In our study so far we have been concerned mainly with the introduction of
materials into the body. We are now to consider the removal of materials
from the body. The structures most directly concerned in this work are
known as
*Glands.*--As generally understood, glands are organs that prepare special
liquids in the body and pour them out upon free surfaces. These liquids,
known as _secretions_, are used for protecting exposed parts, lubricating
surfaces that rub against each other, digesting food, and for other
purposes. They differ widely in properties as well as in function, but are
all alike in being composed chiefly of water. The water, in addition to
being necessary to the work of particular fluids, serves in all cases as a
carrier of solid substances which are dissolved in it.
*General Structure of Glands.*--While the various glands differ greatly in
size, form, and purpose, they present striking similarities in structure.
All glands contain the following parts:
1. Gland, or secreting, cells. These are _specialized_ cells for the work
of secretion and are the active agents in the work of the gland. They are
usually cubical in shape.
2. A basement membrane. This is a thin, connective tissue support upon
which the secreting cells rest.
3. A network of capillary and lymph vessels. These penetrate the tissues
immediately beneath the secreting cells.
4. A system of nerve fibers which terminate in the secreting cells and in
the walls of the blood vessels passing to the glands.
These structures--secreting cells, basement membrane, capillary and lymph
vessels, and nerve fibers--form the essential parts of all glands. The
capillaries and the lymph vessels supply the secreting cells with fluid,
and the nerves control their activities.
*Kinds of Glands.*--Glands differ from one another chiefly in the
arrangement of their essential parts.(73) The most common plan is that of
arranging the parts around a central cavity formed by the folding or
pitting of an exposed surface. Many such glands are found in the mucous
membrane, especially that lining the alimentary canal, and are most
numerous in the stomach, where they supply the gastric juice. If these
glands have the general form of tubes, they are called _tubular_ glands;
if sac-like in shape, they are called _saccular_ glands. Both the tubular
and the saccular glands may, by branching, form a great number of similar
divisions which are connected with one another, and which communicate by a
common opening with the place where the secretion is used. This forms a
_compound_ gland which, depending on the structure of the minute parts,
may be either a _compound tubular_ or a _compound saccular_ gland. The
larger of the compound saccular glands are also called _racemose_ glands,
on account of their having the general form of a cluster, or raceme,
similar to that of a bunch of grapes. The general structure of the
different kinds of glands is shown in Fig. 85.
[Fig. 85]
Fig. 85--*Diagram illustrating evolution of glands.* _A._ Simple secreting
surface. 1. Gland cells. 2. Basement membrane. 3. Blood vessel. 4. Nerve.
_B._ Simple tubular gland. _C._ Simple saccular gland. _D._ Compound
tubular gland. _E._ Compound saccular gland. _F._ A compound racemose
gland with duct passing to a free surface. _G._ Relation of food canal to
different forms of glands. The serous coat has a secreting surface.
*Nature of the Secretory Process.*--At one time the gland was regarded
merely as a kind of filter which separated from the blood the ingredients
found in its secretions. Recent study, however, of several facts relating
to secretion has led to important modifications of this view. The
secretions of many glands are known to contain substances that are not
found in the blood, or, if present, are there in exceedingly small
amounts. Then again the cells of certain glands have been found to undergo
marked changes during the process of secretion. If, for example, the cells
of the pancreas be examined after a period of rest, they are found to
contain small granular bodies. On the other hand, if they are examined
after a period of activity, the granules have disappeared and the cells
themselves have become smaller (Fig. 86). The granules have no doubt been
used up in forming the secretion. These and other facts have led to the
conclusion that secretion is, in part, the separation of materials without
change from the blood, and, in part, a process by which special substances
are prepared and added to the secretion. According to this view the gland
plays the double role of a _filtering apparatus_ and of a _manufacturing
organ_.
[Fig. 86]
Fig. 86--*Secreting cells from the pancreas* (after Langley). _A._ After a
period of rest. _B._ After a short period of activity. C. After a period
of prolonged activity. In _A_ and _B_ the nuclei are concealed by the
granules that accumulate during the resting period.
*Kinds of Secretion.*--In a general way all the liquids produced by glands
may be considered as belonging to one or the other of two classes, known
as the _useful_ and the _useless_ secretions. To the first class belong
all the secretions that serve some purpose in the body, while the second
includes all those liquids that are separated as waste from the blood. The
first are usually called _true secretions_, or secretions proper, while
the second are called _excretions_. The most important glands producing
liquids of the first class are those of digestion (Chapter X).
*Excretory Work of Glands.*--The process of removing wastes from the body
is called _excretion_. While in theory excretion may be regarded as a
distinct physiological act, it is, in fact, leaving out the work of the
lungs, but a phase of the work of glands. From the cells where they are
formed, the waste materials pass into the lymph and from the lymph they
find their way into the blood. They are removed from the blood by glands
and then passed to the exterior of the body.
*The Necessity for Excretion* is found in the results attending oxidation
and other chemical changes at the cells (page 107). Through these changes
large quantities of materials are produced that can no longer take any
part in the vital processes. They correspond to the ashes and gases of
ordinary combustion and form wastes that must be removed. The most
important of these substances, as already noted (page 110), are carbon
dioxide, water, and urea.(74) A number of mineral salts are also to be
included with the waste materials. Some of these are formed in the body,
while others, like common salt, enter as a part of the food. They are
solids, but, like the urea, leave the body dissolved in water.
Waste products, if left in the body, interfere with its work (some of,
them being poisons), and if allowed to accumulate, cause death. Their
removal, therefore, is as important as the introduction of food and oxygen
into the body. The most important of the excretory glands are
*The Kidneys.*--The kidneys are two bean-shaped glands, situated in the
back and upper portion of the abdominal cavity, one on each side of the
spinal column. They weigh from four to six ounces each, and lie between
the abdominal wall and the peritoneum. Two large arteries from the aorta,
called the _renal arteries_, supply them with blood, and they are
connected with the inferior vena cava by the _renal veins_. They remove
from the blood an exceedingly complex liquid, called the _urine_, the
principal constituents of which are water, salts of different kinds,
coloring matter, and urea. The kidneys pass their secretion by two slender
tubes, the _ureters_, to a reservoir called the _bladder_ (Fig. 87).
[Fig. 87]
Fig. 87--*Relations of the kidneys.* (Back view.) 1. The kidneys. 2.
Ureters. 3. Bladder. 4. Aorta. 5. Inferior vena cava. 6. Renal arteries.
7. Renal veins.
*Structure of the Kidneys.*--Each kidney is a compound tubular gland and is
composed chiefly of the parts concerned in secretion. The ureter serves as
a duct for removing the secretion, while the blood supplies the materials
from which the secretion is formed. On making a longitudinal section of
the kidney, the upper end of the ureter is found to expand into a
basin-like enlargement which is embedded in the concave side of the
kidney. The cavity within this enlargement is called the _pelvis of the
kidney_, and into it project a number of cone-shaped elevations from the
kidney substance, called the _pyramids_ (Fig. 88).
From the summits of the pyramids extend great numbers of very small tubes
which, by branching, penetrate to all parts of the kidneys. These are the
_uriniferous tubules_, and they have their beginnings at the outer margin
of the kidney in many small, rounded bodies called the _Malpighian
capsules_ (_A_, Fig. 88). Each capsule incloses a cluster of looped
capillaries and connects with a single tubule (Fig. 89). From the capsule
the tubule extends toward the concave side of the kidney and, after
uniting with similar tubules from other parts, finally terminates at the
pyramid. Between its origin and termination, however, are several
convolutions and one or more loops or turns. After passing a distance many
times greater than from the surface to the center of the kidney, the
tubule empties its contents into the expanded portion of the ureter.
[Fig. 88]
Fig. 88--*Sectional view of kidney.* 1. Outer portion or cortex. 2.
Medullary portion. 3. Pyramids. 4. Pelvis. 5. Ureter. _A._ Small section
enlarged to show the tubules and their connection with the capsules.
[Fig. 89]
Fig. 89--*Malpighian capsule* highly magnified (Landois). _a._ Small artery
entering capsule and forming cluster of capillaries within. _e._ Small
vein leaving capsule and branching into _c_, a second set of capillaries,
_h._ Beginning of uriniferous tubule.
The uriniferous tubules are lined with secreting cells. These differ
greatly at different places, but they all rest upon a basement membrane
and are well supplied with capillaries. These cells provide one means of
separating wastes from the blood (Fig. 90).
[Fig. 90]
Fig. 90--*Diagram illustrating renal circulation.* 1. Branch from renal
artery. 2. Branch from renal vein. 3. Small artery branches, one of which
enters a Malpighian capsule (5). 6. Small vein leaving the capsule and
branching into the capillaries (7) which surround the uriniferous tubules.
4. Small veins which receive blood from the second set of capillaries. 8.
Tubule showing lining of secreting cells.
*Blood Supply to the Kidneys.*--The method by which the kidneys do their
work is suggested by the way in which the blood circulates through them.
