerebrum having convolutions, the cerebellum transverse ridges.
2. _That of collections of cell-bodies into rounded masses._ Such masses
are found in the bulb, the pons, the midbrain, and the base of the
cerebrum.
3. _That of arrangement in a continuous column._ This is the plan in the
spinal cord. It matters not at what place the spinal cord be cut, a
central area of gray matter, resembling in form the capital letter H, is
always found.
The fibers connecting with the cell-bodies in the brain and spinal cord
are gathered into bundles or tracts, and these pass through different
parts somewhat as follows:
1. _In the cerebrum_ they extend in three general directions, forming
three classes of fibers. The first connect different localities in the
same hemisphere, and are known as _association_ fibers (_A_, Fig. 134).
The second make connection between the two hemispheres, and form the
corpus callosum. These are known as _commissural_ fibers (_C_, Fig. 134).
The third connect the cerebrum with the parts of the nervous system below,
and are called _projection_ fibers (_P_, Fig. 134).
2. _In the cerebellum_ both association and commissural fibers are found.
Bands of fibers, passing upward toward the cerebrum and downward toward
the cord, connect this part of the brain with other parts of the nervous
system.
[Fig. 134]
Fig. 134--*Semi-diagrammatic representation of a section through the right
cerebral hemisphere*, showing fiber tracts. _A._ Association fibers. _C._
Commissural fibers. _P._ Projection fibers. The cell-bodies with which the
fiber bundles connect are in the surface layer or cortex.
3. _In the midbrain, bulb, and spinal cord_ fibers are found: first, that
connect these parts with the cerebrum(101) and cerebellum above; second,
that pass into and become a part of the nerves of the body; and third,
that connect the opposite sides of these parts together.
*The Peripheral Division.*--The peripheral division of the nervous system
includes all the nervous structures found outside of the brain and spinal
cord. These consist of the cranial, spinal, and sympathetic nerves, and of
various small ganglia, all of which are closely connected with the central
system.
*Spinal Nerves and Dorsal-root Ganglia.*--The spinal nerves comprise a
group of thirty-one pairs, which connect the spinal cord with different
parts of the trunk, with the upper, and with the lower extremities. Each
nerve joins the cord by two roots, these being named from their positions
the _ventral_, or anterior, root and the _dorsal_, or posterior, root. The
two roots blend together within the spinal cavity to form a single nerve
trunk, which passes out between the vertebrae. On the dorsal root of each
spinal nerve is a small ganglion which is named, from its position, the
_dorsal-root ganglion_. (Consult Figs. 133 and 135, and also Fig. 125.)
*Double Nature of Spinal Nerves.*--Charles Bell, in 1811, made the
remarkable discovery that each spinal nerve is double in function. He
found the portion connecting with the cord by the dorsal root to be
concerned in the _production of feeling_ and the portion connecting by the
ventral root to be concerned in the _production of motion_. In keeping
with these functions, the two divisions of the nerve are made up of
different kinds of fibers, as follows:
1. The dorsal-root divisions, of the fibers of di-axonic neurons, the
cell-bodies of which form the dorsal-root ganglia (Fig. 135).
2. The ventral-root divisions, of the fibers of mon-axonic neurons, the
cell-bodies of which are in the gray matter of the cord.
The first convey impulses to the cord and are called _afferent_
neurons;(102) the second convey impulses from the cord and are known as
_efferent_ neurons. Thus, by forming a part of the nerve pathways between
the skin and the brain, the dorsal divisions of these nerves aid in the
production of feeling; and by completing pathways to the muscles, the
ventral divisions aid in the production of motion (Figs. 129, 135, and
141).
[Fig. 135]
Fig. 135--*Connection of spinal nerves with the cord.* On the right is
shown a nerve pathway from the skin to the muscle. A division of this
pathway reaches the brain.
*The Cranial Nerves.*--From the under front surface of the brain, twelve
pairs of nerves emerge and pass to the head, neck, and upper portions of
the trunk. These, the cranial nerves, have names suggestive of their
function or distribution and, in addition, are given numbers which
indicate the order in which they leave the brain (Fig. 136). Unlike the
spinal nerves, the cranial nerves present great variety among themselves,
scarcely any two of them being alike in function or in their connection
with different parts of the body. Several of them have to do with the
special senses, and are for this reason very important. They connect the
brain with the different parts of the head, neck, and trunk, as follows:
1. The first pair (_olfactory_ nerves; nerves of smell; afferent) connect
with the mucous membrane of the nostrils (Fig. 136).
2. The second pair (_optic_ nerves; nerves of sight; afferent) connect
with the retina of the eyes.
3. The third, fourth, and sixth pairs (_motores oculi;_ control muscles of
the eyes; efferent) connect with the internal and external muscles of the
eyeballs (Fig. 136).
[Fig. 136]
Fig. 136--*Diagram suggesting the distribution and functions of the cranial
nerves* (Colton). See also Fig. 132.
4. The fifth pair (_trigeminal_ nerves; nerves of feeling to the face, of
taste to the front of the tongue, and of control of muscles of
mastication; afferent and efferent) connect with the skin of the face, the
mucous membrane of the mouth, the teeth, and the muscles of mastication.
5. The seventh pair (_facial_ nerves; control muscles that give the facial
expressions; efferent) connect with the muscles just beneath the skin of
the face.
6. The eighth pair (_auditory_ nerves; nerves of hearing; afferent)
connect with the internal ear.
7. The ninth pair (_glossopharyngeal_ nerves; nerves of taste to back of
tongue and of muscular control of pharynx; afferent and efferent) connect
with the back surface of the tongue and with the muscles of the pharynx.
8. The tenth pair (_vagus_, or pneumogastric, nerves; nerves of feeling
and of muscular control; afferent and efferent) connect with the heart,
larynx, lungs, and stomach. They have the widest distribution of any of
the cranial nerves.
9. The eleventh pair (_spinal accessory_ nerves; control muscles of neck;
efferent) connect with the muscles of the neck.
10. The twelfth pair (_hypoglossal_ nerves; control muscles of the tongue;
efferent) connect with the muscles of the tongue.
*Sympathetic Ganglia and Nerves.*--The sympathetic ganglia are found in
different parts of the body, and vary in size from those which are half an
inch in diameter to those that are smaller than the heads of pins. The
largest and most important ones are found in two chains which lie in
front, and a little to either side, of the spinal column, and extend from
the neck to the region of the pelvis (Figs. 125 and 133). The number of
ganglia in each of these chains is about twenty-four. They are connected
on either side by the right and left sympathetic nerves which extend
vertically from ganglion to ganglion. In addition to the ganglia forming
these chains, important ones are found in the head (outside of the cranial
cavity) and in the plexuses of the thorax and the abdomen.
The sympathetic ganglia receive nerves from the central division of the
nervous system, but connect with glands, blood vessels, and the intestinal
walls through fibers from their own cell-bodies. Some of these latter
fibers join the spinal nerves, and some blend with each other to form
small sympathetic nerves.
*Protection of Brain and Spinal Cord.*--On account of their delicate
structure, the brain and spinal cord require the most complete protection.
In the first place, they are surrounded by the bones of the head and
spinal column; these not only shield them from the direct effects of
physical force, but by their peculiar construction prevent, to a large
degree, the passage of jars and shocks to the parts within. In the second
place, they are surrounded by three separate membranes, as follows:
1. The _dura_, or dura mater, a thick, dense, and tough membrane which
lines the bony cavities and forms supporting partitions.
2. The _pia_, or pia mater, a thin, delicate membrane, containing numerous
blood vessels, that covers the surface of the brain and cord.
3. The _arachnoid_, a membrane of loose texture, that lies between the
dura and the pin.
Finally, within the spaces of the arachnoid is a lymph-like liquid which
completely envelops the brain and the cord, and which, by serving as a
watery cushion, protects them from jars and shocks. Thus the brain and
cord are directly shielded by bones, by membranes, and by the liquid which
surrounds them. They are also protected from jars resulting from the
movements of the body by the general elasticity of the skeleton.
*Summary.*--The nervous system establishes connections between all parts of
the body, and provides a stimulus by means of which they are controlled.
It is made up of a special form of cells, called neurons. The neurons form
the different divisions of the nervous system, and also serve as the
active agents in carrying on its work. Through a side-by-side method of
joining they form the nerves, ganglia, spinal cord, and brain; and by a
method of end-to-end joining they connect places remote from each other,
and provide for nervous movements through the body. The nervous system,
may in some respects be compared to a complicated system of telephony, in
which the chains of neurons correspond to the wires, and the brain and
spinal cord to the central station.
Exercises.--1. Give the meaning of the term "cooerdination." Supply
illustrations.
2. What two general conditions are supplied in the body by the nervous
system?
3. Compare the skeleton outline of the nervous system with the bony
skeleton.
4. Sketch outlines of mon-axonic and di-axonic neurons.
5. Give two differences between the neurons and the other cells of the
body.
6. Describe the two general methods of connecting neurons in the body.
What purpose is accomplished by each method?
7. Name and locate the principal divisions of the nervous system.
8. Draw an outline of the brain (side view), locating each of its
principal divisions.
9. If a pencil were placed over the ear, what portions of the brain would
be above it and what below?
10. Describe briefly the cerebrum, the cerebellum, the midbrain, the pons,
and the bulb.
11. Locate and describe the cortex. State purpose of the convolutions.
12. State the general differences between the cranial and the spinal
nerves.
13. Locate and give the number of the dorsal-root ganglia. Locate and give
the approximate number of the sympathetic ganglia.
