THE NERVOUS SYSTEM.
_Animal and Vegetative Functions--Sensation, Motion, and Volition--The
Structure of the Nervous System--The White and Gray Substances--The
Brain--Its Convolutions--The Cerebellum--The Spinal Cord and its System
of Nerves--The Anterior and Posterior Roots--The Sympathetic System of
Nerves--The Properties of Nervous Tissue--Excitability of Nervous
Tissues--The Functions of the Spinal Nerves and Cord--The Direction of
the Fibres of the Cord--Reflex Activity, and its Uses--The Functions of
the Medulla Oblongata and the Cranial Ganglia--The Reflex Action of the
Brain._
[Sidenote: 1. What processes are known as the vegetative functions? Why so
called? What properties and functions does the plant possess? Their
object?]
1. ANIMAL FUNCTIONS.--The vital processes which we have been considering,
in the three previous chapters, of digestion, circulation, and
respiration--belong to the class of functions known as _vegetative_
functions. That is, they are common to vegetables as well as animals; for
the plant, like the animal, can originate nothing, not even the smallest
particle of matter; and yet it grows, blossoms, and bears fruit, by reason
of obtaining and digesting the nutriment which the air and soil provide.
The plant has its circulatory fluid and channels, by which the nutriment is
distributed to all its parts. It has, also, a curious apparatus in its
foliage, by which it abstracts from the air those gaseous elements so
necessary to its support; and thus it accomplishes vegetable respiration.
These vegetative functions have their beginning and end within the organism
of the plant; and their object is the preservation of the plant itself, as
well as of the entire species.
[Sidenote: 2. What second set of powers has the animal? What functions are
mentioned? The advantage they give?]
2. The animal, in addition to these vegetative functions, has another set
of powers, by the use of which he becomes conscious of a world external to
himself, and brings {149} himself into active relations with it. By means
of the vegetative processes, his life and species are maintained; while, by
means of certain animal functions, he feels, acts, and thinks. These
functions, among which are sensation, motion, and volition, not only
distinguish the animal from the plant, but, in proportion to their
development, elevate one creature above another; and it is by virtue of his
pre-eminent endowment, in these respects, that man holds his position at
the head of the animal creation.
[Sidenote: 3. Animals whose structure is simple? As we approach man?
Dependence of the animal functions of man?]
3. Among animals whose structure is very simple, the hydra, or fresh-water
polyp, being an example, no special organs are empowered to perform
separate functions; but every part is endowed alike, so that if the animal
be cut into pieces, each portion has all the properties of the entire
original; and, if the circumstances be favorable, each of the pieces will
soon become a complete hydra. As we approach man, in the scale of beings,
we find that the organs multiply, and the functions become more complete.
The function of motion, the instruments of which--the muscles and
bones--have been considered in former chapters, and all the other animal
functions of man, depend upon the set of organs known as the nervous
system.
[Sidenote: 4. The nervous tissues, of what composed? When examined by the
aid of the microscope? The white substance? The gray substance?]
4. THE NERVOUS SYSTEM.--The intimate structure of this system differs from
any tissue which we have before examined. It is composed of a soft, pulpy
substance, which, early in life, is almost fluid, but which gradually
hardens with the growth of the body. When examined under the microscope, it
is found to be composed of two distinct elements:--(1) the white substance,
composing the larger proportion of the nervous organs of the body, which is
formed of delicate cylindrical filaments, about 1/6000 of an inch in
diameter, termed the nerve-fibres; and (2) the gray substance, composed of
grayish-red, or {150} ashen-colored cells, of various sizes, generally
possessing one or more off-shoots, which are continuous with the
nerve-fibres just mentioned.
[Sidenote: 5. Nervous centres and ganglia? Nerves? What do they serve?
Cerebro-spinal system?]
5. The gray, cellular substance constitutes the larger portion of those
important masses, which bear the name of _nervous centres_ and _ganglia_
(from _ganglion_, a knot), and in which all the nerve-fibres unite. These
white nerve-fibres are found combined together in long and dense cords,
called _nerves_ (from _neuron_, a cord), which serve to connect the nervous
centres with each other, and to place them in communication with all the
other parts of the body which have sensibility or power of motion. That
part of the nervous system which is concerned in the animal functions,
comprises the brain, the spinal cord, and the nerves which are derived
therefrom; these are, together, called the _cerebro-spinal_ system (Fig.
40); while that other set of organs, which presides over, and regulates the
vegetative functions, is called the sympathetic system of nerves.
[Sidenote: 6. Location of the brain? Its weight? Its shape? Of what it
consists? What organs at the base?]
6. THE BRAIN.--The brain is the great volume of nervous tissue that is
lodged within the skull. It is the largest and most complex of the nervous
centres, its weight, in the adult, being about fifty ounces, or
one-fortieth of that of the whole body. The shape of the brain is oval, or
egg-shaped, with one extremity larger than the other, which is placed
posteriorly in the skull, to the concavity of which it very closely
conforms. The brain consists chiefly of two parts; the _cerebrum_, or brain
proper, and the _cerebellum_, or "little brain." In addition to these,
there are several smaller organs at the base, among which is the
commencement or expansion of the spinal cord, termed the _medulla
oblongata_, or oblong marrow.
{151}
[Illustration: FIG. 40.--THE CEREBRO-SPINAL SYSTEM.]
{152} [Sidenote: 7. The tissue of the brain? What, therefore, is required?
Blows on the head? Membranes of the brain? Blood sent to the brain?]
7. The tissue of the brain is soft and easily altered in shape by pressure;
it therefore requires to be placed in a well-protected position, such as is
afforded by the skull, or _cranium_, which is strong without being
cumbrous. In the course of an ordinary lifetime, this bony box sustains
many blows, with little inconvenience; while, if they fell directly upon
the brain, they would at once, and completely, disorganize that structure.