The renal artery entering each kidney divides into four branches and these
send smaller divisions to all parts of the kidney. At the outer margin of
the kidney, called the _cortex_, the blood is passed through _two sets of
capillaries_. The first forms the clusters in the Malpighian capsules and
receives the blood directly from the smallest arteries. The second forms a
network around the uriniferous tubules and receives the blood which has
passed from the capillary clusters into a system of small veins (Fig. 90).
From the last set of capillaries the blood is passed into veins which
leave the kidneys where the artery branches enter, uniting there to form
the main renal veins.
*Work of the Kidneys.*--Why should the blood pass through two systems of
capillaries in the kidneys? This is because the separation of waste is
done in part by the Malpighian capsules and in part by the uriniferous
tubules. Water and salts are removed chiefly at the capsules, while the
remaining solid constituents of the urine pass through the secreting cells
that line the tubules. It was formerly believed that the kidneys obtained
their secretion by a process of filtration from the blood, but this belief
has been gradually modified. The prevailing view now is that the processes
of filtration and secretion are both carried on by the kidneys,--that the
capillary clusters in the Malpighian bodies serve as delicate filters for
the separation of water and salts, while the secreting cells of the
tubules separate substances by the process of secretion.
On account of the large volume of blood passing through the kidneys this
liquid is still a bright red color as it flows into the renal veins (Fig.
90). The kidney cells require oxygen, but the amount which they remove
from the blood is not sufficient to affect its color noticeably. The blood
in the renal veins, having given up most of its impurities and still
retaining its oxygen, is considered the purest blood in the body.
*Urea* is the most abundant solid constituent of the urine and is the
chief waste product arising from the oxidation of nitrogenous substances
in the body. Although secreted by the cells lining the uriniferous
tubules, it is not formed in the kidneys. The secreting cells simply
separate it from the blood where it already exists. The muscles also have
been suggested as a likely source of urea, for here the proteids are
broken down in largest quantities; but the muscles produce little if any
urea. Its production has been found to be the _work of the liver_. In the
muscular tissue, and in the other tissues as well, the proteids are
reduced to a lower order of compounds, such as the compounds of ammonia,
which pass into the blood and are then taken up by the liver. By the
action of the liver cells these are converted into urea and this is turned
back into the blood. From the blood the urea is separated by the secreting
cells of the kidneys.
*Work of the Liver.*--The liver, already described as an organ of digestion
(page 152), assists in the work of excretion both by changing waste
nitrogenous compounds into urea and by removing from the blood the wastes
found in the bile. While the chief work of the liver is perhaps not that
of excretion, its functions may here be summarized. The liver is, first of
all, a _manufacturing organ_, producing, as we have seen, three distinct
products--bile, glycogen, and urea. On account of the nature of the urea
and the bile, the liver is properly classed as an _excretory organ_; but
in the formation of the glycogen it plays the part of a _storage organ_.
Then, on account of the use made of the bile after it is passed into the
food canal, the liver is also classed as a _digestive organ_. These
different functions make of the liver an organ of the first importance.
*Excretory Work of the Food Canal.*--The glands connected with the food
canal, other than the liver, while secreting liquids that aid in
digestion, also separate waste materials from the blood. These are passed
into the canal, whence they leave the body with the undigested portions of
the food and the waste from the liver. Though the nature and quantity of
the materials removed by these glands have not been fully determined,
recent investigations have tended to enhance the importance attached to
this mode of excretion.
*The Perspiratory Glands.*--The perspiratory, or sweat, glands are located
in the skin. They belong to the type of simple tubular glands and are very
numerous over the entire surface of the body. A typical sweat gland
consists of a tube which, starting at the surface of the cuticle,
penetrates to the under portion of the true skin and there forms a
ball-shaped coil. The coiled extremity, which forms the secreting portion,
is lined with secreting cells and surrounded by a network of capillaries.
The portion of the tube passing from the coil to the surface serves as a
duct (Figs. 91 and 121).
[Fig. 91]
Fig. 91--*Diagram of section through a sweat gland.* _a._ Outer layer of
skin or cuticle. _b._ Dermis or true skin. _d, e._ Sections of the tube
forming the coiled portion of the gland. _c._ Duct passing to the surface.
The other structures of the skin not shown.
The sweat glands secrete a thin, colorless fluid, called _perspiration_,
or sweat. This consists chiefly of water, but contains a small per cent of
salts and of urea. The excretory work of these glands seems not to be so
great as was formerly supposed, but they supplement in a practical way the
work of the kidneys and, during diseases of these organs, show an increase
in excretory function to a marked degree. The perspiration also aids in
the regulation of the temperature of the body (Chapter XVI).
*Excretory Work of the Lungs.*--While the lungs cannot be regarded as
glands, they do a work in the removal of waste from the body which must be
considered in the general process of excretion. They are especially
adapted to the removal of gaseous substances from the blood, and it is
through them that most of the carbon dioxide leaves the body. The lungs
remove also a considerable quantity of water. This is of course in the
gaseous form, being known as water vapor.
*Ductless Glands and Internal Secretion.*--Midway in function between the
glands that secrete useful liquids and those that remove waste materials
from the blood is a class of bodies, found at various places, known as the
_ductless glands._ They are so named from their having the general form of
glands and from the fact that they have no external openings or ducts.
They prepare special materials which are passed into the blood and which
are supposed to exert some beneficial effect either upon the blood or upon
the tissues through which the blood circulates. The most important of the
ductless glands are the thyroid gland, located in the neck; the suprarenal
bodies, situated one just over each kidney; and the thymus gland, a
temporary gland in the upper part of the chest. The spleen and the
lymphatic glands (page 68) are also classed with the ductless glands. The
liver, the pancreas, and (according to some authorities) the kidneys, in
addition to their external secretions, produce materials that pass into
the blood. They perform in this way a function like that of the ductless
glands. The work of glands in preparing substances that enter the blood is
known as _internal secretion._
*Quantity of Excretory Products.*--If the weight of the normal body be
taken at intervals, after growth has been attained, there will be found to
be practically no gain or loss from time to time. This shows that
materials are leaving the body as fast as they enter and that the tissues
are being torn down as fast as they are built up. It also shows that
substances do not remain in the body _permanently_, but only so long
perhaps as is necessary for them to give up their energy, or serve some
additional purpose in the ever changing protoplasm. The excretory organs
then remove from the body a quantity of material that is equal in weight
to the materials absorbed by the organs of digestion and respiration. This
is estimated for the average individual to be about five pounds daily. The
passage of waste from the body is summarized in Table III.
TABLE III. THE PASSAGE OF WASTE MATERIALS FROM THE BODY
Materials State How Formed Condition in How Removed
in the Body the Blood from the
Blood
Carbon Gas By the Dissolved in Separated
dioxide oxidation of the plasma from the
the carbon and in loose blood at the
of proteids, combination alveoli of
carbohydrates, with salts the lungs
and fats. in the and then
blood. forced
through the
air passages
into the
atmosphere.
Urea Solid By the Dissolved in Removed by
oxidation in the plasma. the
the liver of uriniferous
nitrogenous tubules of
compounds. the kidneys
and to a
small extent
by the
perspiratory
glands.
Water Liquid By the As water. Removed by
oxidation of all the
the hydrogen organs of
of proteids, excretion,
carbohydrates, but in the
and fats. largest
Amount formed quantities
in the body is by the
small. kidneys and
the skin.
Salts Solid Dissolved in By the
the plasma. kidneys,
liver, and
skin.
HYGIENE
The separation of wastes from the body has such a close relation to the
health that all conditions affecting it should receive the most careful
attention. Their retention beyond the time when they should be discharged
undoubtedly does harm and is the cause of many bodily disorders.
*Value of Water.*--As a rule the work of excretion is aided by drinking
_freely_ of pure water. As water is the natural dissolver and transporter
of materials in the body, it is generally conceded by hygienists and
physicians that the taking of plenty of water is a healthful practice.
People do not as a rule drink a sufficient amount of water, about three
pints per day being required by the average adult, in addition to that
contained in the food. Most of the water should, of course, be taken
between meals, although the sipping of a small amount during meals does
not interfere with digestion. As stated elsewhere, the taking of a cup of
water on retiring at night and again on rising in the morning is very
generally recommended.
*Protection of Kidneys and Liver.*--The kidneys and liver are closely
related in their work and in many instances are injured or benefited by
the same causes. Both, as already stated (page 124), are liable to injury
from an _excess of proteid food_, especially meats, and also by a
condition of inactivity of the bowels (page 166). The free use of alcohol
also has an injurious effect on both of these organs.(75) On the other
hand, increasing the activity of the skin has a beneficial effect upon
them, especially the kidneys. Exercise and bathing, which tend to make the
skin more active, are valuable aids both in ridding the body of impurities
and in lessening the work of the other excretory organs. One having a
disease of the kidneys, however, needs to exercise great care in bathing
on account of the bad results which follow getting chilled.