14. Show how the two portions of the spinal nerves are formed--the one from
the mon-axonic and the other from the di-axonic neurons.
15. Enumerate the different agencies through which the brain and spinal
cord are protected.
16. What cranial nerves contain afferent fibers? What ones contain
efferent fibers? What ones contain both afferent and efferent fibers?
17. In what respects is the nervous system similar to a system of
telephony? In what respects is it different?
PRACTICAL WORK
Examine a model of the brain, identifying the different divisions and
noting the position and relative size of the different parts (Fig. 137).
Observe the convolutions of the cerebrum and compare these with the
parallel ridges of the cerebellum. If the model is dissectible, study the
arrangement of the cell-bodies (gray matter) and the distribution of the
fiber bundles (white matter). Note the connection of the cranial nerves
with the under side.
[Fig. 137]
Fig. 137--Model for demonstrating the brain (dissectible).
A prepared nervous system of a frog (such as may be obtained from supply
houses) should also be examined. Observe the appearance and general
distribution of the nerves and their connection with the brain and spinal
cord. If such a preparation is not at hand, some small animal may be
dissected to show the main divisions of the nervous system, as follows:
*Dissection of the Nervous System* (by the teacher).--For this purpose a
half-grown cat is generally the best available material. This should be
killed with chloroform and secured to a board as in the dissection of the
abdomen (page 169). Open the abdominal cavity and remove the contents,
tying the alimentary canal where it is cut, and washing out any blood
which may escape. Dissect for the nervous system in the following order:
1. Cut away the front of the chest, exposing the heart and lungs. Find on
each side of the heart a nerve which passes by the side of the pericardium
to the diaphragm. These nerves assist in controlling respiration and are
called the _phrenic_ nerves. Find other nerves going to different parts of
the thorax.
2. Remove the heart and lungs. Find in the back part of the thoracic
cavity, on each side of the spinal column, a number of small "knots" of
nervous matter joined together by a single nerve. These are sympathetic
ganglia. Where the neck joins the thorax, find two sympathetic ganglia
much larger than the others.
3. Cut away the skin from the shoulder and upper side of the fore leg. By
separating the muscles and connective tissue where the leg joins the
thorax, find several nerves of considerable size. These connect with each
other, forming a network called the _brachial plexus_. From here nerves
pass to the thorax and to the fore leg.
4. From the brachial plexus trace out the nerves which pass to different
parts of the fore leg. In doing this separate the muscles with the fingers
and use the knife only where it is necessary to expose the nerves. Note
that some of the branches pass into the muscles, while others connect with
the skin.
5. Remove the skin from the upper portion of one of the hind legs and
separate the muscles carefully until a large nerve is found. This is one
of the divisions of the _sciatic_ nerve. Carefully trace it to the spinal
cord, cutting away the bone where necessary, and find the connections of
its branches with the cord. Then trace it toward the foot, discovering its
branches to different muscles and to the skin.
6. Unjoint the neck and remove the head. Examine the spinal cord where
exposed. Cut away the bone sufficiently to show the connection between the
cord and one of the spinal nerves. On the dorsal root of one of the nerves
find a small ganglion. What is it called?
7. Fasten the head to a small board and remove the scalp. Saw through the
skull bones in several directions. Pry off the small pieces of bones,
exposing the upper surface of the brain. Study its membranes,
convolutions, and divisions.
8. With a pair of bone forceps, or nippers, break away the skull until the
entire brain can be removed from the cavity. Examine the different
divisions, noting the relative position and size of the parts.
9. With a sharp knife cut sections through the different parts, showing
the positions of the "gray matter" and of the "white matter."
NOTE.--If the entire class is to examine one specimen, it is generally
better to have the dissecting done beforehand and the parts separated and
tacked to small boards. This will permit of individual examination.
Sketches of the sciatic nerve, brachial plexus, and of sections through
the brain and spinal cord should be made.
*Location of Nerves in the Body.*--Several of the nerves of the body lie
sufficiently near the surface to be located by pressure and are easily
recognized as sensitive cords. Slight pressure from the fingers reveals
the presence of nerves in the grooves of the elbow (the crazy bone),
between the muscles on the inner side of the arm near the shoulder, and in
the hollow part of the leg back of the knee. These are all large nerves.
Small nerves may be located in the same manner in the face and neck.
CHAPTER XVIII - PHYSIOLOGY OF THE NERVOUS SYSTEM
In the preceding chapter was pointed out the method by which the different
parts of the body are brought into communication by the neurons or nerve
cells. We are now to study the means whereby the neurons are made to
control and cooerdinate the different parts of the body and bring about the
necessary adjustment of the body to its surroundings. This work of the
neurons naturally has some relation to their properties.
*Properties of Neurons.*--The work of the neurons seems to depend mainly
upon two properties--the property of irritability and the property of
conductivity. _Irritability_ was explained, in the study of the muscles
(page 243), as the ability to respond to a stimulus. It has the same
meaning here. The neurons, however, respond more readily to stimuli than
do the muscles and are therefore more irritable. Moreover, they are
stimulated by all the forces that induce muscular contraction and by many
others besides. They are by far the most irritable portions of the body.
_Conductivity_ is the property which enables the effect of a stimulus to
be transferred from one part of a neuron to another. On account of this
property, an excitation, or disturbance, in any part of a neuron is
conducted or carried to all the other parts. Thus a disturbance at the
distant ends of the dendrites causes a movement toward the cell-body and,
reaching the cell-body, the disturbance is passed through it into the
axon. This movement through the neuron is called the _nervous impulse_.
*Purpose of the Impulse. *--Though the nature of the nervous impulse is not
understood, (103) its purpose is quite apparent. It is the means employed
by the nervous system for controlling and cooerdinating the different parts
of the body. The arrangement of the neurons enables impulses to be started
in certain parts of the nervous system, and the property of conductivity
causes them to be passed _as stimuli _to other parts. This enables
excitation at one place to bring about action at another place.
Acting as stimuli, the impulses seem able to produce two distinct effects:
first, to throw resting organs into action and to increase the activity of
organs already at work; and second, to diminish the rate, or check
entirely, the activity of organs. Impulses producing the first effect are
called _excitant_ impulses; those producing the second effect,
_inhibitory_ impulses.
*Functions of the Parts of Neurons.*--The _cell-body_ serves as a nutritive
center from which the other parts derive nourishment. Proof of this is
found in the fact that when any part of the neuron is separated from the
cell-body, it dies, while the cell-body and the parts attached to the
cell-body may continue to live. In addition to this the cell-body probably
reenforces the nervous impulse.
The _dendrites_ serve two purposes: first, they extend the surface of the
cell-body, thereby enabling it to absorb a greater amount of nourishment
from the surrounding lymph; second, they act as _receivers of stimuli_
from other neurons. The same impulse does not pass from one neuron to
another. An impulse in one neuron, however, is able to excite the neuron
with which it makes an end-to-end connection, so that a series of impulses
is produced along a given nerve path (Fig. 129).
The special _function of the axon_ is to transmit the impulse. By its
length, structure, and property of conductivity it is especially adapted
to this purpose. The axis cylinder, however, is the only part of the axon
concerned in the transmission. The primitive sheath and the medullary
layer protect the axis cylinder, and, according to some authorities, serve
to insulate it. The medullary sheath may also aid in the nourishment of
the axis cylinder.
*Nerve Stimuli.*--While the properties of irritability and conductivity
supply a necessary cause for the production and transmission of nervous
impulses, these alone are not sufficient to account for their origin. An
additional cause is necessary--a force not found in the nerve protoplasm,
but one which, by its action on the protoplasm, makes it produce the
impulse. In this respect, the neuron does not differ essentially from the
cell of a muscle. Just as the muscle cell requires a stimulus to make it
contract, so does the neuron require a stimulus to start the impulse.
Hence, in accounting for the activities of the body, it is not sufficient
to say they are caused by nervous impulses. We must also investigate the
_nerve stimuli_--the means through which the nervous impulses are started.
Most of these are found outside of the body and are known as external
stimuli.
*Action of External Stimuli.*--In the arrangement of the nervous system the
most favorable conditions are provided for the reception of external
stimuli. Not only do vast numbers of neurons terminate at the surface of
the body,(104) but they connect there with delicate structures, called
_sense organs_. The purpose of the sense organs is to _sensitize_ (make
sensitive) the terminations of the neurons. This they do by supplying
special structures through which the stimuli can act to the best advantage
upon the nerve endings. Moreover, there are different kinds of sense
organs, and these cause the neurons to be sensitive to different kinds of
stimuli. Acting through the sense organs adapted for receiving them,
light, sound, heat, cold, and odors all act as stimuli for starting
impulses. Indeed, the arrangement is so complete that the nervous system
is subjected to the action of external stimuli in some form practically
all the time. The work of the sense organs is further considered in
Chapters XX, XXI, and XXII.
*How External Stimuli act on Internal Organs.*--For stimulating the neurons
not connected with the body surface we are dependent, so far as known,
upon the nervous impulses. An impulse started by the external stimulus
goes only so far as its neuron extends. But it serves as a stimulus for
the neuron with which the first connects and starts an impulse in this
connecting neuron, the point of stimulation being where the fiber
terminations of the first neuron make connection with the dendrites of the
second. This impulse in turn stimulates the next neuron, and so on,
producing a series of impulses along a given nerve path. In this way the
effect of an external stimulus may reach and bring about action in any
part of the body. This is in brief the general plan of inducing action in
the various organs of the body. This plan, however, is varied according to
circumstances, and at least three well-defined forms of action are easily
made out. These are known as _reflex action, voluntary action_, and
_secondary reflex action_.