Within the skull, the brain is enveloped by certain membranes, which at
once protect it from friction, and furnish it with a supply of nutrient
vessels; they are called the _arachnoid_, or "spider's web," the _dura
mater_ and the _pia mater_, or the "tough" and "delicate coverings." The
supply of blood sent to the brain is very liberal, amounting to one-fifth
of all that the entire body possesses. The brain of man is heavier than
that of any other animal, except the elephant and whale.
[Sidenote: 8. Size of the brain proper? How divided? The exterior of the
hemispheres? The interior?]
8. THE CEREBRUM.--The brain proper, or _cerebrum_, is the largest of the
intracranial organs, and occupies the entire upper and front portion of the
skull. It is almost completely bisected, by a fissure, or cleft, running
through it lengthwise, into two equal parts called _hemispheres_. The
exterior of these hemispheres is gray in color, consisting chiefly of
nerve-cells, arranged so as to form a layer of gray matter one-fifth of an
inch in thickness, and is abundantly supplied with blood-vessels. The
interior of the brain, however, is composed almost wholly of white
substance, or nerve-fibres.
[Sidenote: 9. The surface of the cerebrum, how marked? The gray matter of
the surface? Extent of the entire brain surface? Source of nervous power?
What further?]
9. The surface of the cerebrum is divided by a considerable number of
tortuous and irregular furrows, about an inch deep, into "convolutions," as
shown in Fig. 41. Into these furrows the gray matter of the surface is
extended, and, in this manner, its quantity is vastly increased. The extent
of the entire surface of the brain, {153} with the convolutions unfolded,
is computed to be equal to four square feet; and yet it is easily enclosed
within the narrow limits of the skull. When it is stated that the gray
matter is the true source of nervous power, it becomes evident that this
arrangement has an important bearing on the mental capacity of the
individual. And it is noticed that in children, before the mind is brought
into vigorous use, these markings or furrows on the surface are
comparatively shallow and indistinct; the same fact is true of the brain in
the less civilized races of mankind and in the lower animals. It is also
noticeable, that among animals, those are the most capable of being
educated which have the best development of the cerebrum.
[Illustration: FIG. 41.--UPPER SURFACE OF THE CEREBRUM.
A, Longitudinal Fissure. B, The Hemispheres.]
[Sidenote: 10. Location of the "little brain?" How divided? Its surface and
interior? Its subdivisions? Its size?]
10. THE CEREBELLUM.--The "little brain" is placed beneath the posterior
part of the cerebrum, and, like the latter, is divided into hemispheres.
Like it, also, the surface of the cerebellum is composed of gray matter,
and its interior is chiefly white matter. It has, however, no convolutions,
but is subdivided by many crescentic, parallel ridges, which, sending down
gray matter deeply into the {154} white, central portion, gives the latter
a somewhat branched appearance. This peculiar appearance has been called
the _arbor vitæ_, or the "tree of life," from the fact that when a section
of the organ is made, it bears some resemblance to the trunk and branches
of a tree (Fig. 42, F). In size, this cerebellum, or "little brain," is
less than one-eighth of the cerebrum.
[Illustration: FIG. 42.--VERTICAL SECTION OF THE BRAIN.
A, Left Hemisphere of Cerebrum. B, Corpus Callosum. C, Optic Thalamus.
D, The Pons Varolii. E, Upper extremity of the Spinal Cord. F, The Arbor
Vitæ.]
[Sidenote: 11. Medulla oblongata? Cranial nerves? Their shape and
position?]
11. From the under surface of the cerebrum, and from the front margin of
the cerebellum, fibres collect together to form the _medulla oblongata_
(Fig. 43, MA), which, on issuing from the skull, enters the spinal column,
and then becomes known as the spinal cord. From the base of the brain, and
from the sides of the medulla originate, also, the _cranial nerves_, of
which there are twelve pairs. These nerves are round cords of glistening
white appearance, and, {155} like the arteries, generally lie remote from
the surface of the body, and are well protected from injury.
[Illustration: FIG. 43.--THE BASE OF THE BRAIN.]
[Sidenote: 12. The spinal cord? Of what composed? How divided? Each half?]
12. THE SPINAL CORD.--The spinal cord, or "marrow," is a cylindrical mass
of soft nervous tissue, which occupies a chamber, or tunnel, fashioned for
it in the spinal column (Fig. 44). It is composed of the same substances as
the brain; but the arrangement is exactly reversed, the white matter
encompassing or surrounding the gray matter instead of being encompassed by
it. The amount of the white substance is also greatly in excess of the
other material. A vertical fissure partly separates the cord into two
lateral halves, and each half is composed of two separate bundles of
fibres, which are named the anterior and posterior columns.
{156} [Illustration: FIG. 44.
A, Cerebrum. B, Cerebellum. D, D, Spinal Cord.]
[Sidenote: 13. Uses of these columns? Importance of this part of the
nervous system? How protected?]
13. These columns have entirely different uses, and each of them unites
with a different portion of the nerves which have their origin in the
spinal cord. The importance of this part of the nervous system is apparent
from the extreme care taken to protect it from external injury. For, while
a very slight disturbance of its structure suffices to disarm it of its
power, yet so staunch is its bony enclosure, that only by very severe
injuries is it put in peril. The three membranes that cover the brain are
continued downward so as to envelope and still further shield this delicate
organism.
[Sidenote: 14. The spinal nerves? The posterior root? The nerves, how
arranged? Their office?]
14. THE SPINAL NERVES.--The spinal nerves, thirty-one pairs in number,
spring from each side of the cord by two roots, an anterior and a posterior
root, which have the same functions as the columns bearing similar names.
The posterior root is distinguished by possessing a ganglion of gray
matter, and by a somewhat larger size. The successive points of departure,
or the off-shooting of these nerves, occur at short and nearly regular
intervals along the course of the spinal cord. Soon after leaving these
points, {157} the anterior and posterior roots unite to form the trunk of a
nerve, which is distributed, by means of branches, to the various organs of
that part of the body which this nerve is designed to serve. The spinal
nerves supply chiefly the muscles of the trunk and limbs and the external
surface of the body.