*Special Care after Certain Diseases.*--Certain diseases, as measles,
diphtheria, scarlet fever, and typhoid fever, sometimes have the effect of
weakening the kidneys (and other vital organs) and of starting disease in
them. When this occurs it is usually the result of exposure or of
over-exertion while the body is in a weakened condition. Severe chilling
at such a time, by driving blood from the surface to the parts within,
often causes inflammation of the kidneys. On recovering from any wasting
disease one should exercise great caution both in resuming his regular
work and in exposing his body to wet or cold.
*Misunderstood Symptoms.*--Pains in the small of the back, an increase in
the secretions of the kidneys, and a sediment in the urine very naturally
suggest some disorder of the kidneys. It is a fact, however, that these
symptoms have little or no relation to the state of the kidneys and may
occur when the kidneys are in a perfectly healthy condition. The kidneys
are not located in the small of the back, but above this place, so that
pains in this region are evidently not from the kidneys, while the
increase in the flow of the urine may arise from a number of causes, one
of which is an increase of certain waste products passed into the blood.
The symptoms referred to are frequently the results of nervous exhaustion,
resulting from overstudy, worry, eye strain, or some other condition that
overtaxes the nervous system. When this is the case, relief is obtained
through resting the nerves. Actual disease of the kidneys can only be
determined through a chemical and microscopic examination of the urine. To
resort to some patent medicine for kidney trouble without knowing that
such trouble exists, as is sometimes done, is both foolish and unhygienic.
*Alcoholic Beverages and the Elimination of Waste.*--Causing as it does
such serious diseases as cirrhosis of the liver and Bright's disease of
the kidneys (footnote, page 210), alcohol will greatly interfere in this
way with the elimination of waste. There is also evidence to the effect
that it interferes with waste elimination before the stage is reached of
causing disease of these organs. Researches have shown that alcohol
increases the amount of uric acid in the body and decreases the amount of
urea found in the urine. The conclusion to be drawn is that alcohol
interferes in some way with the change of the harmful uric acid into the
comparatively harmless urea--an interference which in some instances
results in great harm. It has also been shown that malted liquors, such as
beer and ale, contain substances which, like the caffein of tea and coffee
(page 167), are readily converted into uric acid.(76) Wines contain acids
which may also act injuriously. The harm which such substances do is, of
course, additional to that caused by the alcohol.
*Summary.*--As a result of the oxidations and other changes at the cells,
substances are produced that can no longer serve a purpose in the body.
They are of the nature of waste, and their continuous removal from the
body is as necessary to the maintenance of life as the introduction of
food and oxygen. The organs whose work it is to remove the waste,
excepting the lungs, are glands; and the material which they remove are of
the nature of secretions. From the cells, the waste passes through the
lymph in the blood. From the blood it is separated by the excretory organs
and passed to the exterior of the body.
*Exercises.*--1. What general purposes are served by the glands in the
body?
2. What are the parts common to all glands? What purpose is served by each
of these parts?
3. How do tubular glands differ in structure from saccular glands? What is
a racemose gland? Why so called?
4. Describe the nature of the secretory process.
5. What conditions render necessary the formation of waste materials in
the body? Why must these be removed?
6. How do the waste materials get from the cells to the organs of
excretion?
7. Show by a drawing the connections of the kidneys with the large blood
vessels and the bladder. Name parts of drawing.
8. In what do the uriniferous tubes have their beginning? In what do they
terminate? With what are they lined?
9. Why should the blood pass through two sets of capillaries in the
kidneys?
10. Bright's disease of the kidneys affects the uriniferous tubes and
interferes with their work. What impurity is then left in the blood?
11. Trace water and salts from the Malpighian capsules to the bladder,
naming parts through which they pass.
12. Trace carbon dioxide from the cells to the outside atmosphere.
13. How does the quantity of material introduced into the body compare
with that which is removed by the organs of excretion?
14. Name two ways of lessening the work of the kidneys.
15. Why is the drinking of plenty of pure water a healthful practice?
PRACTICAL WORK
*To suggest the Double Work of Glands.*--Prepare a simple filter by fitting
a piece of porous paper into a glass funnel. Through this pass pure water
and also water having salt dissolved in it and containing some sediment,
as sand. The water and the dissolved salt pass through, while the sediment
remains on the filter. Now substitute a fresh piece of paper in the funnel
and drop on its surface a little solid coloring matter, such as cochineal.
Again pass the liquid through the funnel. This time it comes through
colored, the color being added by the filter. Compare the filter and
materials filtered to the gland and the materials concerned in secretion
(blood, the liquid secreted, substances added by the gland, etc.).
[Fig. 92]
Fig. 92--*The physiological scheme.* Diagram suggesting the essential
relation of the bodily activities. See Summary of Part I, page 215, and
Summary of Part II, page 413.
SUMMARY OF PART I
The body is an organization of different kinds of cells; it grows through
the growth and reproduction of these cells; and its life as a whole is
maintained by providing such conditions as will enable the cells to keep
alive. Of chief importance in the work of the body is a nutrient fluid
which supplies the cells with food and oxygen and relieves them of waste.
A moving portion of this fluid, called the blood, serves as a transporting
agent, while another portion, called the lymph, passes the materials
between the blood and the cells. Through their effects upon the blood and
the lymph, the organs of circulation, respiration, digestion, and
excretion minister in different ways to the cells, and aid in the
maintenance of life. By their combined action two distinct movements are
kept up in the body, as follows:
1. An _inward_ movement which carries materials from the outside of the
body toward the cells.
2. An _outward_ movement which carries materials from the cells to the
outside of the body.
Passing _inward_ are the oxygen and food materials _in a condition to
unite with each other_ and thereby change their potential into kinetic
energy. Passing _outward_ are the oxygen and the elements that formed the
food materials _after having united_ at the cells and liberated their
energy.
As a final and all-important result, there is kept up a _continuous series
of chemical changes_ in the cells. These liberate the energy, provide
special substances needed by the cells, and preserve the life of the body
(Fig. 92).
In the chapters which follow, we are to consider the problem of adjusting
the body to and of bringing it into proper relations with its
surroundings.
PART II: MOTION, COORDINATION, AND SENSATION
CHAPTER XIV - THE SKELETON
One necessary means of establishing proper relations between the body and
its surroundings is _motion_.(77) Not only can the body move itself from
place to place, but it is able to move surrounding objects as well. In the
production of motion three important systems are employed--the muscular
system, the nervous system, and a system of mechanical devices which are
found mainly in the skeleton. The muscular system supplies the energy for
operating the mechanical devices, while the nervous system controls the
movements.(78) Although the skeleton serves other purposes, such as giving
shape to the body and protecting certain organs, its main use is that of
an aid in the production of motion.
*Skeleton Tissues.*--The tissues employed in the construction of the
skeleton are the osseous, the cartilaginous, and the connective tissues.
These are known as the supporting tissues of the body. They form the
bones, supply the elastic pads at the ends of the bones, and furnish
strong bands, called ligaments, for fastening the bones together. The
skeleton forms about 16 per cent of the weight of the body. Its tissues,
being of a more durable nature than the rest of the body, do not so
readily decay. Especially is this true of the osseous tissue, which may be
preserved indefinitely, after removal from the body, by simply keeping it
dry.
*The Bones.*--The separate units, or parts, of which the skeleton is
constructed are called bones. They are the hard structures that can be
felt in all parts of the body, and they comprise nearly the entire amount
of material found in the prepared skeleton. As usually estimated, the
bones are 208 in number. They vary greatly in size and shape in different
parts of the body.
*Composition and Properties of Bones.*--The most noticeable and important
properties of the bones are those of hardness, stiffness, and toughness.
Upon these properties the uses of the bones depend. These properties may,
in turn, be shown to depend upon the presence in osseous tissue of two
essentially different kinds of substance, known as the _animal matter_ and
the _mineral matter_. If a bone is soaked in an acid, the mineral matter
is dissolved out, and as a result it loses its properties of hardness and
stiffness. (See Practical Work.) This is because the mineral matter
supplies these properties, being composed of substances which are hard and
closely resemble certain kinds of rock. The chief materials forming the
mineral matter are calcium phosphate and calcium carbonate.
On the other hand, burning a bone destroys the animal matter. When this is
done the bone loses its toughness, and becomes quite brittle. The property
of toughness is, therefore, supplied by the animal matter. This consists
mainly of a substance called _ossein_, which may be dissolved out of the
bones by boiling them. Separated from the bones it is known as _gelatine_.
The blood vessels and nerves in the bones, and the protoplasm of the bone
cells, are also counted in with the animal matter.
[Fig. 93]
Fig. 93--*Section of a long bone* (_tibia_), showing the gross structure.
If a dry bone from a full-grown, but not old, animal be weighed before and
after being burned, it is found to lose about one third of its weight.
From this we may conclude that about one third of the bone by weight is
animal matter and two thirds is mineral matter. This proportion, however,
varies with age, the mineral matter increasing with advance of years.
*Gross Structure of Bones.*--The gross structure of the bones is best
learned by studying both dry and fresh specimens. (See Practical Work.)