*Reflex Action.*--When some sudden or strong stimulus acts upon the nerve
terminations at the surface of the body, an immediate response is
frequently observed in some quick movement. The jerking away of the hand
on accidentally touching a hot stove, the winking of the eyes on sudden
exposure to danger, and the quick movements from slight electrical shocks
are familiar examples. The explanation of reflex action is that external
stimuli start impulses in neurons terminating at the surface of the body
and these, in turn, excite impulses in neurons which pass from the spinal
cord or brain to the muscles (Fig. 138). Since there is an apparent
turning back of the impulses by the cord or brain, the resulting movements
are termed _reflex_.(105)
[Fig. 138]
Fig. 138--*Diagram illustrating reflex action of an external organ.*
*Reflex Action and the Mind.*--If one carefully studies the reflex actions
of his own body, he will find that they occur at the time, or even a
little before the time, that he realizes what has happened. If a feather
is brought in contact with the more sensitive parts of the face of a
sleeping person, there is a twitching of the skin and sometimes a movement
of the hand to remove the offending substance. Surgeons operating upon
patients completely under the influence of chloroform, and therefore
completely unconscious, have observed strong reflex actions. These and
other similar cases indicate clearly that reflex action occurs
_independently_ of the mind--that the mind neither causes nor controls it.
If a further proof of this fact were needed, it is supplied by experiments
upon certain of the lower animals,(106) which live for a while after the
removal of the brain. These experiments show that the nervous impulses
that produce reflex action need only pass through the spinal cord and do
not reach the cerebrum, the organ of the mind.
*The Reflex Action Pathway.*--By study of the impulses that produce any
reflex action, a rather definite pathway may be made out, having the
following divisions:
1. _From the surface of the body to the central nervous system_ (usually
the spinal cord). This, the _afferent_ division, is made up of di-axonic
neurons, and these have (in the case of the spinal nerves) their
cell-bodies in the dorsal root ganglia (page 295). They are acted upon by
external stimuli, while their impulses in turn act on the neurons in the
spinal cord.
2. _Through the central system_ (spinal cord or base of brain). This, the
_intermediate_ division, may be composed of mon-axonic neurons, or it may
consist of branches from the afferent neurons. In the case of separate
neurons, these are acted upon by impulses from the afferent neurons, while
their impulses serve in turn as stimuli to other neurons within the cord
(Fig. 129).
3. _From the central nervous system to the muscles._ This, the _efferent_
division, is made up of mon-axonic neurons. Most of these have their
cell-bodies in the gray matter of the cord, while their fibers pass into
the spinal nerves by the ventral roots.(107) They may be stimulated by
impulses either from the intermediate neurons, or from branches of the
afferent neurons. Their impulses reach and stimulate the muscles.
*Reflex Action in Digestion.*--The flowing of the saliva, when food is
present in the mouth, is an example of reflex action. In this case,
however, the organ excited to activity is a gland instead of a muscle. The
food starts the impulses, and these, acting through the bulb, reach and
stimulate the salivary glands. In a similar manner food excites the glands
that empty their fluids into the stomach and intestines, and stimulates
the muscular coats of these organs to do their part in the digestive
process. To a considerable extent, neurons having their cell-bodies in the
sympathetic ganglia are concerned in these actions (Fig. 139).
[Fig. 139]
Fig. 139--Diagram illustrating reflex action in its relation to the food
canal. The nerve path in this case includes sympathetic neurons.
*Reflex Action in the Circulation of the Blood.*--On sudden exposure to
cold, the small arteries going to the skin quickly diminish in size, check
the flow of blood to the surface, and prevent too great a loss of heat. In
this case, impulses starting at the surface of the body are transmitted to
the bulb and then through the efferent neurons to the muscles in the walls
of the arteries. In a somewhat similar manner, heat leads to a relaxation
of the arterial walls and an increase in the blood supply to the skin.
Other changes in the blood supply to different parts of the body are also
of the nature of reflex actions. As in the work of digestion, neurons
having their cell-bodies in the sympathetic ganglia aid in the control of
the circulation.
*Purposes of Reflex Action.*--The examples of reflex action so far
considered illustrate its two main purposes--(1) protection, and (2) a
means of controlling important processes.
The pupil has but to study carefully the reflex actions of his own body
for a period, say of two or three weeks, in order to be convinced of their
protective value. He will observe that portions of his body have, on
exposure to danger, been moved to places of safety, while in some
instances, like falling, his entire body has been adjusted to new
conditions. He will also find that reflex action is quicker, and for that
reason offers in some cases better protection, than movements directed by
the mind. In digestion and circulation are found the best examples of the
control of important processes through reflex action.
*Voluntary Action.*--It is observed that reflex action, in the sense that
it has so far been considered, is not the usual mode of action of the
external organs, but is, instead, a kind of emergency action, due to
unusual conditions and excitation by strong stimuli. Voluntary actions, on
the other hand, represent the ordinary, or normal, action of these organs.
They comprise the movements of the body of which we are conscious and
which are _controlled by the mind_. But while they are of a higher order
than reflex actions and are under _intelligent_ direction, they are
brought about in much the same manner.
*Voluntary Action Pathways* differ in but one essential respect from those
of reflex action. They pass through the cerebrum, the organ of the mind
(Fig. 140). This is necessary in order that the mind may control the
action. From all portions of the body surface, afferent pathways may be
traced to the cerebrum; and from the cerebrum efferent pathways extend to
all the voluntary organs. A complex system of intermediate neurons, found
mostly in the brain, join the afferent with the efferent pathways. The
voluntary pathways are not distinct from, but include, reflex pathways, a
fact which explains why the same external stimulus may excite both reflex
and voluntary action (Fig. 141).
[Fig. 140]
Fig. 140--*Diagram of a voluntary action pathway.*
*Choice in Voluntary Action.*--In reflex action a given stimulus, acting in
a certain way; produces each time the same result. This is not the case
with voluntary action, the difference being _due to the mind_. In these
actions the external stimulus first excites the mind, and the resulting
mental processes--perhaps as memory of previous experiences--supply a
variety of facts, any of which may act as stimuli to action. Before the
action takes place, however, some one fact must be singled out from among
the mental processes excited. This fact becomes the _exciting stimulus_
and leads to action. It follows, therefore, that the action which finally
occurs is not necessarily the result of an immediate external stimulus,
but of a _selected_ stimulus--one which is the result of choice.
[Fig. 141]
Fig. 141--*Diagram of voluntary action pathways* including reflex pathways.
Not only does the element of choice enter into the selection of the proper
stimulus, but it also enters into the time, nature, and intensity of the
action. For these reasons it is frequently impossible to trace voluntary
actions back to their actual stimuli. The pupil will recognize the element
of choice in such simple acts as picking up some object from the street,
complying with a request, and purchasing some article from a store.
*Reflex and Voluntary Action Compared.*--Certain likenesses and
differences, already suggested in these two forms of action, may now be
more fully pointed out. Reflex and voluntary action are alike in that the
primary cause of each is some outside force or condition which has
impressed itself upon the nervous system. They are also alike in the
general direction taken by the impulses in producing the action. The
impulses are, first, from the surface of the body to the central nervous
system; second, through the central system; and third, from the central
nervous system to the active tissues of the body.
Their chief differences are to be found, first, in the pathways followed
by the impulses, which are through the cerebrum (the organ of the mind) in
voluntary action, but in reflex action are only through the spinal cord or
the lower parts of the brain; and second, in the fact that voluntary
action is under the direction of the mind, while reflex action is not. It
would seem, therefore, that the statement sometimes made that "voluntary
action is reflex action plus the mind" is not far from correct. Mind,
however, is the important factor in this kind of action.
*Secondary Reflex Action.*--Everyday experience teaches that any voluntary
action becomes easier by repetition. A given act performed a number of
times under conscious direction establishes a condition in the nervous
system that enables it to occur without that direction and very much as
reflex actions occur. Actions of this kind are known as secondary reflex
actions, or as _acquired reflexes_. Walking, writing, and numerous other
movements pertaining to the occupation which one follows are examples of
such reflexes. These activities are at first entirely voluntary, but by
repetition they gradually become reflex, requiring only the stimulus to
start them.
The advantages to the body of its acquired reflexes are quite apparent.
The mind does not have to attend to the selection and direction of stimuli
and, to that extent, is left free for other work. A good example of this
is found in writing, where the mind apparently gives no heed to the
movements of the hand and is only concerned in what is being written. The
student will easily supply other illustrations of the advantages of
secondary reflex action.
The development of secondary reflexes probably consists in the
establishment of fixed pathways for impulses through the nervous system.
Through the branching of the nerve fibers many pathways are open to the
impulses. But in repeating the same kind of action the impulses are guided
into particular paths, or channels. In time these paths become so well
established that the impulses flow along them without conscious direction
and it is then simply necessary that some stimulus starts the impulses. By
following the established pathways, these reach the right destination and
produce the desired result. According to this view, secondary reflex
action is but a higher phase of ordinary reflex action--a kind of reflex
action, the conditions of which have been established by the mind through
repetition. (See functions of the cerebellum, page 317.)
*Habits.*--People are observed to act differently when exposed to the same
conditions, or when acted upon by the same stimuli. This is explained by
saying they have different habits. By _habits_ are meant certain general
modes of action that have been acquired by repetition. Certain acts
repeated again and again have established conditions in the nervous system
which enable definite forms of action to be excited, somewhat after the
manner of reflex action. On account of habits, therefore, the actions of
the individual are more or less _predisposed_. What he will do under
certain conditions may be foretold from his habits. Habits simply
represent, a higher order of secondary reflexes--those more closely
associated with the mental life and character than are the lower forms.