[Sidenote: 15. The nerve tissue? Its character? Course of each nerve
fibre?]
15. The tissue composing the nerves is entirely of the white variety, or,
in other words, the nerve-fibres; the same as we have observed forming a
part of the brain. But the nerves, instead of being soft and pulpy, as in
the case of the brain, are dense in structure, being hardened and
strengthened by means of a fibrous tissue which surrounds each of these
delicate fibres, and binds them together in glistening, silvery bundles.
Delicate and minutely fine as are these nerve-fibres, it is probable that
each of them pursues an unbroken, isolated course, from its origin, in the
brain or elsewhere, to that particular point which it is intended to serve.
For, although their extremities are often only a hair's breadth distant
from each other, the impression which any one of them communicates is
perfectly distinct, and is referred to the exact point whence it came.
[Sidenote: 16. How may we illustrate the fact? The fibre connecting the
brain with a point in the foot?]
16. This may be illustrated in a simple manner, thus: if two fingers be
pressed closely together, and the point of a pin be carried lightly across
from one to the other, the eyes may be closed, and yet we can easily note
the precise instant when the pin passes from one finger to the other. If
the nerve-fibres were less independent, and if it were necessary that they
should blend with and support each other, all accuracy of perception would
be lost, and all information thus afforded would be pointless and confused.
These silvery threads must, therefore, be spun out with an infinite degree
of nicety. Imagine, for instance, the fibre which {158} connects the brain
with some point on the foot,--its length cannot be less than one hundred
thousand times greater than its diameter; and yet it performs its work with
as much precision as fibres that are comparatively much stronger and less
exposed.
[Sidenote: 17. The sympathetic system of nerves? Of what does it consist?]
17. THE SYMPATHETIC SYSTEM.--The _sympathetic system_ of nerves remains to
be described. It consists of a double chain of ganglia, situated on each
side of the spinal column, and extending through the cavities of the trunk,
and along the neck into the head. These ganglia are made up for the most
part of small collections of gray nerve-cells, and are the nerve-centres of
this system. From these, numerous small nerves are derived, which connect
the ganglia together, send out branches to the cranial and spinal nerves,
and form networks in the vicinity of the stomach and other large organs. A
considerable portion of them also follows the distribution of the large and
small blood-vessels, in which the muscular tunic appears. Branches also
ascend into the head, and supply the muscles of the eye and ear, and other
organs of sense.
[Sidenote: 18. Association of the various regions of the body? If one
member suffers? Blushing?]
18. In this manner, the various regions of the body are associated with
each other by a nervous apparatus, which is only indirectly connected with
the brain and spinal cord; and thus it is arranged that the most widely
separated organs of the body are brought into close and active sympathy
with each other, so that, "if one member suffers, all the other members
suffer with it." From this fact, the name _sympathetic system_, or the
_great sympathetic nerve_, has been given to the complicated apparatus we
have briefly described. Blushing and pallor are caused by mental emotions,
as modesty and fear, which produce opposite conditions of the capillaries
of the face by means of these sympathetic nerves. {159}
[Sidenote: 19. Properties of nervous tissue? Office of the gray substance?
Of the white? The nervous centres? White fibres?]
19. THE PROPERTIES OF NERVOUS TISSUE.--We have seen that in all parts of
this system, there are only two forms of nervous tissue; namely, the gray
substance and the white substance, so called from their difference of color
as seen by the naked eye; or the nerve-cell, and the nerve-fibre, so called
from their microscopic appearance. Now these two tissues are not commonly
mingled together, but either form separate organs, or distinct parts of the
same organs. This leads us to the conclusion that their respective uses are
distinct. And this proves to be the simple fact; wherever we find the gray
substance, we must look upon it as performing an active part in the system,
that is, it originates nervous impulses; the white matter, on the contrary,
is a passive agent, and serves merely as a conductor of nervous influences.
Accordingly, the nervous centres, composed so largely of the gray cells,
are the great centres of power, and the white fibres are simply the
instruments by which the former communicate with the near and distant
regions of the body under their control.
[Sidenote: 20. What comparison is made between the brain and the nation's
capitol? The vital property, excitability? What example is given?]
20. We may compare the brain, then, to the capital, or seat of government,
while the various ganglia, including the gray matter of the cord, like so
many subordinate official posts, are invested with authority over the
outlying provinces; and the nerves, with the white matter of the cord, are
the highways over which messages go and return between these provinces and
the local or central governments. But both forms of nervous tissue possess
the same vital property, called excitability; by which term is meant, that
when a nerve-cell or fibre is stimulated by some external agent, it is
capable of receiving an impression and of being by it excited into
activity. A ray of light, for example, falling upon one extremity of a
fibre in the eye, excites it throughout its whole length; and its {160}
other extremity, within the brain, communicating with a nerve-cell, the
latter, in its turn, is excited, and the sensation of sight is produced.
[Sidenote: 21. Change in the nervous tissues? Nerve force and electricity?]
21. What sort of change takes place in the nervous tissue when its
excitability is aroused, is not known; certainly none is visible. On this
account, it has been thought by some, that the nerve-fibre acts after the
manner of a telegraph wire; that is, it transmits its messages without
undergoing any material change of form. But, though the comparison is a
convenient one, it is far from being strictly applicable; and the notion
that nerve-force is identical with electricity has been fully proved to be
incorrect.
[Sidenote: 22. Functions of the nerves? In the case of the nerve of a
living animal? Of the human body?]