The ends of the bones are capped by a layer of smooth, elastic cartilage,
while all the remaining surface is covered by a rather dense sheath of
connective tissue, called the _periosteum_. Usually the central part of
the long bones is hollow, being filled with a fatty substance known as the
_yellow marrow_. Around the marrow cavity the bone is very dense and
compact, but most of the material forming the ends is porous and spongy.
These materials are usually referred to as the _compact substance_ and the
_cancellous_, or _spongy, substance_ of the bones (Fig. 93).
The arrangement of the compact and spongy substance varies with the
different bones. In the short bones (wrist and ankle bones, vertebrae,
etc.) and also in the flat bones (skull bones, ribs, shoulder blades,
etc.) there is no cavity for the yellow marrow, all of the interior space
being filled with the spongy substance. The _red marrow_, relations of
which to the red corpuscles of the blood have already been noted (page
27), occupies the minute spaces in the spongy substance.
[Fig. 94]
Fig. 94--*Cross section of bone showing minute structure.* Magnified. 1.
Surface layer of bone. 2. Deeper portion. 3. Haversian canals from which
pass the canaliculi. 4. A lacuna. Observe arrangement of lacunae at surface
and in deeper portion.
*Minute Structure of Bone.*--A microscopic examination of a thin slice of
bone taken from the compact substance shows this to be porous as well as
the spongy substance. Two kinds of small channels are found running
through it in different directions, known as the Haversian canals and the
canaliculi (Fig. 94). These serve the general purpose of distributing
nourishment through the bone. The _Haversian canals_ are larger than the
canaliculi and contain small nerves and blood vessels, chiefly capillaries
(Fig. 95). They extend lengthwise through the bone. The _canaliculi_ are
channels for conveying lymph. They pass out from the Haversian canals at
right angles, going to all portions of the compact substance except a thin
layer at the surface. In the surface layer of the bone the canaliculi are
in communication with the periosteum.
[Fig. 95]
Fig. 95--*Section showing Haversian canal and contents*, highly magnified
(after Schaefer). 1. Arterial capillary. 2. Venous capillary. 3. Nerve
fibers. 4. Lymph vessel.
*The Bone Cells.*--Surrounding the Haversian canals are thin layers of bone
substance called the _laminae_, and within these are great numbers of
irregular bodies, known as the _lacunae_. The walls of the lacunae are hard
and dense, but within each is an open space. In this lies a flattened
body, having a nucleus, which is recognized as the _bone cell_, or the
bone corpuscle (Fig. 96). It appears to be the work of the bone cells to
deposit mineral matter in the walls surrounding them and in this way to
supply the properties of hardness and stiffness to the bones. The
canaliculi connect with the lacunae in all parts of the bone, causing them
to appear under the microscope like so many burs fastened together by
their projecting spines (Fig. 94).
[Fig. 96]
Fig. 96--*Bone cell* removed from the lacuna and very highly magnified.
(From Quain's _Anatomy_.)
*How the Bone Cells are Nourished.*--The bone cells, like all the other
cells of the body, are nourished by the lymph that escapes from the blood.
This passes through the canaliculi to the cells in the different parts of
the bone, as follows:
1. The cells in the surface layer of the bone receive lymph from the
capillaries in the periosteum.(79) It gets to them through the short
canaliculi that run out to the surface.
2. The cells within the interior of the bone receive their nourishment
from the small blood vessels in the Haversian canals. Lymph from these
vessels is conveyed to the cells through the canaliculi that connect with
the Haversian canals.
*Plan and Purpose of the Skeleton.*--The framework of the body is such as
to adapt it to a _movable_ structure. Obviously the different parts of the
body cannot be secured to a foundation, as are those of a stationary
building, but must be arranged after a plan that is conducive to motion. A
moving structure, as a wagon or a bicycle, has within it some strong
central part to which the remainder is joined. The same is true of the
skeleton. That part to which the others are attached is a long, bony axis,
known as the _spinal column_. Certain parts, as the ribs and the skull,
are attached directly to the spinal column, while others are attached
indirectly to it. The arrangement of all the parts is such that the spinal
column is made the central, cohering portion of the skeleton and also of
the whole body.
Besides the general arrangement of the parts of the skeleton, there is
such a grouping of the bones in each of its main divisions as will enable
them to serve definite purposes. In most places they form mechanical
devices for supplying special movements, and in certain places they
provide for the support or protection of important organs. In most cases
there is a definite combination of different bones, forming what is called
the bone group.
[Fig. 97]
Fig. 97--The human skeleton.
*Bone Groups.*--On account of the close relation between the bones of the
same group, they cannot profitably be studied as individual bones, but
each must be considered as a part of the group to which it belongs. By
first making out the relation of a given bone to its group, its value to
the whole body can be determined. The most important of the groups of
bones are as follows:
1. _The Spinal Column._--This group consists of twenty-four similarly
shaped bones, placed one above the other, called the _vertebrae_, and two
bones found below the vertebrae, known as the sacrum and the coccyx (Fig.
98). These twenty-six bones supply the central axis of the body, support
the head and upper extremities, and inclose and protect the spinal cord.
[Fig. 98]
Fig. 98--The spinal column.
The upper seven vertebrae form the neck and are called the _cervical_
vertebrae. They are smaller and have greater freedom of motion than the
others. The first and second cervical vertebrae, known as the _atlas_ and
the _axis_, are specially modified to form a support for the head and
provide for its movements. The head rests upon the atlas, forming with it
a hinge joint (used in nodding to indicate "yes"); and the atlas turns
upon an upward projection of the axis forming a pivot joint (used in
shaking the head to indicate "no").
The next twelve vertebrae, in order below the cervical, are known as the
_thoracic_ vertebrae. They form the back part of the framework of the
thorax and have little freedom of motion. The five vertebrae below the
thoracic are known as the _lumbar_ vertebrae. These bones are large and
strong and admit of considerable motion. Below the last lumbar vertebra is
a wedge-shaped bone which has the appearance of five vertebrae fused
together. This bone, known as the _sacrum_, connects with the large bones
which form the pelvic girdle. Attached to the lower end of the sacrum is a
group of from two to four small vertebrae, more or less fused, called the
_coccyx_.
[Fig. 99]
Fig. 99--*Two views of a lumbar vertebra.* _A._ From above. _B._ From the
side. 1. Body. 2, 3, 4, 5. Projections from the neural arch.
*The Joining of the Vertebrae.*--A typical vertebra consists of a heavy,
disk-shaped portion in front, called the _body_, which is connected with a
ring-like portion behind, called the _neural arch_. The body and the
neural arch together encircle a round opening which is a part of the canal
that contains the spinal cord (Fig. 99). From the neural arch are seven
bony projections, or processes, three of which serve for the attachment of
muscles and ligaments, while the other four, two above and two below, are
for the interlocking of the vertebrae with each other. The separate
vertebrae are joined together in the spinal column, as follows:
_a._ Between the bodies of adjacent vertebrae are disks of elastic
cartilage. Each disk is about one fourth of an inch thick and is grown
tight onto the face of the vertebra above and also onto the face of the
vertebra below. By means of these disks a very close connection is secured
between the vertebrae on the front side of the column.
_b._ On the back of the column, the downward projections from the neural
arch of each vertebra above fit into depressions found in the neural arch
of the vertebra below. This _interlocking_ of the vertebrae, which is most
marked in the lumbar region, strengthens greatly the back portion of the
column.
_c._ To further secure one bone upon the other, numerous ligaments pass
from vertebra to vertebra on all sides of the column.
2. _The Skull._--The skull is formed by the close union of twenty-two
irregular bones. These fall naturally into two subgroups--the cranium and
the face (Fig. 100). The _cranium_ consists of eight thin, curved bones
which inclose the space, called the _cranial cavity_, that holds the
brain. The _face group_, consisting of fourteen bones, provides cavities
and supports for the different organs of the face, and supplies a movable
part (the inferior maxillary) which, with the bones above (superior
maxillary), forms the machine for masticating the food.
[Fig. 100]
Fig. 100--*The skull (Huxley).* The illustration shows most of the bones of
the skull.
3. _The Thorax._--This group contains twenty-four bones of similar form,
called _ribs_, and a straight flat bone, called the _sternum_, or
breastbone (Fig. 101). The ribs connect with the spinal column behind, and
all but the two lowest ones connect with the sternum in front, and, by so
doing, inclose the thoracic cavity. As already stated (page 85), the bones
of the thorax form a mechanical device, or machine, for breathing. The
ribs are so arranged that the volume of the thorax is increased by
elevating them and diminished by depressing them, enabling the air to be
forced into and out of the lungs.
[Fig. 101]
Fig. 101--*Bone groups of trunk.*
4. _The Shoulder and Pelvic Girdles._--These groups form two bony
supports--one at the upper and the other at the lower portion of the
trunk--which serve for the attachment of the arms and legs (Fig. 101). The
_shoulder girdle_ is formed by four bones--two clavicles, or collar bones,
and two scapulae, or shoulder blades. The clavicle on either side connects
with the upper end of the sternum and serves as a _brace_ for the
shoulder, while the scapula forms a socket for the humerus (the large bone
of the arm) and supplies many places for the attachment of muscles.