Habits, in common with other forms of secondary reflex action, serve the
important purpose of _economizing the nervous energy_. However, if
pernicious habits are formed instead of those that are useful, they are
detrimental from both a moral and physical standpoint. Youth is recognized
as the period in which fundamental habits are formed and character is
largely determined. Therefore parents and teachers do wisely when they
insist upon the formation of right habits by the young.
*Functions of Divisions of the Nervous System.*--The relationship between
the different parts of the nervous system is very close and one part does
not work independently of other parts. At the same time the general work
of the nervous system requires that its different divisions serve
different purposes:
1. The peripheral divisions of the nervous system are concerned in the
transmission of impulses between the surface of the body and the central
system and between the central system and the active tissues. The nerves
are the carriers of the impulses. The ganglia contain the cell-bodies
which serve as nutritive centers; and, in the case of the sympathetic
ganglia, these cell-bodies are the places where the fiber terminations of
one neuron connect with, and stimulate, other neurons.
2. The gray matter in the spinal cord, bulb, pons, and midbrain (through
the cell-bodies, fiber terminations, and short neurons which they contain)
completes the reflex action pathways between the surface of the body and
the voluntary muscles, and also between the surface of the body and the
organs of circulation and digestion.
3. The white matter of the spinal cord, bulb, pons, and midbrain (by means
of the fibers of which they are largely composed) forms connections with,
and passes impulses between, the various parts of the central nervous
system.
4. The bulb, because of certain special reflex-action pathways completed
through it, is the portion of the central nervous system concerned in the
control of respiration, circulation, and the secretion of liquids.
*Work of the Sympathetic Ganglia and Nerves.*--The neurons which form these
ganglia aid in controlling the vital processes, especially digestion and
circulation. These neurons are controlled for the most part by fibers from
the bulb and spinal cord, and cannot for this reason be looked upon as
forming an independent system. Their chief purpose seems to be that of
spreading the influence of neurons from the central system over a wider
area than they would otherwise reach. For example, a single neuron passing
out from the spinal cord may, by terminating in a sympathetic ganglion,
stimulate a large number of neurons, each of which will in turn stimulate
the cells of muscles or of glands. Because of this function, the
sympathetic neurons are sometimes called _distributing_ neurons.
*Functions of the Cerebellum.*--Efforts to discover some _special_ function
of the cerebellum have been in the main unsuccessful. Its removal from
animals, instead of producing definite results, usually interferes in a
mild way with a number of activities. The most noticeable results are a
general weakness of the muscles and an inability on the part of the animal
to balance itself. This and other facts, including the manner of its
connection with other parts of the nervous system, have led to the belief
that the cerebellum is the chief organ for the _reflex_ cooerdination of
muscular movements, especially those having to do with the balancing of
the body. In this connection it is subordinate to and under the control of
the cerebrum. Of the relations which the cerebellum sustains to the
cerebrum and to the different parts of the body, the following view is
quite generally held:
In the development of secondary reflexes, as already described, conditions
are established in the cerebellum, such that given stimuli may act
_reflexively_ through it and produce definite results in the way of
muscular contraction. After the establishment of these conditions,
afferent impulses from the eyes, ears, skin, and other places, under the
general direction of the cerebrum, may cause such actions as the balancing
of the body, walking, etc., as well as the delicate and varied movements
of the hand. This view of its functions makes of the cerebellum the great
center of secondary reflex action.
*Functions of the Cerebrum.*--While the work of the cerebrum is closely
related to that of the general nervous system, it, more than any other
part, exercises functions peculiar to itself. The cerebrum is the part of
the nervous system upon which our varied experiences leave their
impressions and through which these impressions are made to influence the
movements of the body. But the power to alter, postpone, or entirely
inhibit, nervous movements is but a part of the general work ascribed to
the cerebrum as _the organ of the mind_. Numerous experiments performed
upon the lower animals, together with observations on man, show the
cerebrum to be the seat of the mental activities, and to make possible, in
some way, the processes of consciousness, memory, volition, imagination,
emotion, thought, and sensation.
*Localization of Cerebral Functions.*--Many experiments have been performed
with a view to determining whether the entire cerebrum is concerned in
each of its several activities or whether special functions belong to its
different parts. These experiments have been made upon the lower animals
and the results thus obtained compared with observations made upon injured
and imperfectly developed brains in man. The results have led to the
conclusion that certain forms of the work of the cerebrum are _localized_
and that some of its parts are concerned in processes different from those
of others.
[Fig. 142]
Fig. 142--*Location of cerebral functions.* Diagram of cerebrum, showing
most of the areas whose functions are known.
The work of locating the functions of different parts of the cerebrum
forms one of the most interesting chapters in the history of brain
physiology. The portions having to do with sight, voluntary motion,
speech, and hearing have been rather accurately determined, while
considerable evidence as to the location of other functions has been
secured. Much of the cerebral surface, however, is still undetermined
(Fig. 142).
NERVOUS CONTROL OF IMPORTANT PROCESSES
*Circulation of the Blood.*--1. _Control of the Heart._--The ability to
contract at regular intervals has been shown to reside in the heart
muscle. Among other proofs is that furnished by cold-blooded animals, like
the frog, whose heart remains active for quite a while after its removal
from the body. These automatic contractions, however, are not sufficient
to meet all the demands made upon the circulation. The needs of the
tissues for the constituents of the blood vary with their activity, and it
is therefore necessary to vary frequently the force and rapidity of the
heart's contractions. Such changes the heart itself is unable to bring
about.
For the purpose of controlling the rate and force of its contractions, the
heart is connected with the central nervous system by two kinds of fibers:
_a._ Fibers that convey _excitant_ impulses to the heart to quicken its
movements.
_b._ Fibers that convey _inhibitory_ impulses to the heart to retard its
movements.
The cell-bodies of the excitant fibers are found in the sympathetic
ganglia, but fibers from the bulb connect with and control them. The
cell-bodies of the inhibitory fibers are located in the bulb, from where
their fibers pass to the heart as a part of the vagus nerve.
In addition to the fibers above mentioned, are those that convey impulses
_from_ the heart to the bulb. These connect with neurons that in turn
connect with blood vessels and with them act reflexively, when the heart
is likely to be overstrained, to cause a dilation of the blood vessels.
This lessens the pressure which the heart must exert to empty itself of
blood. These fibers serve, in this way, as a kind of safety valve for the
heart.
2. _Control of Arteries._--Changes in the rate and force of the heart's
contractions can be made to correspond only to the _general_ needs of the
body. When the blood supply to a particular organ is to be increased or
diminished, this is accomplished through the muscular coat in the
arteries. The connection of the arterial muscle with the sympathetic
ganglia and the method by which they vary the flow of blood to different
organs has already been explained (pages 311 and 49), so that only the
location of the controlling neurons need be noted here. These, like the
controlling neurons of the heart, have their cell-bodies in the bulb. It
thus appears that the entire control of the circulation is effected in a
reflex manner through the nerve centers in the bulb. These centers are
stimulated by conditions that relate to the movement of the blood through
the body.
*Respiration.*--Efferent fibers connect the different muscles of
respiration with a cluster of cell-bodies in the bulb, called the
_respiratory center_. This center together with the nerves and muscles in
question form an automatic, or self-acting, mechanism similar in some
respects to that of the heart. Through the impulses passing from the
respiratory center to the muscles, a rhythmic action is maintained
sufficient to satisfy the usual needs of the body for oxygen. The demand
of the body for oxygen, however, varies with its activities, and to such
variations the respiratory center alone is unable to respond. The
regulating factor in the respiratory movements has been found to be the
condition of the blood with reference to the presence of oxygen and carbon
dioxide. If the blood contains much carbon dioxide and little oxygen, it
acts as a strong stimulus to the respiratory center, causing it, in turn,
to stimulate the respiratory muscles with greater intensity and frequency.
On the other hand, if the blood contains much oxygen and little carbon
dioxide, it acts only as a mild stimulus. This explains how physical
exercise increases the breathing, since the muscles at work consume more
oxygen than when resting and give more carbon dioxide and other wastes to
the blood.
The respiratory center is also connected by afferent nerves with the
mucous membrane of the air passages. Irritation of the nerve endings in
this membrane causes impulses to pass to the center, and this leads, by
reflex action, to such modifications of the respiratory acts as sneezing
and coughing. There is also a connection between the cerebrum and the
respiratory center. This is shown by the fact that one can voluntarily
change the rate and force of the respiratory movements, and further by the
fact that emotions affect the breathing.
*Regulation of the Body Temperature.*--As explained in the study of the
skin (page 270), the nervous system regulates the body temperature by
controlling the circulation of the blood through the skin and the internal
organs. This is accomplished by stimulating in a reflex manner the muscles
in the walls of certain arteries. To prevent the body from getting too
hot, muscles in the arteries going to the skin relax, thereby allowing
more blood to flow to the surface, where the heat can be disposed of
through radiation and through the evaporation of the perspiration. On the
other hand, if the body is in danger of losing too much heat, the muscles
in the walls of arteries going to the skin are made to contract and those
to internal organs relax, so that less blood flows to the skin and more to
the internal organs. In this way the nervous system adjusts the
circulation to suit the conditions of temperature outside of and within
the body and, in so doing, maintains the normal body temperature.