22. THE FUNCTIONS OF THE NERVES.--The nerves are the instruments of the two
grand functions of the nervous system, Sensation and Motion. They are not
the true centres of either function, but they are the conductors of
influences which occasion both. If the nerve in a limb of a living animal
be laid bare, and irritated by pinching, galvanizing, or the like, two
results follow, namely: the animal experiences a sensation, that of pain,
in the part to which the nerve is distributed, and the limb is thrown into
convulsive action. When a nerve in a human body is cut by accident, or
destroyed by disease, the part in which it ramifies loses both sensation
and power of motion; or, in other words, it is paralyzed. We accordingly
say that the nerves have a twofold use, a _sensory_ and _motor_ function.
[Sidenote: 23. If an exposed nerve be divided? What is proved? The course
of the sensory set of fibres? Of the motor set? To what are they likened?]
23. If a nerve that has been exposed be divided, and the inner end, or that
still in connection with the nerve-centres, be irritated, sensation is
produced, but no movement takes place. But if the outer end, or that still
connected with the limb, be irritated, then no pain is felt, but {161}
muscular contractions are produced. Thus we prove that there are two
distinct sets of fibres in the nerves; one of which, the _sensory_ fibres,
conduct toward the brain, and another, the _motor_ fibres, conduct to the
muscles. The former may be said to begin in the skin and other organs, and
end in the brain; while the latter begin in the nervous centres and end in
the muscles. They are like a double line of telegraph wires, one for
inquiries, the other for responses.
[Sidenote: 24. The two roots of the spinal nerves? What has been found?
Difference of the two sorts of fibres? Result of their union?]
24. We have already spoken of the two roots of the spinal nerves, called
from their points of origin in the spinal cord, the anterior and posterior
roots. These have been separately cut and irritated in the living animal,
and it has been found that the posterior root contains only sensory fibres,
and the anterior root has only motor fibres. So that the nerves of a limb
may be injured in such a way that it will retain power of motion and yet
lose sensation; or the reverse condition, feeling without motion, may
exist. Between these two sorts of fibres, no difference of structure can be
found; and where they have joined to form a nerve it is impossible to
distinguish one sort from the other.
[Sidenote: 25. Transient paralysis? When such is the case with the leg?
What other fact is observed?]
25. Occasionally a nerve is so compressed as to be temporarily unable to
perform its functions: a transient paralysis then takes place. This is the
case when the leg or arm "gets asleep," as it is expressed. When such is
the condition with the leg, and the person suddenly attempts to walk, he is
liable to fall, inasmuch as the motor fibres cannot convey orders to the
muscles of the limb. Another fact is observed: there is no sensation in
this nerve at the point of its compression; but the whole limb is numb, and
tingling sensations are felt in the foot, the point from which the sensory
fibres arise.
[Sidenote: 26. What does this illustrate? Sensation? The feeling after a
limb has been amputated? Striking of the "funny bone?"]
26. This illustrates the manner in which the brain {162} interprets all
injuries of the trunk of a nerve. Sensation or pain is not felt at the
point of injury, but is referred to the outer extremities of the nerve,
where impressions are habitually received. This is the reason why, after a
limb has been amputated by the surgeon, the patient appears to suffer pain
in the member that has been severed from the body; while some form of
irritation at the end of the nerve in the wound, or stump, is the real
source of his distress. Again, when the "funny-bone"--that is, the ulnar
nerve at the elbow,--is accidentally struck, the tingling sensations thus
produced are referred to the outer side of the hand and the little finger,
the parts to which that nerve is distributed.
[Sidenote: 27. The spinal nerves, and two from the brain? Of the remainder?
Difference in the nerves? How accounted for? The rate of conduction along a
nerve? As compared with electricity?]
27. All the spinal nerves, and two from the brain, are concerned in both
sensation and motion. Of the remainder of the cranial nerves, some are
exclusively motor, others exclusively sensory; and still others convey, not
ordinary sensations, but special impressions, such as sight, hearing, and
smell, which we have yet to consider. However much the functions of the
nerves seem to vary, there is but little difference discoverable in the
nerves themselves, when examined under the microscope. Whatever difference
exists must be accounted for in consequence of the nerves communicating
with different portions of the gray matter of the brain. The rate of motion
of a message, to or from the brain along a nerve, has been measured by
experiment upon the lower animals, and estimated in the case of man at
about two hundred feet per second. As compared with that of electricity,
this is a very slow rate, but, in respect to the size of the human body, it
is practically instantaneous.
[Sidenote: 28. Functions of the anterior and posterior columns of the cord?
If the cord be divided?]
28. THE FUNCTIONS OF THE SPINAL CORD.--As the {163} anterior and posterior
roots of the spinal nerves have separate functions, so the anterior and
posterior columns of the cord are distinct in function. The former are
concerned in the production of motion, the latter in sensation. If the cord
be divided, as before in the case of the nerve, it is found that the parts
below the point of injury are deprived of sensation and of the power of
voluntary motion on both sides of the body, a form of paralysis which is
called _paraplegia_.
[Sidenote: 29. Paraplegia? Result and danger to life? When the injury
occurs in the neck?]
29. This form of disease, paraplegia, is sometimes seen among men,
generally as the result of a fall, or some other severe accident, by which
the bones of the spine are broken, and the cord is crushed, or pierced by
fragments of bone. The parts which are supplied by nerves from the cord
above the point of injury are as sensitive and mobile as before. The
results are similar, whether the division happens at a higher or lower
portion of the spinal cord; but the danger to life increases proportionally
as the injury approaches the brain. When it occurs in the neck, the muscles
of inspiration are paralyzed, since they are supplied by nerves issuing
from that region; and as a result of this paralysis, the lungs are unable
to act, and life is speedily brought to a close.
[Sidenote: 30. Experiment of cutting the spinal cord of an animal? What
inference is drawn?]