The _pelvic girdle_ consists of two large bones of irregular shape, called
the _innominate_ bones. They connect behind with the sacrum and in front
they connect, through a small pad of cartilage, with each other. On the
inside of the girdle is a smooth, basin-shaped support for the contents of
the abdomen, but on the outside the bones are rough and irregular and
provide many places for the attachment of muscles and ligaments. Each
innominate bone has a deep, round socket into which the end of the femur
(the long bone of the leg) accurately fits.
5. _The Arm and Hand Groups._--A long bone, the _humerus_, connects the arm
with the shoulder and gives form to the upper arm. In the forearm are two
bones, the _radius_ and the _ulna_, which connect at one end with the
humerus and at the other with the bones of the wrist (Fig. 102).
[Fig. 102]
Fig. 102--*Bone groups of arm and leg.*
A group of eight small, round bones is found in the wrist, known as the
_carpal_ bones. These are arranged in two rows and are movable upon one
another. Five straight bones, the _metacarpals_, connect with the wrist
bones and form the framework for the palm of the hand. Attached to the
metacarpals are the bones of the fingers and thumb. These form an
interesting group of fourteen bones, called the _phalanges of the fingers_
(Fig. 102).
The bones of the hand provide a mechanical device, or machine, for
grasping, and the arm serves as a device for moving this grasping machine
from place to place. The work of the arm, in this respect, is not unlike
that of a revolving crane upon the end of which is a grab-hook. The hand
without the arm to move it about would be of little use.
6. _The Leg and Foot Groups._--These correspond in form and arrangement to
the bones of the arm and hand. Since, however, the leg and foot are used
for purposes different from those of the arm and hand, certain differences
in structure are to be found. The _patella_, or kneepan, has no
corresponding bone in the arm; and the _carpus_, or ankle, which
corresponds to the wrist, contains seven instead of eight bones. The bones
of the foot and toes are the same in number as those of the hand and
fingers, but they differ greatly in size and form and have less freedom of
motion. The _femur_, which gives form to the thigh, is the longest bone of
the body. The _tibia_, or shin bone, and the _fibula_, the slender bone by
its side, give form to the lower part of the leg (Fig. 102).
The legs are mechanical devices (walking machines) for moving the body
from place to place. The feet serve both as supports for the body and as
levers for pushing the body forward. By their attachment to the legs they
may be placed in all necessary positions for supporting and moving the
body.
The different bone groups are shown in Fig. 97 and named in Table IV.
*Adaptation to Special Needs.*--When any single bone is studied in its
relation to the other members of the group to which it belongs or with
particular reference to its purpose in the body, its adaptation to some
special place or use is at once apparent. Each bone serves some special
purpose, and to this purpose it is adapted by its form and structure. Long
bones, like the humerus and femur, are suited to giving strength, form,
and stiffness to certain parts, while irregular bones, like the vertebrae
and the pelvic bones, are fitted for supporting and protecting organs.
Others, like the wrist and ear bones, make possible a peculiar kind of
motion, and still others, like the ribs, are adapted to more than one
purpose. The vast differences in shape, size, structure, and surface among
the various bones are but the conditions that adapt them to particular
forms of service in the body.
TABLE IV - THE PRINCIPAL BONES AND THEIR GROUPING IN THE BODY
I. AXIAL SKELETON
A. _Skull_, 28.
1. Cranium, 8.
_ a._ Frontal, forehead 1
_ b._ Parietal 2
_ c._ Temporal, temple 2
_ d._ Occipital 1
_ e._ Sphenoid 1
_ f._ Ethmoid 1
2. Face, 14.
_ a._ Inferior maxillary 1
_ b._ Superior maxillary 2
_ c._ Palatine, palate 2
_ d._ Nasal bones 2
_ e._ Vomer 1
_ f._ Inferior turbinated 2
_ g._ Lachrymal 2
_ h._ Malar, cheek bones 2
3. Bones of the Ears, 6.
_ a._ Malleus 2
_ b._ Incus 2
_ c._ Stapes 2
B. _Spinal Column_, 26.
1. Cervical, or neck, vertebrae 7
2. Dorsal, or thoracic, vertebrae 12
3. Lumbar vertebrae 5
4. Sacrum 1
5. Coccyx 1
C. _Thorax_, 25.
1. Ribs 24
2. Sternum 1
D. _Hyoid_, 1 (at base of tongue).
II. APPENDICULAR SKELETON
A. _Shoulder girdle_ 4.
1. Clavicle, collarbone. 2
2. Scapula, shoulder blade 2
B. _Upper extremities_, 60.
1. Humerus 2
2. Radius 2
3. Ulna 2
4. Carpal, wrist bones 16
5. Metacarpal 10
6. Phalanges of fingers 28
C. _Pelvic girdle_, 2.
1. Osinnominatum 2
D. _Lower extremities_, 60.
1. Femur, thigh bone 2
2. Tibia, shin bone 2
3. Fibula 2
4. Patella, kneepan 2
5. Tarsal, ankle bones 14
6. Metatarsal, instep bones 10
7. Phalanges of toes 28
ARTICULATIONS
Any place in the body where two or more bones meet is called an
articulation, or joint. At the place of meeting the bones are firmly
attached to each other, thereby securing the necessary coherence of the
skeleton. The large number of bones, and consequently of articulations,
are necessary for the different movements of the body and also on account
of the manner in which the skeleton develops, or grows. Articulations are
classed with reference to their freedom of motion, as _movable_, _slightly
movable_, and _immovable_ articulations.
Most of the _immovable_ articulations are found in the skull. Here
irregular, tooth-like projections from the different bones enable them to
interlock with one another, while they are held firmly together by a thin
layer of connective tissue. The wavy lines formed by articulations of this
kind are called _sutures_ (Fig. 100).
The best examples of joints that are _slightly_, but not freely, _movable_
are found in the front of the spinal column. The cartilaginous pads
between the vertebrae permit, by their elasticity, of a slight bending of
the column in different directions. These movements are caused, not by one
bone gliding over another, but by compressions and extensions of the
cartilage. Between the vertebrae in the back of the spinal column, however,
there is a slight movement of the bone surfaces upon one another.
*Structure of the Movable Joints.*--By far the most numerous and important
of the joints are those that are freely movable. Such joints are strongly
constructed and endure great strain without dislocation, and yet their
parts move over each other easily and without friction. The ends of the
bones are usually enlarged and have specially formed projections or
depressions which fit into corresponding depressions or elevations on the
bones with which they articulate. In addition to this the articular
surfaces are quite smooth and dense, having no Haversian canals, and they
are covered with a layer of cartilage. Strong ligaments pass from one bone
to the other to hold each in its place (_A, _Fig. 103). Some of these
consist simply of bands, connecting the joint on its different sides,
while others form continuous sheaths around the joint.
[Fig. 103]
Fig. 103--*Outside and inside view of knee joint.* 1. Tendons. 2.
Ligaments. 3. Cartilage. 4. Space containing synovial fluid. This space is
lined, except upon the articular surfaces, by the synovial membrane.
The interior of the joint, except where the bone surfaces rub upon each
other, is covered with a serous lining, called the _synovial membrane_
(_B_, Fig. 103). This secretes a thick, viscid liquid, the _synovial
fluid_, which prevents friction. The synovial membrane does not cover the
ends of the bones, but passes around the joint and connects with the bones
at their edges so as to form a closed sac in which the fluid is retained.
*Kinds of Movable Joints.--*The different kinds of movable joints are the
ball and socket joint, the hinge joint, the pivot joint, the condyloid
joint, and the gliding joint. These are constructed and admit of motion,
as follows:
1. In the _ball and socket_ joint the ball-shaped end of one bone fits
into a cup-shaped cavity in another bone, called the socket. The best
examples of such joints are found at the hips and shoulders. The ball and
socket joint admits of motion in all directions.
2. In the _hinge_ joint the bones are grooved and fit together after the
manner of a hinge. Hinge joints are found at the elbows and knees and also
in the fingers. The hinge joint gives motion in but two directions--forward
and backward.
3. A _pivot_ joint is formed by the fitting of a pivot-like projection of
one bone into a ring-like receptacle of a second bone, so that one, or the
other, is free to turn. A good example of the pivot joint is found at the
elbow, where the radius turns upon the humerus. Another example is the
articulation of the atlas with the axis vertebra as already noted. The
pivot joint admits of motion around an axis.
4. The _condyloid_ joint is formed by the fitting of the ovoid
(egg-shaped) end of one bone into an elliptical cavity of a second bone.
Examples of condyloid joints are found at the knuckles and where the wrist
bones articulate with the radius and ulna. They move easily in two
directions, like hinge joints, and slightly in other directions.
5. _Gliding_ joints are formed by the articulation of plain (almost flat)
surfaces. Examples of gliding joints are found in the articulations
between the bones of the wrist and those of the ankle. They are the
simplest of the movable joints and are formed by one bone gliding, or
slipping, upon the surface of another.