*Summary.*--The nervous system is able to control, cooerdinate, and adjust
the different organs of the body through its intimate connection with all
parts and through a stimulus (the nervous impulse) which it supplies and
transmits. Nervous impulses, excited by external stimuli, follow definite
paths and cause activity in the different parts of the body. All such
pathways are through the central nervous system. In reflex action the
impulses are mainly through the spinal cord, but to some extent through
the bulb, pons, and midbrain. In voluntary action they pass through the
cerebrum--a condition that leads to important modifications in the results.
The cerebrum, in addition to controlling the voluntary movements, is able
to establish the necessary conditions for secondary reflex actions, such
as walking, writing, etc. Although certain of the divisions of the nervous
system exercise special functions, all parts of it are closely related.
Exercises.--1. Give the function of each of the parts of a neuron.
2. State the purpose of the nervous impulse.
3. Show that the exciting cause of bodily action is outside of the nervous
system and, to a large extent, outside of the body.
4. Describe the arrangement that enables stimuli outside of the body to
cause action within the body.
5. Describe a reflex action and show how it is brought about.
6. Distinguish between afferent, efferent, and intermediate neurons.
7. Draw diagrams showing the impulse pathways in voluntary and in reflex
action.
8. What purposes are served by the sympathetic neurons?
9. Describe the method of control of the circulatory and digestive
processes. How do reflex actions protect the body?
10. Compare voluntary and reflex action. In what sense are all the
activities of the body reflex?
11. In what sense is walking voluntary? In what sense is it reflex?
12. How does secondary reflex action lessen the work of the nervous
system?
13. State the special functions of the nerves, ganglia, spinal cord, bulb,
cerebellum, and cerebrum.
14. State the importance of the formation of correct habits.
[Fig. 143]
Fig. 143--Nerve board for demonstrating nerve pathways.
PRACTICAL WORK
*To demonstrate Nerve Pathways.*--A smooth board, 2x6 ft., is painted
black, and upon this is drawn in white a life-size outline of the body.
Pieces of cord of different colors and lengths are knotted to represent
mon-axonic and di-axonic neurons. These are then pinned or tacked to the
board in such a manner as to represent the connections in the different
kinds of nerve pathways. Fig. 143 shows such a board with connections for
a reflex action and a voluntary action of the same muscle.
*Study of the "Knee Jerk" Reflex.*--A boy is seated on a chair with the
legs crossed. With a small pointer he is given a light, quick blow on the
upper margin of the patella at the point of connection of the tendon. The
stroke will usually be followed by a reflex movement of the foot. Does
this take place independently of the mind? (The one upon whom the
experiment is being performed should assume a relaxed condition and make
no effort either to cause or prevent the movement.) Can the movement be
inhibited (prevented)? Repeat the experiment, effort being made to prevent
the movement, but not by contracting opposing muscles.
Other reflex actions adapted to class study are those of the eyes, such as
the closing of the lids on moving objects near them and the dilating of
the pupils when the eyes are shaded. The involuntary jerking of the head
on bringing the prongs of a vibrating tuning fork in contact with the end
of the nose is also a reflex action which can be studied to advantage.
*To determine the Reaction Time.*--Have several pupils join hands, facing
outwards, making a complete circle, excepting one gap. Give a signal by
touching the hand of one pupil at the end of the line. Let this pupil
communicate the signal, by pressure of the other hand, to the next pupil
and so on around, having the last pupil raise the free hand at close of
the experiment. Note carefully the time, preferably with a stop watch,
required to complete the experiment and divide this by the number of
pupils, to get the average reaction time. The experiment may be repeated
with boys only and then with girls, comparing their average reaction time.
*Reflex Action of the Salivary Glands.*--Place a small pinch of salt upon
the tongue and note the flow of saliva into the mouth. Try other
substances, as starch, bits of wood, and sugar. What appears to be the
natural stimulus for these glands? Compare with reflex actions of the
muscles.
CHAPTER XIX - HYGIENE OF THE NERVOUS SYSTEM
The far-reaching effects and serious nature of disorders of the nervous
system are sufficient reasons for considering carefully those conditions
that make or mar its efficiency. Controlling all the activities of the
body and affecting through its own condition the welfare of all the
organs, the hygiene of the nervous system is, in a large measure, the
hygiene of the entire body. Moreover, it is known that some of our worst
diseases, including paralysis and insanity, are disorders of the nervous
system and are prevented in many instances by a proper mode of living.
*The Main Problem.*--Many of our nervous disorders are undoubtedly due to
the age in which we live. Our modern civilization, with all its facilities
for human advancement and enjoyment, throws an extra strain upon the
nervous system. Educational and social standards are higher than ever
before and life in all its phases is more complex. Since we can hardly
change the conditions under which we live, and probably would not if we
could, we must learn to adapt or adjust ourselves to them so as to secure
for the nervous system such relief as it requires. This adjustment is
sometimes difficult, even when the actual needs of the nervous system are
known.
The healthful action of the nervous system requires, on the one hand,
exercise, but on the other hand, a certain condition of quietude, or
_poise_--a state which is directly opposed to that of restlessness. The
conditions of modern life seem able to force upon the nervous system all
the exercise that it needs, and more (whether it be of the right kind or
not), so that the main problem of to-day seems to be that of conserving,
or economizing, the nervous energy and of preventing nervous waste.
*Wasteful Forms of Nervous Activity.*--There are without doubt many forms
of activity that waste the vital forces of the body and lead to nervous
exhaustion. Take, for example, the rather common habit of worrying over
the trivial things of life. Certainly the nervous energy spent in this way
cannot be used in doing useful work, but must be counted as so much loss
to the body. One who would use his nervous system to the best advantage
must find some way of preventing waste of this kind.(108)
Undue excitement, as well as pleasurable dissipations, also tend toward
nervous exhaustion. And while the fact is recognized that pleasurable
activities supply a necessary mental exercise, the limit of healthful
endurance must be watched and _excesses of all kinds avoided_. Intense
emotional states are found to be exhausting in the extreme; and the
suppression of such undesirable feelings as anger, fear, jealousy, and
resentment are of immense value in the hygiene of the nervous system.
*The Habit of Self-control.*--Much of the needless waste of nervous energy,
including that of worrying over trivial matters, may be prevented through
the exercise of self-control. From the standpoint of the nervous system,
the present age differs from the past mainly in supplying a greater number
and variety of nerve stimuli. Self-control means the ability to suppress
activities that would result from undesirable stimuli and to direct the
bodily activities into channels that are profitable. Self-control,
therefore, is not only to be exercised on occasions of great emergency,
but in the everyday affairs of life as well. It is even more important
that the daily toiler at his task be able to keep the petty annoyances of
life from acting as irritants to his nervous system than that he keep cool
during some great calamity. The habit of self-control is acquired mainly
through the persistent effort to prevent any and all kinds of petty
annoyances from affecting the nerves or the temper.
*Nervousness.*--Self-control is much more easily practiced by some than by
others. This is due partly to habit, but is also due to an actual
difference in the degree of sensitiveness, or irritability, of the nervous
systems of different people. One whose nervous system tends to respond too
readily to any and all kinds of stimuli is said to be "nervous." This
condition is in some instances inherited, but is in most cases due to the
wasteful expenditure of nervous energy or to the action of some drug upon
the body. Excess of mental work, too much reading, long-continued anxiety,
eye strain, and the use of tea, coffee, alcohol, tobacco, or other drugs,
including many of those taken as medicines, are known to cause
nervousness. Nervousness is not only a source of great annoyance, both to
one's self and to others, but is a menace to the general health.
The first step toward securing relief from such a condition is the removal
of the cause. The habits should be inquired into and excesses of all kinds
discontinued. In some instances it may be necessary to _have the eyes
__examined_ and glasses fitted by a competent oculist.(109) The nervous
energy should be carefully economized and the habit of self-control
diligently cultivated. Special exercises that have for their purpose the
equalizing of the circulation and the strengthening of the blood vessels
of the neck and the brain also have beneficial effects.
*Nervous Overstrain.*--Both mental and physical overwork tends to weaken
the nervous system and to produce nervousness. Where hard mental work is
long continued, or where it is carried on under excitement, a tense
nervous condition is developed which is decidedly weakening in its
effects. The causes which lead to such a condition, and in fact overwork
of all kinds, should if possible be avoided. Where this is not possible,
and in many cases it is not, the period of overwork should be followed by
one of rest, recreation, and plenty of sleep. To the overworked in body or
in mind, nothing is more important from a hygienic, as well as moral,
standpoint, than the right use of the _one rest day in seven_. The best
interests of our modern civilization _require_ that the Sabbath be kept as
a quiet, rest-giving day.
*Disturbed Circulation of the Brain.*--Nervousness not infrequently is
accompanied by an increase in the circulation of the brain and disappears
when this condition is relieved. Though mental work and excitement tend
naturally to increase the circulation in the brain, this should subside
with rest and relief from excitement. When there is a tendency for this
condition to become permanent, effort should be made looking for relief.
Increasing the circulation in the lower extremities by hot or cold foot
baths, or by much walking, is found to be most beneficial. Special
exercises of the muscles of the neck are also recommended as a means of
relieving this condition.(110)
*Hygienic Value of Work.*--Within reasonable limits, both mental and
physical work are conducive to the vigor of the nervous system. Through
work the energies of the body find their natural outlet, and this prevents
dissipation and the formation of bad habits. Even hard work does not
injure the nervous system, and severe mental exertion may be undergone,
provided the proper hygienic conditions are observed. The nervous
disorders suffered by brain workers are not, as a rule, due to the work
which the brain does, but to violation of the laws of health, especially
the law of exercise. Such persons should observe the general laws of
hygiene and especially should they practice daily those forms of physical
exercise that tend to counteract the effects of mental work.