30. When the spinal cord of an animal has been cut, in experiment, it may
be irritated in a manner similar to that alluded to when considering the
nerves. If, then, the upper cut surface be excited, it is found that pain,
referable to the parts below the cut, is produced; but when the lower cut
surface is irritated, no feeling is manifested. So we conclude that in
respect to sensation, the spinal cord is not its true centre, but that it
is merely a conductor, and is therefore the great sensory nerve of the
body. When the lower surface of the cut is irritated, the muscles of the
{164} parts below the section are violently contracted. Hence, we conclude
that, in respect to the movements ordered by the will, the spinal cord is
not their source; but that it acts only as a conductor, and is,
accordingly, the great motor nerve of the body.
[Sidenote: 31. What singular fact is noticed? What does the result show?]
31. DIRECTION OF THE FIBRES OF THE CORD.--If one lateral half of the spinal
cord be cut, or injured, a very singular fact is observed. All voluntary
power over the muscles of the corresponding half of the body is lost, but
the sensibility of that side remains undiminished. This result seems to
show that the motor fibres of the cord pursue a direct course, while its
sensory fibres are bent from their course. And this has been proved to be
the fact; for immediately after the posterior roots--the conductors of
sensory impressions--join the posterior columns, they enter the gray matter
of the cord, and passing over, ascend to the brain on the opposite side.
Accordingly, the sensory fibres from the right and left sides interlace
each other in the gray matter; this arrangement has been termed the
_decussation_, or crossing of these fibres. This condition serves to
explain how a disease or injury of the cord may cause a paralysis of motion
in one leg, and a loss of sensation in the other.
[Sidenote: 32. Direction of the anterior or motor columns? In the cord
itself? In the medulla oblongata? The decussation?]
32. The direction of the anterior, or motor columns of the cord, is
downward from the brain. In the cord itself, the course of the motor fibres
is for the most part, a direct one; but in the medulla oblongata, or upper
extremity of the cord, and therefore early in their career, these fibres
decussate, or cross from side to side in a mass; and not separately, as in
the case of the posterior fibres just mentioned. This arrangement is termed
the _decussation_ of the anterior columns of the medulla.
[Sidenote: 33. Result of the double interlacing of fibres? Where is the
seat of pain when the right hand is hurt? The moving of the foot? Loss of
sensation in one side of the body?]
33. From this double interlacing of fibres results a {165} crossed action
between the original and terminal extremity of all nerve-fibres which pass
through the medulla; namely, those of all the spinal nerves. Consequently,
if the right hand be hurt, the left side of the brain feels the pain; and
if the left foot move, it is the right hemisphere which dictates its
movement. For the same reason, when a loss of sensation and power of motion
affecting the right side of the body alone is observed, the physiologist
understands that the brain has been invaded by disease upon its left side.
This affection is termed _hemiplegia_, or the "half-stroke." The
full-stroke, which often follows the rupture of a blood-vessel in the
brain, is commonly called _paralysis_.
[Sidenote: 34. What other important use has the cord? What is the activity
denominated?]
34. THE REFLEX ACTION OF THE CORD.--We have already considered the cord as
the great motor and sensory nerve of the body, but it has another and
extremely important use. By virtue of the gray matter, which occupies its
central portion, it plays the part of an independent nerve centre. The
spinal cord not only conducts some impressions to the brain, but it also
arrests others; and, as it is expressed, "reflects" them into movements by
its own power. This mode of nervous activity is denominated the _Reflex
Action_ of the cord.
[Sidenote: 35. Example of the fowl? Centipede? Frog? What do they prove?]
35. A familiar example of this power of the cord is found in the violent
movements which agitate a fowl after its head has been cut off. The
cold-blooded animals also exhibit reflex movements in an astonishing
degree. A decapitated centipede will run rapidly forward, and will
seemingly strive to overturn, or else climb over obstacles placed in its
way. A frog similarly mutilated will sustain its headless body upon its
feet, in the standing posture, just as it might do if it were still alive.
If pushed over, it will regain its feet; and if the feet are irritated, it
will {166} jump forward. There can be no doubt that, in the lower animals,
movements may take place which are completely divorced from the will,
sensation, and consciousness; for in those animals, as well as in man,
these faculties have their principal seat within the brain.
[Sidenote: 36. What is necessary in most cases to awaken reflex movements?
In the case of the fowl? Convulsions which follow decapitation?]
36. An irritation is necessary, in most instances, to awaken reflex
movements. In the case of the decapitated fowl, its muscles are excited to
convulsive action by reason of its being thrown upon the hard ground and
roughly handled. Let it be treated differently, and the convulsions will
not take place: let it be laid gently upon soft cotton, and the body will
remain comparatively quiet. It may comfort some people to know that the
convulsions which follow decapitation are not attended with pain; nor are
they a necessary part of the "act of death," as some suppose.
[Sidenote: 37. Actions in the human body distinct from voluntary efforts?]
37. In the human body, likewise, actions are excited that are entirely
distinct from the ordinary voluntary efforts. It is not permissible,
desirable, nor even necessary to decapitate a man that the body may be
disconnected from his brain, in order to test the effect of irritation upon
the spinal cord; although the bodies of beheaded criminals have been
experimented upon, and caused to move by powerful galvanic batteries. The
resort to such means of experiment is rendered unnecessary by the
occurrence of certain deplorable cases of disease and injury, which
effectually sever all communication between the brain and a large part of
the body.
[Sidenote: 38. Reflex action after injury of the cord? Why not due to the
muscles?]
38. Thus, the cord may be so far injured, as the result of accident, as to
terminate all sensation and voluntary motion in the lower half of the body,
the patient seemingly becoming lifeless and powerless from the waist
downward. And yet, by tickling or pinching either foot, the leg {167} of
the same side may be made to jerk, or even to kick with considerable force;
but, unless the patient is observing his limbs, he is wholly unconscious of
these movements, which are, therefore, performed independently of the
brain. And they are in nowise due to the muscles of the limb; for, if the
cord itself becomes diseased below the point of injury, the muscles cease
to contract.