*The Machinery of the Body.*--A machine is a contrivance for directing
energy in doing work. A sewing machine, for example, so directs the energy
of the foot that it is made to sew. Through its construction the machine
is able to produce just that form of motion needed for its work, and no
other forms, so that energy is not wasted in the production of useless
motion. The places in machines where parts rub or turn upon each other are
called _bearings_, and extra precautions are taken in the construction and
care of the bearings to prevent friction.
The body cannot properly be compared to any single machine, but must be
looked upon as a complex organization which employs a number of different
kinds of machines in carrying on its work. The majority of these machines
are found in the skeleton. The bones are the parts that are moved, and the
joints serve as bearings. Connected with the bones are the muscles that
supply energy, and attached to the muscles are the nerves that control the
motion. Other parts also are required for rendering the machines of the
body effective in doing work. These are supplied by the tissues connected
with the bones and the muscles.
HYGIENE OF THE SKELETON
Of chief concern in the hygiene of the skeleton is the proper _adjustment_
of its parts. The efficiency of any of the body machines is impaired by
lack of proper adjustment. Not only this, but because of the fact that the
skeleton forms the groundwork of the whole body--muscles, blood vessels,
nerves, everything in fact, being arranged with reference to it--any lack
of proper adjustment of the bones interferes generally with the
arrangement and work of tissues and organs. The displaced bones may even
compress blood vessels and nerves and interfere, in this way, with the
nourishment and control of organs remote from the places where the
displacements occur. For these reasons the proper adjustment of the
different parts of the skeleton supplies one of the essential conditions
for preserving the health.
*Hygienic Importance of the Spinal Column.*--What has been said about the
adjustment of the skeleton in general applies with particular force to the
spinal column. The spinal column serves both as the central axis of the
body and as the container of the spinal cord. Thirty-one pairs of nerves
pass between the vertebrae to connect the spinal cord with different parts
of the body, and two important arteries (the vertebral) pass through a
series of small openings in the bones of the neck to reach the brain.
Unnatural curves of the spine throw different parts of the body out of
their natural positions, diminish the thoracic and abdominal cavities,
and, according to the belief of certain physicians, compress the nerves
that pass from the cord to other parts of the body. Slightly misplaced
vertebrae in the neck, by compressing the vertebral arteries, may also
interfere with the supply of blood
[Fig. 104]
Fig. 104--A tendency toward spinal curvature (after Mosher)
[Fig. 105]
Fig. 105--Effect on spinal column of improper position in writing. (From
Pyle's _Personal Hygiene._)
*How the Skeleton becomes Deformed*--We are accustomed to look upon the
skeleton as a rigid framework which can get out of its natural form only
through severe strain or by violence. This view is far from being correct.
On account of their necessary freedom of motion, the bones, especially
those of the spinal column, are easily slipped from their normal
positions; and where improper attitudes are frequently assumed, or
continued through long periods of time, the skeleton gradually becomes
deformed (Fig. 104). For example, the habit of always sleeping on the same
side with a high pillow may develop a bad crook in the neck; and the ugly
curves, assumed so frequently in writing (80) (Fig. 105), and also in
standing, when the weight is shifted too much on one foot, may become
permanent. Then the habit of reclining in a chair with the hips resting on
the front of the seat often deforms the back and causes a drooping of the
shoulders. In fact, slight displacements of the vertebrae come about so
easily _through incorrect positions_, that they may almost be said to
"occur of themselves" where active measures are not taken to preserve the
natural form of the body. The very few people who have perfectly formed
bodies show to what an extent has been overlooked an essential law of
hygiene.
*Prevention of Skeletal Deformities.*--Those deformities of the skeleton
that are acquired through improper positions are prevented by giving
sufficient attention to the positions assumed in sitting, standing, and
sleeping, and also to the posture in various kinds of work. In sitting the
trunk should be erect and the hips should touch the back of the chair. One
should not lounge in the ordinary chair. In standing the body should be
erect, the shoulders back and down, the chest pushed slightly up and
forward, and the chin slightly depressed, while the weight should, as a
rule, rest about equally on the two feet. The habit of leaning against
some object when standing (the pupil in reciting often leans on his desk)
should be avoided. In sleeping the pillow should be of the right thickness
to support the head on a level with the spinal column and should not be
too soft. If one sleeps on his back, no pillow is required. It is best not
to acquire the habit of sleeping always on the same side.
Where one is compelled by his work to assume harmful positions, these
should be corrected by proper exercises, and by cultivating opposing
positions during the leisure hours. Much is to be accomplished through
those forms of physical exercise which develop the muscles whose work it
is to keep the body in an upright position.
*School Furniture.*--It has long been observed that school children are
more subject to curvature of the spine and other deformities of the
skeleton than the children who do not attend school. While this is due
largely to faulty positions assumed by the pupils at their work, it has
been suggested that the school furniture may be in part to blame for these
positions. Investigations of this problem have shown that most of the
school desks and seats in use in our public schools are unhygienically
constructed, in that they _force_ pupils into unnatural positions. School
seats should support the pupil in a natural position, both in the use of
his books and in writing, and there are many arguments in favor of the
so-called "adjustable" school furniture. Fig. 106 shows the seat and desk
designed by the Boston, Mass., Schoolhouse Commission after much study and
experimenting and used in the Boston schools. This furniture, which
provides a seat adjustable for height, having a back rest also adjustable
for height, and a desk which is likewise provided with a vertical
adjustment, supplies all essential hygienic requirements. It is to be
hoped that school furniture of this character may in the near future come
into general use.
[Fig. 106]
Fig. 106--Adjustable seat and desk used in schools of Boston, Mass.
*Correction of Skeletal Deformities.*--It is, of course, easier to prevent
deformities of the skeleton by giving attention to proper positions, than
to correct them after they have occurred. It should also be noted that
severe deformities cannot be corrected by the individual for himself, but
these must come under the treatment of specialists in this line of medical
work. In mild cases of spinal curvature, drooping of the head, and round
shoulders, the individual _can_ benefit his condition. By working to
"substitute a correct attitude for the faulty one,"(81) he can by
persistence bring about marked improvements. It is better, however, to
have the advice and aid of a physical director, where this is possible. It
should also be borne in mind that the correction of skeletal deformities
requires effort through a long period of time, especially where the
deformities are pronounced; and one lacking the will power to persist will
not secure all the results which he seeks.
*"Setting Up" Exercises.*--The splendid carriage of students from military
schools shows what may be accomplished in securing erectness of form where
proper attention is given to this matter. The military student gets his
fine form partly through his exercises in handling arms, but mainly
through his so-called "setting up" drill. As a suggestion to one desiring
to improve the form of his body, a modification of the usual "setting up"
drill is here given:
1. Standing erect, with the heels together, the feet at an angle of 45 deg.,
and hands at the sides, bring the arms to a horizontal position in front,
little fingers touching and nails down. From this position raise the hands
straight over the head, bringing the palms gradually together. Then with a
backward sweeping movement, return the hands again to the sides. Repeat
several times.
2. With the feet as in the above exercise, bring the hands and the arms to
a level with the shoulders, palms down, elbows bent, middle fingers of the
two hands touching, and the extended thumbs touching the chest. Keeping
the palms down and the arms on a level with the shoulders, extend the
hands as far sideward and backward as possible, returning each time to the
first position. As the hands move out, inhale deeply (through the nose),
and as they are brought back, exhale quickly (through the mouth). Repeat
several times.
3. With the arms at the sides and the feet side by side and touching,
bring the hands in a circular movement to a vertical position over the
head, and lock the thumbs. Keeping the knees straight and the thumbs
locked, bend forward, letting the hands touch the ground if possible, and
then bring the body and hands again to the vertical position. Then by a
backward sweeping movement, return the hands again to the sides. Repeat.
While these exercises may be practiced whenever convenient, it is best to
set apart some special time each day for them, as on retiring at night or
on rising in the morning.
*Hygienic Footwear.*--A necessary aid to erectness of position in standing
and walking is a properly fitting shoe. Heels that are too high tilt the
body unnaturally forward, and shoes that cause any kind of discomfort in
walking lead to unnatural positions in order to protect the feet. Shoes
should fit snugly, being neither too large nor too small. Many shoes,
however, are unhygienically constructed, and no attempt should be made to
wear them. Certainly is this true of styles that approach the "French
heel" or the "toothpick toe" (Fig. 107). However, many styles of shoes are
manufactured that are both hygienic and neat fitting. Rubber heels, on
account of their elasticity, are to be preferred to those made of leather.
[Fig. 107]
Fig. 107--Heels and toes of unhygienic and of hygienic footwear.