*Physical Exercise* properly taken is beneficial to the nervous system
through both direct and indirect effects. A large proportion of the nerve
cells have for their function the production of motion, and these are
called into play only through muscular activity. Then, as already
suggested, physical exercise counteracts the unpleasant effects of mental
work. Hard study causes an excess of blood to be sent to the brain and a
diminished amount to the arms and to the legs. Physical exercise
redistributes the blood and equalizes the circulation. Light exercise
should, therefore, follow hard study. The student before retiring at night
is greatly aided in getting to sleep and is put in a better condition for
the next day's work by ten to fifteen minutes of light gymnastics. A daily
walk of two or three miles is also an excellent means of counteracting the
effects of mental work. The brain worker should, however, avoid violent
exercise or the carrying of any kind of exercise to exhaustion.
*Sleep*, and plenty of it, is one of the first requirements of the nervous
system. It is during sleep that the exhausted brain cells are replenished.
To shorten the time for sleep is to weaken the brain and to lessen its
working force. No one should attempt to get along with less than eight
hours of sleep each day and most people require more. Children require
more sleep than adults. Those under six years should have from eleven to
twelve hours of sleep per day. Children between six and ten years should
have at least ten hours.
*Insomnia*, or sleeplessness, on account of its effects upon the nervous
system, is to be regarded as a serious condition, and hygienic means for
relieving it should be diligently sought. Having its cause in nervousness,
a disturbed circulation of the brain, or some form of nervous exhaustion,
it is benefited through relieving these conditions and in the manner
already described. Of course the external conditions for aiding sleep
should not be overlooked. The bed should be comfortable, and the room
should be cool, well ventilated, dark, and quiet. The inducing of sleep by
means of drugs is a dangerous practice and should never be resorted to
except under the direction of the physician.
*Effects of Heat and Cold.*--Heat and cold both have their effects upon the
nervous system. Heat increases the nervous irritability, while cold acts
as a natural sedative to the nerves. A nervous person is made more nervous
by an overheated atmosphere, but derives beneficial effects from exposing
the body freely to cold air and water. The tonic cold bath (page 273), if
taken with the usual precautions, can be used to good advantage in
diminishing nervousness. The taking of outdoor exercise in cold weather
is, for the same reason, an excellent practice.
*Effect of Emotional States.*--We have already noted the effect of certain
emotional states upon the digestion of the food (page 162). Emotional
states are also known to interfere with breathing and with the action of
the heart. Such effects are explained through the close relation of the
mind to the work of the nervous system in general. While certain emotional
states, such as fear, anger, melancholia, and the impulse to worry,
interfere seriously with the normal action of the nervous system, others,
such as contentment, cheerfulness, and joy, are decidedly beneficial in
their effects. How important, then, is the habit of suppressing the states
that are harmful and of cultivating those that are beneficial. From a
hygienic, as well as social, standpoint a cheerful, happy disposition is
worth all the effort necessary for its attainment.
*The Nervous Condition of Children* should be a matter of deep concern on
the part of both parents and teachers. In the home, as well as in the
school, the child may be "pushed" until the nervous system receives
permanent injury. Exhaustion of nerve cells is produced through too many
and too vivid impressions being made upon the immature brain. The child
should be protected from undue excitement. He should have the benefit of
outdoor exercise and should be early inured to cold. He should be shielded
from the poisoning effects of tea, coffee, tobacco, alcohol, and other
drugs. He should have impressed upon him the habit of self-control. He
should not be indulged in foolish caprices or whims, but should be taught
to be content with plain, wholesome food and with the simple forms of
enjoyment.
*Influences at School.*--School life is necessarily a great strain upon the
child. Night study added to the work of the day makes a heavy burden for
elementary pupils to bear. Though the legal school age is usually fixed at
six years, delicate children should be kept out of school until they are
seven or eight years old, provided they have good homes. In addition to
the excitation incident to studying and reciting lessons, conditions
frequently arise both in the schoolroom and upon the playground that
create a feeling of fear or dread in the minds of children. Quarrels and
feuds among the children and the bullying of big boys on the playground
may work untold harm. All conditions tending to develop fear, uneasiness,
or undue excitement on the part of children should receive the attention
of those in authority.
*Excessive Reading* is a frequent cause of injury to the nervous systems
of children. This has a bad effect, both on account of too many
impressions being made upon the mind and also on account of the strain to
the eyes. Then if the reading consists mostly of light fiction, the mind
is directed away from the really important things of life. The reading of
children should be thoughtfully controlled, both as to quality and
quantity. Exciting stories should, as a rule, be excluded, but a taste for
biography, historical and scientific writings, and for the great works of
literature should be cultivated. Simple fairy tales which have a
recognized value in developing the imagination of the child need not be
omitted, but it is of vital importance that the "story-reading habit" be
not formed.
*Effects of Drugs.*--Because of its delicacy of structure a number of
chemical compounds, or drugs, are able to produce injurious effects upon
the nervous system. Some of these are violent poisons, while others, in
small quantities, are mild in their action. Certain drugs, in addition to
their immediate effects, bring about changes in the nervous system which
cause an unnatural appetite, or craving, that leads to their continued
use. This is the case with alcohol, the intoxicating substance in the
usual saloon drinks, and with nicotine, the stimulating drug in tobacco.
The same is also true of morphine, chloral, and several other drugs used
as medicines. The _danger of becoming a slave_ to some useless and
pernicious habit should dissuade one from the use of drugs except in cases
of positive emergency.
*Alcohol and the Nervous System.*--Alcohol, as already shown, injures
practically all portions of the body; but it has its worst effects upon
the nervous system. Through its action on this system, it interferes with
the circulation of the blood, produces a condition of "temporary insanity"
called intoxication, weakens the will, and eventually dethrones the
reason. Worst of all, it produces a condition of "chronic poisoning" which
manifests itself in an unnatural craving, and this causes it to be used by
the victim even when he knows he is "drinking to his own destruction."
Though its use in small quantities does not, as a rule, produce such
marked effects upon the nervous system, it develops the "craving," and
this is apt in time to lead to its use in larger quantities. But even if
this does not occur, the practice is objectionable for its unhygienic
effects in general.(111) Tippling with such mild solutions of alcohol as
light wine, beer, and hard cider is, for these reasons, a dangerous
pastime.
*Alcohol and Crime.*--It is sometimes stated that no one who leaves alcohol
alone will be injured by it. This is true only of its direct effects; not
of its indirect effects. Whenever a crime is committed somebody is
injured, and alcohol is known to be a chief cause of crime. Alcohol causes
crime through the loss of self-control, seen especially in intoxication,
and also because of the moroseness and quarrelsomeness which it developes
in certain individuals. Indirectly it causes crime through the poverty
which it engenders and through its influence in bringing about social
conditions out of which crime develops. Everything considered, the free
use of alcohol is incompatible with the nervous health and moral tone of a
community.
*Nicotine and the Nervous System.*--Nicotine is an oily substance which is
extracted from the tobacco plant. Its action on the nervous system is in
general that of a poison. Taken in small quantities, it is a mild
stimulant and, if the doses are repeated, a habit is formed which is
difficult to break. Tobacco is used mainly for the stimulating effect of
this drug. While not so serious in its results as the alcohol and other
drug habits, the use of tobacco is of no benefit, is a continual and
useless expense, and, in many instances, causes a derangement of the
healthy action of the body.(112) With the bad effects of the nicotine must
be included those of questionable substances added to the tobacco by the
manufacturer, either for their agreeable flavor or for adulteration.
*Relation of Age to the Effects of Nicotine.*--The use of tobacco by the
young is especially to be deplored. In addition to the harmful effects
observed in those of mature years, nicotine interferes with the normal
development of the body and lays, in many instances, the foundation for
physical and mental weakness in later life. The cigarette is decidedly
harmful, especially when inhalation is practiced, its deadening effects
being in part due to the wrappers, some of which have been shown to
contain arsenic and other poisonous drugs. While dulling the intellect and
weakening the body, cigarette smoking also tends to make criminals of
boys.(113) Parents, teachers, school officers, and all who have the good
of mankind at heart should take every precaution, including that of
setting a good example, to prevent the formation of the tobacco habit by
those of immature years.
*Habit versus Self-control.*--The power of self-control, already emphasized
for its importance in the economical expenditure of the nervous energy, is
of vital importance in its relation to the habits of the body.
Self-control is the chief safeguard against the formation of bad habits
and is the only means of redemption from such habits after they have once
been formed. The persistent cultivation of the power to control the
appetites and the passions, as well as all forms of activity which tend to
injure the body or debase the character, gives a tone to the nervous
system which increases the self-respect and raises the individual to a
_higher plane of life_. The worst habits _can_ be broken and good ones
formed in their stead, if only there is sufficient determination to
accomplish these results. Failure comes from not having the mind
thoroughly "made up" and from not having, back of the desire to do better,
"the strong will of a righteous determination."
*Effects of External Conditions.*--While the inner life and habits have
most to do with the hygiene of the nervous system, a certain amount of
attention may properly be given to those conditions outside of the body
which affect directly or indirectly the state of this system. Noise,
disorder, and confusion act as nervous irritants, but quiet, order, and
system have the opposite effect. There is, therefore, much in the
management of the office, factory, schoolroom, or home that has to do with
the real hygiene of the nerves as well as with the efficiency of the work
that is being done. The suppression of distracting influences not only
enables the mind to be given fully to the work in hand, but actually
prevents waste of nervous energy. Although the responsibility for securing
the best conditions for work rests primarily with those in charge, it is
also true that each individual in every organization may contribute to the
order or disorder that prevails.