[Sidenote: 39. What are the requisites for the production of this form of
nervous action?]
39. For the production of this form of nervous action three things are
requisite--(1) a nerve to conduct messages from the surface of the body,
one of that variety formerly described as sensory, but which are now
incapable of awakening sensation; (2) a portion of uninjured spinal cord
which shall reflect or convert impressions into impulses; and (3) a motor
nerve to conduct impulses outward to the muscles. The power of the cord to
enforce reflex acts resides in the gray matter, into which the reflex
nerves enter and from which they depart, by means of their posterior and
anterior roots respectively.
[Sidenote: 40. Why do we not readily recognize the reflex activity of the
cord in our own bodies? How best studied in others? Example?]
40. THE USES OF THE REFLEX ACTION.--The reflex activity of the cord is
exhibited in the healthy body in many ways, but since it is never
accompanied with sensation, we do not readily recognize it in our own
bodies. Reflex movements are best studied in the cases of other persons,
when the conditions enable us to distinguish between acts that are
consciously, and those that are unconsciously performed. For example, if
the foot of a person soundly asleep be tickled or pinched, it will be
quickly withdrawn from the irritation.
[Sidenote: 41. Similar movements? Arm of a person? Melted wax or heated
coin on the hand?]
41. Similar movements may be observed in cases where the consciousness and
sensation are temporarily obliterated by disease, or by means of narcotic
poisons. If the arm of a person who has been rendered insensible by {168}
chloroform, be raised, and then allowed to fall, it will be noticed that
the limb does not drop instantly, like a lifeless member, but a certain
amount of rigidity remains in its muscles, which resists or breaks the
force of its descent. Again, when a substance like melted sealing-wax, or a
heated coin, falls upon the hand, the limb is snatched away at once, even
before the feeling of pain has been recognized by the brain. When jolted in
a rapidly moving car, we involuntarily step forward or backward, so as to
preserve the centre of gravity of the body.
[Sidenote: 42. Result of healthful reflex activity? When may the reflex
energy be deficient?]
42. These and similar acts are executed by the same mechanism as that
previously described in the case of paralysis from an injury of the spinal
cord. The muscles thus called into play, are those which are ordinarily
under the sway of the will, but which in these cases act through this
reflex action of the cord, altogether independently of the will. A
healthful reflex activity produces an elasticity, or "tone," of the
voluntary muscular system, which, in a great measure, explains the
existence in the young and vigorous of a feeling of buoyancy and reserve
power. Its possessor is restlessly active, and it may appropriately be said
of him, "he rejoiceth as a strong man to run a race." But this reflex
energy may be deficient. This is true when the blood is poor and wanting in
its solid ingredients, or the circulation is feeble; the muscles, then, are
flabby and weak, and the person himself is said to be "nerveless," or
indisposed to exertion. Shivering from cold, and trembling from fear, may,
in part, be referred to a temporary loss of tone, resulting from a powerful
impression upon the brain.
[Sidenote: 43. Excess of this activity in disease? Hydrophobia, etc.? The
difference in severity of the convulsions?]
43. An excess of this activity may also be observed in disease. In this
condition, the excitability of the cord is unnaturally aroused, and
frequent and violent movements {169} of the limbs and body, called
convulsions, are the result. The convulsions of young children, and the
nervous agitation of _chorea_, or St. Vitus's dance, are reflex in
character; as are also the symptoms attending poisoning by strychnine, and
those terrible diseases, _tetanus_, or "locked jaw," and _hydrophobia_. The
severity of the convulsions is not the same in all cases of these
disorders; but, in those last mentioned the most violent spasmodic
movements are provoked by the slightest form of irritation--such as the
sound of pouring water, the sight of any glittering object, the glancing of
a mirror, the contact of cool air, or even the touch of the bedclothes.
[Sidenote: 44. Another variety of reflex motions? What are they? What is
stated of the mind in connection with these movements?]
44. Another variety of reflex motions takes place in certain involuntary
muscles, and over these the cord exercises supreme control. They are
principally those movements which aid the performance of digestion and
nutrition, the valve-action of the pylorus, and other movements of the
stomach and intestines. In these movements the mind shares no part. And it
is well that this is so; for since the mind is largely occupied with
affairs external to the body, it acts irregularly, becomes fatigued, and
needs frequent rest. The spinal cord, on the contrary, is well fitted for
the form of work on which depends the growth and support of the body, as it
acts uniformly, and with a machine-like regularity.
[Sidenote: 45. Consciousness in these operations? Physical wants?]
45. These operations are not accompanied by consciousness; for, as a
general rule, the attention is only called to them when they become
disordered. Many a person does not know where his stomach is situated,
until he discovers its position by reason of a feeling of distress within
it, produced by giving that organ improper work to perform. In this manner
the higher and nobler faculties of the mind are liberated from the simply
routine duties of the {170} body; and we are thus left to direct the
attention, the reason, and the will to the accomplishment of the great ends
of our existence. If it were otherwise, we could only find time to attend
to our ordinary physical wants.
[Sidenote: 46. How many objects may the reflex activity be said to have?
State the first. The second. The third.]
46. The objects of the reflex activity of the cord are threefold. In the
first place, it acts as the protector of man, in his unconscious moments.
It is his unseen guardian, always ready to act, never growing weary, and
never requiring sleep. Nor does its faithful action wholly cease with the
cessation of life in other parts. In the second place, it is the regulator
of numerous involuntary motions that are necessary to the nutrition of the
body. Here its actions are entirely independent of the brain, and are
performed in a secret and automatic manner. And, thirdly, it acts as a
substitute, and regulates involuntary movements in the muscles usually
under the influence of the will. It thus takes the place of the higher
faculties in performing habitual acts, and permits them to extend their
operations more and more beyond the body and its material wants.
[Sidenote: 47. How does the medulla oblongata resemble the cord?]