*The Skeleton in Childhood and Old Age.*--Certain peculiarities are found
to exist in the bones of children and of old people which call for special
care of the skeleton during the first and last periods of life. The bones
of children are soft, lacking mineral matter, and are liable to become
bent For this reason, children who are encouraged to walk at too early an
age may bend the thigh bones, causing the too familiar "bow-legs." These
bones may also be bent by having children sit on benches and chairs which
are too high for the feet to reach the floor, and which do not provide
supports for the feet. Wholesome food, fresh air, sunlight, and exercise
are also necessary to the proper development of the bones of children.
Where these natural conditions are lacking, as in the crowded districts of
cities, children often suffer from a disease known as "rickets," on
account of which their bones are unnaturally soft and easily bent.
On account of the accumulation of mineral matter, the bones of elderly
people become brittle and are easily broken, and from lack of vigor of the
bone cells they heal slowly after such injuries occur. This makes the
breaking of a bone by an aged person a serious matter. Old people should,
as far as possible, avoid liabilities to falls, such as going rapidly up
and down stairs, or walking on icy sidewalks, and should use the utmost
care in getting about. In old people also the cartilage between the bones
softens, increasing the liability of getting misshaped. Special attention,
therefore, should be given to erectness of form, and to such exercises as
tend to preserve the natural shape of the body.
*Treatment of Fractures.*--A fractured bone always requires the aid of a
surgeon, and no time should be lost in securing his services. In the
meantime the patient should be put in a comfortable position, and the
broken limb supported above the rest of the body. Though the breaking of a
bone is not, as a rule, a serious mishap, it is necessary that the very
best skill be employed in setting it. Any failure to bring the ends of the
broken bone into their normal relations permanently deforms the limb and
interferes with its use.
*Dislocations and Sprains.*--Dislocations, if they be of the larger joints,
also require the aid of the surgeon in their reduction and sometimes in
their subsequent treatment. Simple dislocations of the finger joints,
however, may be reduced by pulling the parts until the bones can be
slipped into position.
_A sprain_, which is an overstrained condition of the ligaments
surrounding a joint, frequently requires very careful treatment. When the
sprain is at all serious, a physician should be called. Because of the
limited supply of blood to the ligaments, they are slow to heal, and the
temptation to use the joint before it is fully recovered is always great.
Massage(82) judiciously applied to a sprained joint, by bringing about a
more rapid change in the blood and the lymph, is beneficial both in
relieving the pain, and in hastening recovery.
*Summary.*--The skeleton, or framework of the body, is a structure which is
movable as a whole and in most of its parts. It preserves the form of the
body, protects important organs, and supplies the mechanical devices, or
machines, upon which the muscles act in the production of motion. The
skeleton is adapted to its purposes through the number and properties of
the bones, and through the cartilage and connective tissue associated with
the bones. The places where the different bones connect one with another
are known as joints, and most of these admit of motion. The preservation
of the natural form of the skeleton is necessary, both for its proper
action and for the health of the body.
*Exercises.*--1. State the main purpose of the skeleton. What is the
necessity for so many bones in its construction?
2. How may the per cent of animal and of mineral matter in a bone be
determined?
3. What properties are given the bones by the animal matter? What by the
mineral matter?
4. Locate the bone cells. What is their special function?
5. State the plan by which nourishment is supplied to the bone cells in
different parts of the bone.
6. Give the uses of the periosteum.
7. State the purpose of the Haversian canals. Of the canaliculi.
8. Give functions of the spinal column.
9. Name the different materials used in the construction of a joint and
the purpose served by each.
10. Name four mechanical devices, or machines, found in the skeleton and
state the purpose served by each.
11. Name one or more of the body machines not located in the skeleton.
12. Of what advantage is the peculiar shape of the lower jaw? Of the ribs?
Of the bones of the pelvic girdle?
13. State the importance of preserving the natural form of the skeleton.
How are unnatural curves produced in the spinal column?
14. How may slight deformities of the skeleton be corrected?
15. What different systems are employed in the body in the production of
motion? What is the special function of each?
PRACTICAL WORK
To obtain clear ideas of the form and functions of the bones, a careful
examination of a prepared and mounted skeleton is necessary. Many of the
bones, however, may be located and their general form made out from the
living body. Bones of the lower animals may also be studied to advantage.
*Experiments to show the Composition of Bone.*--1. Examine a slender bone,
like that in a chicken's leg. Note that it resists bending and is
difficult to break. Note also that it is elastic--that, when slightly bent,
it will spring back.
2. Soak such a bone over night in a mixture of one part hydrochloric acid
and four parts water. Then ascertain by bending, stretching, and twisting
what properties the bone has lost. The acid has dissolved out the mineral
matter.
3. Burn a small piece of bone in a clear gas flame, or on a bed of coals,
until it ceases to blaze and turns a white color. Can the bone now be bent
or twisted? What properties has it lost and what retained? What substance
has been removed from the bone by burning?
*Observation on the Gross Structure of Bone.*--1. Procure a long, dry bone.
(One that has lain out in the field until it has bleached will answer the
purpose excellently.) Test its hardness, strength, and stiffness. Saw it
in two a third of the distance from one end, and saw the shorter piece in
two lengthwise. Compare the structure at different places. Find rough
elevations on the outside for the attachment of muscles, and small
openings into the bone for the entrance of blood vessels and nerves. Make
drawings to represent the sections.
2. Procure a fresh bone from the butcher shop. Note the difference between
it and the dry bone. Examine the materials surrounding the sides and
covering the ends of the bone. Saw through the enlarged portion at the end
and examine the red marrow. Saw through the middle of the bone and observe
the yellow marrow.
*To show the Minute Structure of the Bone.*--Prepare a section of bone for
microscopic study as follows: With a jeweler's saw cut as thin a slice as
possible. Place this upon a good-sized whetstone, not having too much
grit, and keeping it wet rub it under the finger, or a piece of leather,
until it is thin enough to let the light shine through. The section may
then be washed and examined with the microscope. If the specimen is to be
preserved for future study, it may be mounted in the usual way, but with
_hard_ balsam. Prepare and study both transverse and longitudinal
sections, making drawings. The sections should be prepared from bones that
are thoroughly dry but which have not begun to decay.
*To show the Structure of a Joint.*--Procure from a butcher the joint of
some small animal (hog or sheep). Cut it open and locate the cartilage,
synovial membrane, and ligaments. Observe the shape and surface of the
rubbing parts and the strength of the ligaments.
CHAPTER XV - THE MUSCULAR SYSTEM
As already stated, the skeleton, the nervous system, and the muscular
system are concerned in the production of motion. The skeleton and the
nervous system, however, serve other purposes in the body, while the
muscular system is devoted exclusively to the production of motion. For
this reason it is looked upon as the special _motor_ system. The muscular
tissue is the most abundant of all the tissues, forming about 41 per cent
of the weight of the body.
*Properties of Muscles.*--The ability of muscular tissue to produce motion
depends primarily upon two properties--the property of irritability and the
property of contractility. _Irritability_ is that property of a substance
which enables it to respond to a stimulus, or to act when acted upon.
_Contractility_ is the property which enables the muscle when stimulated
to draw up, thereby becoming shorter and thicker (a condition called
contraction), and when the stimulation ceases, to return to its former
condition (of relaxation). The property of contractility enables the
muscles to produce motion. Irritability is a condition necessary to their
control in the body.
*Kinds of Muscular Tissue.*--Three kinds of muscular tissue are found in
the body. These are known as the _striated_, or striped, muscular tissue;
the _non-striated_, or plain, muscular tissue; and the _muscular tissue of
the heart_. These are made up of different kinds of muscle cells and act
in different ways to cause motion. The striated muscular tissue far
exceeds the others in amount and forms all those muscles that can be felt
from the surface of the body. The non-striated muscle is found in the
walls of the food canal, blood vessels, air passages, and other tubes of
the body; while the muscular tissue of the heart is confined entirely to
that organ.
*Striated Muscle Cells.*--The cells of the striated muscles are slender,
thread-like structures, having an average length of 1-1/2 inches (35
millimeters) and a diameter of about 1/400 of an inch (60 {~GREEK SMALL LETTER MU~}). Because of
their great length they are called fibers, or fiber cells. They are marked
by a number of dark, transverse bands, or stripes, called striations,(83)
which seem to divide them into a number of sections, or disks (Fig. 108).
A thin sac-like covering, called the _sarcolemma_, surrounds the entire
cell and just beneath this are a number of nuclei.(84)
[Fig. 108]
Fig. 108--*A striated muscle cell* highly magnified, showing striations and
nuclei. Attached to the cell is the termination of a nerve fiber.
Within the sarcolemma are minute fibrils and a semiliquid substance,
called the _sarcoplasm_. At each end the cell tapers to a point from which
the sarcolemma appears to continue as a fine thread, and this, by
attaching itself to the inclosing sheath, holds the cell in place. Most of
the muscle cells receive, at some portion of their length, the termination
of a nerve fiber. This penetrates the sarcolemma and spreads out upon a
kind of disk, having several nuclei, known as the _end plate_.