*Social Relations.*--In considering the external conditions that affect the
nervous system, the fact must not be overlooked that man is a social being
and has to adjust himself to an established social order. His relations to
his fellow-men, therefore, affect strongly his nervous condition and
theirs also. For this reason the best hygiene of the nervous system is
based upon _moral_ as well as physical right living. Along with the power
of self-control and the maintenance of a correct nervous poise, there
should be a proper regard for the welfare of others. On account of the
ease with which one individual may disturb the nervous state of another,
those social forms and customs which tend to establish harmonious
relations among men are truly hygienic in their effects, and may well be
carried out in spirit as well as "in letter."
It is also a fact that a given mental state in one person tends to excite
a like state in those with whom he associates. How important, then, that
each and all cultivate, as habits, the qualities of cheerfulness,
kindness, and good-will, instead of the opposite states of mind.
Especially in the family, and other groups of closely associated
individuals, should the nervous effect of one member upon the others be
considered and every effort made to secure and maintain harmonious
relations.
*The High Ideal.*--Everything considered, the conditions most favorable to
the healthfulness of the nervous system are in harmony with what our
greatest teachers have pointed to as the higher plane of living. On this
account a true conception of the value and meaning of life is of the
greatest importance. _An ever present, strong desire to live a vigorous,
but simple and noble, life_ will suggest the proper course to pursue when
in doubt and will stimulate the power of self-control. It will lead to the
stopping of "nerve leaks" and to the maintenance of harmonious relations
with one's fellows. It will cause one to recoil from the use of alcohol
and other nerve poisons, as from a deadly serpent, seeing the end in the
beginning, and will be the means eventually of leading the body into its
greatest accomplishments.
*Summary.*--The nervous system, on account of its delicate structure, is
liable to injury through wrong methods of using it and also through the
introduction of drugs, or poisons, into the body. There are also found in
our methods of living and systems of education conditions that tend to
waste the nervous energy. To protect the nervous system from all these
threatened dangers requires, among other things, the power of
self-control. This enables the individual to direct his life according to
his highest ideals and to free himself from habits known to be injurious.
Children must have their nervous systems safeguarded by parents and
teachers. Especially must they be kept from becoming enslaved to some
drug, such as alcohol or the nicotine of tobacco.
*Exercises.*--1. In what respect is the hygiene of the nervous system the
hygiene of the entire body?
2. Of what value in the hygiene of the nervous system is the power of
self-control? How is the habit of self-control formed?
3. Name several forms of activity that waste the nervous energy.
4. Name several influences that react unfavorably on the nervous systems
of children.
5. How may too much reading prove injurious to the nervous system?
6. What forms of physical exercise are beneficial to the brain worker?
7. Why is the use of alcohol even in small quantities to be regarded as a
dangerous practice?
8. Name several causes of nervousness.
9. What are the unanswerable arguments for preventing the use of tobacco
by the young?
10. Why do cigarettes have a more harmful effect upon the body than other
forms of tobacco?
11. Enumerate conditions in the schoolroom that dissipate the nervous
energy of pupils; that economize it.
CHAPTER XX - PRODUCTION OF SENSATIONS
Our study of the nervous system has shown that impulses arising at the
surface of the body are able, through connecting neurons, to bring about
various activities. Moving along definite pathways, they induce motion in
the muscles, and in the glands the secretion of liquids. It is now our
purpose to consider the effect produced by afferent impulses upon the
brain and, through the brain, upon the mind.(114) This effect is
manifested in a variety of similar forms, known as
*The Sensations.*--Sensations constitute the lowest forms of mental
activity. Roughly speaking, they are the states of mind experienced as the
_direct_ result of impulses reaching the brain. In a sense, just as
impulses passing to the muscles cause motion, impulses passing to the
brain cause sensations. The feeling which results from the hand's touching
a table is a sensation and so also is the pain which is caused by an
injury to the body. The mental action in each case is due to impulses
passing to the brain. Care must be exercised by the beginner, however, not
to confuse sensations with the nervous impulses, on the one hand, or with
_secondary_ mental effects, such as emotion or imagination, on the other.
Sensations are properly regarded as the first conscious effects of the
afferent impulses and as the _beginning stage_ in the series of mental
processes that may take place on account of them.
In some way, not understood, the mind associates the sensation with the
part of the body from which the impulses come. Pain, for example, is not
felt at the brain where the sensation is produced, but at the place where
the injury occurs. This association, by the mind, of the sensations with
different parts of the body, is known as "localizing the sensation."
*Sensation Stimuli.*--While the sensations are dependent upon the afferent
impulses, the afferent impulses are in turn dependent upon causes outside
of the nervous system. If these are removed, the sensations cease and they
do not start up again unless the exciting influences are again applied.
Any agency, such as heat or pressure, which, by acting on the neurons of
the body, is able to produce a sensation, may be called a _sensation
stimulus_. It has perhaps already been observed that the stimuli that lead
to voluntary action, as well as those that produce reflex action of the
muscles, cause sensations at the same time. From this we may conclude that
sensation stimuli are the same in character as those that excite motion.
On the other hand, it should be noted that sensations are constantly
resulting from stimuli that are of too mild a nature to cause motion.
*Classes of Sensations.*--Perhaps as many as twenty distinct sensations,
such as pain, hunger, touch, etc., are recognized. If these are studied
with reference to their origin, it will be seen that some of them result
from the action of definite forms of stimuli upon the neurons terminating
in sense organs; while the others, as a rule, arise from the action of
indefinite stimuli upon neurons in parts of the body that do not possess
sense organs. The members of the first class--and these include the
sensations of touch, temperature, taste, smell, hearing, and sight--are
known as the _special_ sensations. The others, including the sensations of
pain, hunger, thirst, nausea, fatigue, comfort, discomfort, and those of
disease, are known as _organic_, or general, sensations. These two classes
of sensations differ in their purpose in the body as well as in the manner
of their origin.
*Purposes of Sensations.*--Any given sensation is related to the stimulus
which excites it as an _effect_ to a _cause_. It starts up or stops,
increases in intensity or diminishes, according to the action of the
exciting stimulus. As the stimuli are outside of the nervous system, and
in the majority of cases outside of the body, the sensations indicate to
the mind what is taking place either in the body itself or in its
surroundings. They supply, in other words, the means through which the
mind acquires information. By means of the special sensations, a knowledge
of the physical surroundings of the body is gained, and through the
organic sensations the needs of the body and the state of the various
organs are indicated. In general, sensations are made to serve two great
purposes in the body, as follows:
1. They provide the necessary conditions for intelligent and purposeful
action on the part of the body.
2. They supply the basis for the higher mental activities, as perception,
memory, thought, imagination, and emotion.
Intelligent action is impossible without a knowledge both of the bodily
organs and of the body's surroundings. Protection and the regulation of
the work of an organ necessitate a knowledge of its condition, while the
adapting and adjusting of the body to its surroundings require a knowledge
of what those surroundings are. The dependence of all the higher forms of
mental activity upon sensations is recognized by psychologists and is
easily demonstrated by a study of the manner in which we acquire
knowledge. "Without sensation there can be no thought."
*Steps in the Production of Sensations.*--The steps in the production of
sensations are not essentially different from those in the production of
reflex action. First of all, external stimuli act upon the fiber
terminations in the sense organs, or elsewhere, starting impulses in the
neurons. These pass into the central nervous system and there excite
neurons which in turn discharge impulses into the cerebrum. The result is
to arouse an activity of the mind--a sensation. The steps in the production
of any _special_ sensation naturally involve the following parts:
1. A sense organ where the terminations of the neurons are acted upon by
the stimulus.
2. A chain of neurons which connect the sense organ with the brain.
3. The part of the cerebrum which produces the sensation.
*Sense Organs.*--The sense organs are not parts of the afferent neurons,
but are structures of various kinds, in which the neurons terminate. Their
function is to enable the sensation stimuli to start the impulses. By
directing, concentrating, or controlling the stimuli, the sense organs
enable them to act to the best advantage upon the neurons. When it is
recognized that such widely different forces as light waves, sound waves,
heat, pressure, and odors are enabled by them to stimulate neurons, the
importance of these organs becomes apparent. As would naturally be
inferred, the construction of any sense organ has particular reference to
the nature of the stimulus which it is to receive. This is most apparent
in the sense organs of sight and hearing.
*Simple Forms of Sense Organs.*--The simplest form of a sense organ (if
such it may be called) is one found among the various tissues. It consists
of the terminal branches of nerve fibers which spread over a small area of
cells, as a network or plexus. Such endings are numerous in the skin and
muscles.
Next in order of complexity are the so-called _end-bulbs_. These consist
of rounded, or elongated, connective tissue capsules, within which the
nerve fibers terminate. On the inside the fibers lose their sheaths and
divide into branches, which wind through the capsule. End-bulbs are
abundant in the lining membrane of the eye, and are found also in the skin
of the lips and in the tissues around the joints.
Slightly more complex than the end-bulbs are the _touch corpuscles_. These
are elongated bulb-like bodies, having a length of about one
three-hundredth of an inch, and occupying the papillae of the skin (Fig.
144). They are composed mainly of connective tissue. Each corpuscle
receives the termination of one or more nerve fibers. These, on entering,
lose the medullary sheath and separate into a number of branches that
penetrate the corpuscle in different directions.
[Fig. 144]
Fig. 144--*A touch corpuscle* highly magnified. (See text.)
The largest of the simple forms of sense organs are bodies visible to the
naked eye and called, from their discoverer Pacini, the _Pacinian
corpuscles_. They lie along the course of nerves in many parts of the
body, and have the general form of grains of wheat. (See Practical Work.)