47. THE FUNCTIONS OF THE MEDULLA OBLONGATA.--The prolongation of the spinal
cord, within the skull, has been previously spoken of as the medulla
oblongata. It resembles the cord, in being composed of both white and gray
matter, and in conducting sensory and motor influences. It likewise gives
rise to certain nerves, which are here called cranial nerves (from
_cranium_, the skull). All except two of these important nerves spring from
the medulla, or the parts immediately adjoining it; the exceptions are the
two nerves taking part in the special senses of sight and smell, which
nerves have their origin at the base of the cerebrum.
[Sidenote: 48. What final fact is observed in the crossing of the motor
columns?]
48. The decussation, or crossing of the motor columns, has been previously
described, when treating of the {171} direction of the nerve-fibres of the
cord; and the singular fact has been alluded to, that when one side of the
brain is injured, its effects are limited to the opposite side of the body.
One more fact remains to be observed in this connection, namely, this
crossed action does not usually take place in the cranial nerves.
Accordingly, when apoplexy, or the rupture of a blood-vessel, occurs in the
right hemisphere of the cerebrum, the left side of the body is paralyzed,
but the right side of the face is affected; this is because that part of
the body is supplied by the cranial nerves.
[Sidenote: 49. The pneumogastric nerve? The feelings aroused by it? The
"vital knot?"]
49. A portion of the medulla presides over the important function of
respiration, and from it arises the _pneumogastric_ nerve, so called
because its branches serve both the lungs and stomach. The feelings of
hunger, thirst, and the desire for air are aroused by means of this nerve.
The wounding of the gray matter of the medulla, even of a small portion of
it, near the origin of the pneumogastric nerve, at once stops the action of
the lungs and causes death. In consequence of the importance of this part,
it has been termed the "vital knot." We find, also, that its location
within the skull is exceedingly well protected, it being quite beyond the
reach of any ordinary form of harm from without.
[Sidenote: 50. The uses of the smaller gray masses at the base of the
brain?]
50. THE FUNCTIONS OF THE CRANIAL GANGLIA.--The uses of the smaller gray
masses lying at the base of the brain are not well ascertained; and, on
account of their position, so remote from the surface, it would, at first,
seem well-nigh impossible to study them. But, from the results following
diseases in these parts, and from experiments upon inferior animals, they
are becoming gradually better understood; and there is reason to believe
that eventually the physiological office of each part will be clearly
ascertained and defined. It is believed, however, but not {172} absolutely
proven, that the anterior masses, like the anterior roots of the spinal
nerves and the anterior columns of the cord, are concerned in the
production of motion; in fact, that they are the central organs of that
function. The posterior gray masses are, on the contrary, supposed to be
the seat of sensation.
[Sidenote: 51. Function of the cerebellum? When it is diseased?]
51. THE FUNCTION OF THE CEREBELLUM.--The function of the cerebellum, or
"little brain," is the direction of the movements of the voluntary muscles.
When this organ is the seat of disease or injury, it is usually observed
that the person is unable to execute orderly and regular acts, but moves in
a confused manner as if in a state of intoxication. Like the larger brain,
or cerebrum, it appears to be devoid of feeling; but it takes no part in
the operations of the mind.
[Sidenote: 52. Where is the seat of the mind? The subordination of the
other organs? The gray matter?]
52. THE FUNCTION OF THE CEREBRUM.--The cerebrum, or brain proper, is the
seat of the mind; or, speaking more exactly, it is the material instrument
by which the mind acts; and, as it occupies the highest position in the
body, so it fulfils the loftiest uses. All the other organs are subordinate
to it: the senses are its messengers, which bring it information from the
outer world, and the organs of motion are its servants, which execute its
commands. Here, as in the nervous apparatus of lower grade already
considered, the gray matter is the element of power; and, in proportion as
this substance increases in extent, and in proportion to the number of
convolutions in the hemispheres, do the mental faculties expand.
[Sidenote: 53. What is stated of men in connection with the size of their
brain? With the brains of other animals?]
53. There have been a few, but only a few, men of distinguished ability
whose brains have been comparatively small in size; the rule being that
great men possess large brains. The relative weight of the brain of man, as
{173} compared with the weight of the body, does not, in all instances,
exceed that of the inferior animals; the canary and other singing-birds
have a greater relative amount of nervous matter than man; but man
surpasses all other creatures in the size of the hemispheres of the
cerebrum, and in the amount of gray substance which they contain.
[Sidenote: 54. Sensitiveness of the brain substance? The removal of a
portion of the brain? State the remarkable case mentioned?]
54. It is a singular fact that this cerebral substance is insensitive, and
may be cut without causing pain. The removal of a considerable quantity of
the brain has taken place, as the result of accident, without causing
death, and without even affecting seriously the intellect. A remarkable
case of injury of the brain is recorded, in which, from the accidental
explosion of gunpowder used in blasting a rock, the "tamping-iron" was
driven directly through the skull of a man. This iron rod, three feet and
seven inches long, an inch and a quarter in diameter, and weighing more
than thirteen pounds, entered the head below the ear and passed out at the
top of the skull, carrying with it portions of the brain and fragments of
bone. The man sustained the loss of sight on one side, but otherwise
recovered his health and the use of his faculties. Moreover, disease has
occurred, compromising a large portion of the brain, without impairing the
faculties of the mind, when the disease was limited to one side only.
[Sidenote: 55. Thought, emotion, and will? What power do they give us?]
55. Impressions conveyed to the hemispheres from the external world arouse
the mental operations called thought, emotion, and the will. These are the
godlike attributes which enable man to subjugate a world, and afterward
cause him to "sigh for other worlds to conquer;" which enable him to
acquaint himself with the properties of planets millions of miles distant
from him, and which give him that creative power by which he builds and
peoples the new worlds of poetry and art. {174}
[Sidenote: 56. Are the brain and the mind identical?]
56. All these mental acts, and many others, are developed through the
action of the brain; not that the brain and the mind are the same, or that
the brain secretes memory, imagination, or the ideas of truth and justice,
as the stomach secretes the gastric juice. But rather, as the nerve of the
eye, stimulated by the subtile waves of light, occasions the notion of
color, so the brain, called into action by the mysterious influences of the
immaterial soul, gives rise to all the intellectual, emotional, and
voluntary activities of mankind.
[Sidenote: 57. What do we know of the cerebrum and its powers?]
57. The cerebrum, according to our present knowledge of it, must be
regarded as a single organ, which produces different results, according as
it is acted upon by the immaterial mind in different ways. Recent
investigations, however, seem to prove that the faculty of language is
dependent upon a small part of the left hemisphere of the cerebrum, near
the temple. At least, in almost every instance where this part is diseased,
the patient can no longer express himself in speech and writing.
[Sidenote: 58. The reflex function of the organs within the skull? The
reflex power of the medulla? Respiration?]
58. THE REFLEX ACTION OF THE BRAIN.--The reflex function of the organs
within the skull is very active and important. Like that of the cord, it
protects the body by involuntary movements, it regulates the so-called
vegetative acts, and it takes the place of the will in controlling the
voluntary muscles, when the attention is turned in other directions. The
reflex power of the medulla governs the acts of respiration, which are
absolutely and continuously essential to life. Respiration is, as we have
seen, partly under the influence of the will; but this is due in part to
the fact that respiration is indirectly concerned in one of the animal
functions, that of speech.
[Sidenote: 59. What else does reflex action occasion? Winking? Other
examples?]
59. Reflex action also occasions coughing and sneezing, {175} whenever
improper substances enter the air-passages. Winking is an act of the same
sort, and serves both to shield the eyes from too great glare of light, and
to preserve them by keeping the cornea moist. Looking at the sun or other
strong light, causes sneezing by reflex action. Laughing, whether caused by
tickling the feet or by some happy thought, and also sobbing, are reflex
acts, taking place by means of the respiratory muscles.
[Sidenote: 60. Muscles called into play by certain reflex movements? The
somnambulist?]
60. Certain of the protective reflex movements call into play a large
number of muscles, as in the balancing of the body when walking along a
narrow ledge, or on a slippery pavement. The dodging motion of the recruit,
when the first cannon ball passes over his head, is reflex and involuntary.
The fact that these involuntary, reflex acts are performed with great
precision, will explain why it is that accidents seldom befall the
somnambulist, or sleep-walker, although he often ventures in most perilous
places.
[Sidenote: 61. What is said of walking and other acts in connection with
the office performed by the medulla and spinal cord?]
61. Walking, sitting, and other acts of daily life, become automatic, or
reflex, from habit: the mind is seldom directed to them, but delegates
their control to the medulla and spinal cord. Thus a person in walking, may
traverse several miles while absorbed in thought, or in argument with a
companion, and yet be conscious of scarcely one in a thousand of the acts
that have been necessary to carry his body from one point to another. By
this admirable and beautiful provision, the mind is released from the
charge of the ordinary mechanical acts of life, and may devote itself to
the exercise of its nobler faculties. And it is worthy of notice, that the
greater the use of these faculties, the more work does the reflex function
assume and perform; and thus the employment of the one insures the
improvement of the other. {176}
QUESTIONS FOR TOPICAL REVIEW.
PAGE
1. State fully what is meant by the term vegetable function. 148
2. To what is man indebted for his position as the head of the
animal creation? 148, 149
3. What can you state on the subject of special organs for
separate functions? 149
4. Describe, as fully as you can, the structure of the nervous
system. 149, 150
5. Describe the brain, its location, size, shape, and structure. 150, 152
6. Describe the brain proper, or cerebrum. 152, 153, 174
7. What connection is noticed between the cerebrum and mental
power? 153, 172, 174
8. Describe the little brain, or cerebellum. 153, 154, 172
9. Describe the spinal cord. 154, 155, 156
10. What are the spinal nerves, and how are they arranged? 156, 157
11. What is the character and substance of their tissues? 157
12. State how the nerve-fibres perform their office, and give
the illustration. 157, 158
13. Describe the sympathetic system of nerves. 158
14. State what is meant by the properties of nervous tissue,
and give the illustration. 159, 160
15. Explain what is meant by the functions of the nerves, and
give the illustration. 160, 161, 162
16. What is meant by a transient paralysis of a nerve? Give the
illustration. 161, 162
17. What can you state of the rate of message-motion along a nerve? 162
18. What are the functions of the spinal cord? 162, 163, 164, 165
19. State what you can of the form of paralysis known as
paraplegia. 163
20. What experiments, with results, upon the spinal cord are
noted? 163, 164
21. Explain how injury of the cord may produce paralysis of
motion in one leg, and at the same time a loss of sensation
in the other. 164
22. Explain how, if the right hand be hurt, the left side of
the brain is made to feel the pain. 165
23. Now, explain as fully as you can the direction of the fibres
of the cord. 164, 165
24. What is understood by the reflex action of the cord? 165
25. What experiments are mentioned to prove this power of the
cord? 165, 166
26. What are the uses of the reflex action of the cord? 167-170
27. What illustrations are mentioned to show such uses? 167-170
28. What is the medulla oblongata? 154, 170
29. What are the functions of the medulla oblongata? 170, 171
30. What can you state of the functions of the cranial ganglia? 171, 172
31. What are the functions of the cerebellum? 172
32. What is the function of the cerebrum? 172, 174
33. In what way does the size of the brain generally indicate
the character of the man? 172, 173
34. What facts show that the gray substance of the brain is
insensitive? 173
35. Upon what does the faculty of language seem to depend? 174
36. What has been observed in support of this statement? 174
37. Of what importance is the reflex action of the brain? 174, 175
38. In what ways is this importance made manifest? 174, 175
* * * * *
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