The Pacinian corpuscles are composed of connective tissue arranged in
separate layers around a narrow central cavity called the core (Fig. 145).
Within the core is the termination of a large nerve fiber. These
corpuscles are found in the connective tissue beneath the skin, along
tendons, around joints, and among the organs of the abdominal cavity.
[Fig. 145]
Fig. 145--*Pacinian corpuscle*, magnified. _A._ Medullated nerve fiber.
_B._ Axis cylinder terminating in small bulb at _C._ _D._ Concentric
layers of connective tissue. _E._ Inner bulb.
The simple forms of sense organs have a more or less general distribution
over the body, and are concerned in the production of at least three
special sensations. These are _touch, temperature_, and the _muscular
sensation_.
*Touch*, or feeling, is perhaps the simplest of the sensations. The sense
organs employed are the touch corpuscles, and the external stimulus is
some form of pressure or impact. Pressure applied to the skin, by acting
on the fiber terminations in the corpuscles, starts the impulses that give
rise to the sensation. The touch corpuscles render the fiber terminations
so sensitive that the slightest pressure is able to arouse sensations of
touch. It is found that _a change of pressure_, rather than pressure that
is constant, is the active stimulus. That all parts of the skin are not
equally sensitive to pressure, and that the mind does not interpret
equally well the sensations from different parts, are facts easily
demonstrated by experiment. (See Practical Work.)
*The Temperature Sensation.*--Temperature sensations, like those of touch,
are limited almost entirely to the skin. They are of two kinds, and are
designated as _heat_ sensations and as _cold_ sensations. Whether the
sense organs for temperature are different from those of touch is not
known. It is known, however, that the same corpuscles do not respond alike
to heat, cold, and pressure.
_A Change of Temperature_, rather than any specific degree of heat or
cold, is the active temperature stimulus. The sensation of warmth is
obtained when the temperature of the skin is being raised, and of cold
when it is being lowered. This explains why in going into a hallway from a
heated room one receives a sensation of cold, while in coming into the
same hallway from the outside air he receives a sensation of warmth. It is
for the same reason that we are able to distinguish only the relative, not
the actual, temperature of bodies.
*Muscular Sensations.*--These are sensations produced by impulses arising
at the muscles. Such impulses originate at the fiber terminations which
are found in both the muscles and their tendons. By muscular sensations
one is conscious of the location of a contracting muscle and of the degree
of its tension. They also make it possible to judge of the weight of
objects.
[Fig. 146]
Fig. 146--*Sense organs of taste.* _A._ Map of upper surface of tongue,
showing on the left the different kinds of papillae, and on the right the
areas of taste (after Hall). Area sensitive to bitter (----); to acid
(....); to salt (--.--.--.--); to sweet (--------). _B._ Section through a
papilla. _n._ Small nerve connecting with taste buds at _d. e._
Epithelium. _C._ Single taste bud magnified. _n._ Nerve, the fibers of
which terminate between the spindle-shaped cells _a. e._ Epithelial cells.
*The Sensation of Taste.*--The sense organs of taste are found chiefly in
the mucous membrane covering the upper surface of the tongue. Scattered
over this surface are a number of rounded elevations, or large papillae (A,
Fig. 146). Toward the back of the tongue two rows of these, larger than
the others, converge to meet at an angle, where is located a papilla of
exceptional size. Surrounding each papilla is a narrow depression, within
which are found the sense organs of taste (B, Fig. 146). These are called,
from their shape, _taste buds_, and each bud contains a central cavity
which communicates with the surface by a small opening--_the gustatory
pore_. Within this cavity are many slender, spindle-shaped cells which
terminate in hair-like projections at the end nearest the pore, but in
short fibers at the other end. Nerve fibers enter at the inner ends of the
buds and spread out between the cells (_C_, Fig. 146). These fibers pass
to the brain as parts of two pairs of nerves--those from the front of the
tongue joining the trigeminal nerve, and those from the back of the
tongue, the glossopharyngeal nerve.
The gustatary, or _taste stimulus_, is some chemical or physical condition
of substances which is manifested only when they are in a liquid state.
For this reason _only liquid substances can be tasted_. Solids to be
tasted must first be dissolved.
The different taste sensations are described as bitter, sweet, sour, and
saline, and in the order named are recognized as the tastes of quinine,
sugar, vinegar, and salt. As to how these different tastes are produced,
little is known. Flavors such as vanilla and lemon, and the flavors of
meats and fruits, are really smelled and not tasted. Taste serves two main
purposes: it is an aid in the selection of food and it is a means of
stimulating the digestive glands (page 161).
[Fig. 147]
Fig. 147--*Sense organ of smell.* _A._ Distribution of nerves in outer wall
of nasal cavity. 1. Turbinated bones. 2. Branch of fifth pair of nerves.
3. Branches of olfactory nerve. 4. Olfactory bulb. _B._ Diagram showing
connection of neurons concerned in smell.
*The Sensation of Smell.*--The sense organs of smell are found in the
mucous membrane lining the upper divisions of the nasal cavities. Here are
found two kinds of cells in great abundance--column-shaped epithelial cells
and the cells which are recognized as the sense organs of smell. These
olfactory cells are spindle-shaped, having at one end a slender,
thread-like projection which reaches the surface, and at the other end a
fiber which joins an olfactory nerve (B, Fig. 147). In fact, the olfactory
cells resemble closely the cell-bodies of neurons, and are thought to be
such. The divisions of the olfactory nerve pass through many small
openings in the ethmoid bone to connect with the olfactory bulbs, which in
turn connect with the cerebrum (A, Fig. 147).
*The Olfactory Stimulus.*--Only substances in the gaseous state can be
smelled. From this it is inferred that the stimulus is supplied by gas
particles. Solids and liquids are smelled because of the gas particles
which separate from them. The substance which is smelled must be kept
moving through the nostrils and made to come in direct contact with the
olfactory cells. There is practically no limit to the number of distinct
odors that may be recognized.
*Value of Smell.*--Although the sense of smell is not so acute in man as in
some of the lower animals, it is, nevertheless, a most important and
useful gift. It is the only sense that responds to matter in the gaseous
state, and is, for this reason, the only natural means of detecting
harmful constituents of the atmosphere. In this connection it has been
likened to a sentinel standing guard over the air passages. Many gases
are, however, without odor, and for this reason cannot be detected by the
nostrils. It is of especial importance that gases which are likely to
become mixed with the air supply to the body have odor, even though the
odor be disagreeable. The bad odors of illuminating gas and of various
compounds of the chemical laboratory, since they serve as danger signals
to put one exposed to them on his guard, are of great protective value.
*Sight and Hearing.*--The sense organs of sight and hearing are highly
complicated structures, and will be considered in the chapters following.
*Summary.*--Sensations are certain activities of the mind that result from
excitations within the body or at its surface. These cause the neurons to
discharge impulses which on reaching the cerebrum cause the sensations.
Sensations are necessary for intelligent and purposeful action and for
acquiring all kinds of knowledge. To enable the stimuli to act to the best
advantage in starting the impulses, special devices, called sense organs,
are employed. These receive the terminations of the neurons, and by their
special structure enable the most delicate stimuli to start impulses. The
simpler forms of sense organs are those of touch, temperature, taste, and
smell.
*Exercises.*--1. Compare sensations and reflex actions with reference to
their nature and cause. Give steps in the production of each.
2. Give examples of sensation stimuli. State the purpose of sense organs.
3. How do general sensations differ from special sensations?
4. Of what value is pain in the protection of the body?
5. Show that sensations lead to the higher forms of mental activity, such
as emotion and imagination.
6. Of what value to the body is the "localizing of the sensation"?
7. What kinds of sense organs are found in the skin? State the purpose of
each.
8. Through what sense avenues is one made aware of solids, of liquids, and
of gases?
9. Of what special protective value is the sense of smell?
PRACTICAL WORK
*To demonstrate the Pacinian Corpuscles.*--Spread out the mesentery from
the intestine of a cat and hold it between the eye and the light: Pacinian
corpuscles will appear as small translucent bodies having the general form
of grains of wheat. Secure a portion of the mesentery over a circular
opening in a thin piece of cork and examine it with a microscope of low
power. Follow the course of the nerve fiber to the nerve from which it
branches.
*To show Relative Sensitiveness of Different Parts of the Skin.*--Holding a
bristle between the fingers, bring the end in contact with the skin,
noting the amount of pressure necessary to cause a sensation of touch.
Test the lips, tongue, tips of fingers, and palm and back of hand, trying
different sizes of bristles. Has the degree of sensitiveness any relation
to the thickness of the cuticle?
*To show Perceptive Differences of Different Portions of the Skin.*--Place
the points of a pair of dividers on the back of the hand of one who looks
in the opposite direction. Is one point felt or two? Repeat several times,
changing the distance between the points until it is fully determined how
near the two points must be placed in order to be felt as one. In like
manner test other parts of the body, as the tips of the fingers and the
back of the neck. Compare results obtained at different places.
*To locate Warm and Cold Sensation Spots.*--Slowly and evenly draw a
blunt-pointed piece of metal over the back of the neck. If it be of the
same temperature as the skin, only touch sensations will be experienced.
If it be a little colder (the temperature of the room) sensations of cold
will be felt at certain spots. If slightly warmer than the body, heat
sensation spots will be found on other parts of the skin. If the heat and
cold sensation spots be marked and tested from day to day they will be
found to remain constant as to position. Inference.
CHAPTER XXI - THE LARYNX AND THE EAR