*The "Muscle-organ."*--We must distinguish between the term "muscle" as
applied to the muscular tissue and the term as applied to a working group
of muscular tissue, which is an organ. In the muscle, or muscle-organ, is
found a definite grouping of muscle fibers such as will enable a large
number of them to act together in the production of the same movement. An
examination of one of the striated muscles shows the individual fibers to
lie parallel in small bundles, each bundle being surrounded by a thin
layer of connective tissue. (See Practical Work.) These small bundles are
bound into larger ones by thicker sheaths and these in turn may be bound
into bundles of still larger size (Fig. 109). The sheaths surrounding the
fiber bundles are connected with one another and also with the outer
covering of the muscle, known as
[Fig. 109]
Fig. 109--*Diagram* of a section of a muscle, showing the perimysium and
the bundles of fiber cells.
[Fig. 110]
Fig. 110--*A muscle-organ in position.* The tendons connect at one end with
the bones and at the other end with the fiber cells and perimysium. (See
text.)
*The Perimysium.*--The plan of the muscle-organ is revealed through a study
of the perimysium. This is not limited to the surface of the muscle, as
the name suggests, but properly includes the sheaths that surround the
bundles of fibers. Furthermore, the surface perimysium and that within the
muscle are both continuous with the strong, white cords, called _tendons_,
that connect the muscles with the bones. By uniting with the bone at one
end and blending with the perimysium and fiber bundles at the other, the
tendon forms a very secure attachment for the muscle. The perimysium and
the tendon are thus the means through which the fiber cells in any
muscle-organ are made to _pull together_ upon the same part of the body
(Fig. 110).
*Purpose of Striated Muscles.*--The striated muscles, by their attachments
to the bones, supply motion to all the mechanical devices, or machines,
located in the skeleton. Through them the body is moved from place to
place and all the external organs are supplied with such motion as they
require. Because of the attachment of the striated muscles to the
skeleton, and their action upon it, they are called _skeletal_ muscles. As
most of them are under the control of the will, they are also called
_voluntary_ muscles. They are of special value in adapting the body to its
surroundings.
*Structure of the Non-striated Muscles.*--The cells of the non-striated
muscles differ from those of the striated muscles in being decidedly
spindle-shaped and in having but a single well-defined nucleus (Fig. 111).
Furthermore, they have no striations, and their connection with the nerve
fibers is less marked. They are also much smaller than the striated cells,
being less than one one-hundredth of an inch in length and one
three-thousandth of an inch in diameter.
In the formation of the non-striated muscles, the cells are attached to
one another by a kind of muscle cement to form thin sheets or slender
bundles. These differ from the striated muscles in several particulars.
They are of a pale, whitish color, and they have no tendons. Instead of
being attached to the bones, they usually form a distinct layer in the
walls of small cavities or of tubes (Fig. 111). Since they are controlled
by the part of the nervous system which acts independently of the will,
they are said to be _involuntary_. They contract and relax slowly.
[Fig. 111]
Fig. 111--*Non-striated muscle cells.* _A._ Cross section of small artery
magnified, showing (1) the layer of non-striated cells. _B._ Three
non-striated cells highly magnified.
*Work of the Non-striated Muscles.*--The work of the non-striated muscles,
both in purpose and in method, is radically different from that of the
striated. They do not change the _position_ of parts of the body, as do
the striated muscles, but they alter the _size_ and _shape_ of the parts
which they surround. Their purpose, as a rule, is to move, or control the
movement of, materials within cavities and tubes, and they do this by
means of the _pressure_ which they exert. Examples of their action have
already been studied in the propulsion of the food through the alimentary
canal and in the regulation of the flow of blood through the arteries
(pages 159 and 49). While they do not contract so quickly, nor with such
great force as the striated muscles, their work is more closely related to
the vital processes.
*Structure of the Heart Muscle.*--The cells of the heart combine the
structure and properties of the striated and the non-striated muscle
cells, and form an intermediate type between the two. They are
cross-striped like the striated cells, and are nearly as wide, but are
rather short (Fig. 112). Each cell has a well-defined nucleus, but the
sarcolemma is absent. They are placed end to end to form fibers, and many
of the cells have branches by which they are united to the cells in
neighboring fibers. In this way they interlace more or less with each
other, but are also cemented together. They contract quickly and with
great force, but are not under control of the will. Muscular tissue of
this variety seems excellently adapted to the work of the heart.
[Fig. 112]
Fig. 112--*Muscle cells from the heart*, highly magnified (after Schaefer).
*The Muscular Stimulus.*--The inactive, or resting, condition of a muscle
is that of relaxation. It does work through contracting. It becomes
active, or contracts, only when it is being acted upon by some force
outside of itself, and it relaxes again when this force is withdrawn. Any
kind of force which, by acting on muscles, causes them to contract, is
called a _muscular stimulus_. Electricity, chemicals of different kinds,
and mechanical force may be so applied to the muscles as to cause them to
contract. These are _artificial_ stimuli. So far as known, muscles are
stimulated _naturally_ in but one way. This is through the nervous system.
The nervous system supplies a stimulus called the _nervous impulse_, which
reaches the muscles by the nerves, causing them to contract. By means of
nervous impulses, all of the muscles (both voluntary and involuntary) are
made to contract as the needs of the body for motion require.
*Energy Transformation in the Muscle.*--The muscle serves as a kind of
engine, doing work by the transformation of potential into kinetic energy.
Evidences of this are found in the changes that accompany contraction.
Careful study shows that during any period of contraction oxygen and food
materials are consumed, waste products, such as carbon dioxide, are
produced, and heat is liberated. Furthermore, the _blood supply to the
muscle_ is such that the materials for providing energy may be carried
rapidly to it and the products of oxidation as rapidly removed. Blood
vessels penetrate the muscles in all directions and the capillaries lie
very near the individual cells (Fig. 113). Provision is made also, through
the nervous system, for _increasing_ the blood supply when the muscle is
at work. From these facts, as well as from the great force with which the
muscle contracts, one must conclude that the muscle is a _transformer of
energy_--that within its protoplasm, chemical changes take place whereby
the potential energy of oxygen and food is converted into the kinetic
energy of motion.
[Fig. 113]
Fig. 113--*Capillaries* of muscles.
*Plan of Using Muscular Force.*--Two difficulties have to be overcome in
the using of muscular force in the body. The first of these is due to the
fact that the muscles exert their force _only when they contract_. They
can pull but not push. Hence, in order to bring about the opposing
movements(85) of the body, each muscle must work against some force that
produces a result directly opposite to that which the muscle produces.
Some of the muscles (those of breathing) work against the elasticity of
certain parts of the body; others (those that hold the body in an upright
position), to some extent against gravity; and others (the non-striated
muscle in arteries), against pressure. But in most cases, _muscles work
against muscles_.
[Fig. 114]
Fig. 114--*The muscle pair* that operates the forearm. For names of these
muscles, see Fig. 119.
The striated, or skeletal, muscles are nearly all arranged after the
last-named plan. As a rule a pair of muscles is so placed, with reference
to a joint, that one moves the part in one direction, and the other moves
it in the opposite direction. From the kinds of motion which the various
muscle pairs produce, they are classified as follows:
1. _Flexors and Extensors._--The flexor muscles bend and the extensors
straighten joints (Fig. 114).
2. _Adductors and Abductors._--The adductors draw the limbs into positions
parallel with the axis of the body and the abductors draw them away.
3. _Rotators_ (two kinds).--The rotators are attached about pivot joints
and bring about twisting movements.
4. _Radiating and Sphincter Muscles. _--The radiating muscles open and the
sphincter muscles close the natural openings of the body, such as the
mouth.
The pupil should locate examples of the different kinds of muscle pairs in
his own body.
*Exchange of Muscular Force for Motion.*--The second difficulty to be
overcome in the use of muscular force in the body is due to the fact that
the muscles contract through _short_ distances, while it is necessary for
most of them to move portions of the body through _long_ distances. It may
be easily shown that the longest muscles of the body do not shorten more
than three or four inches during contraction. To bring about the required
movements of the body, which in some instances amount to four or five
feet, requires that a large proportion of the muscular force be exchanged
for motion. The machines of the skeleton, while providing for motion in
definite directions, also provide the means whereby _strong forces_,
acting through _short distances_, are made to produce movements of _less
force_, through _long distances_. The mechanical device employed for this
purpose is known as
*The Lever.*--The lever may be described as a stiff bar which turns about a
fixed point of support, called the _fulcrum_. The force applied to the bar
to make it turn is called the _power_, and that which is lifted or moved
is termed the _weight_. The weight, the power, and the fulcrum may occupy
different positions along the bar and this gives rise to the three kinds
of levers, known as levers of the first class, the second class, and the
third class (Fig. 115). In levers of the _first class_ the fulcrum
occupies a position somewhere between the power and the weight. In the
_second class_ the weight is between the fulcrum and the power. In the
_third class_ the power is between the fulcrum and the weight.
[Fig. 115]
Fig. 115--*Classes of levers. I.* Two levers of first class showing
fulcrums in different positions. II. Lever of second class. III. Lever of
third class. _F._ Fulcrum. _P._ Power. _W._ Weight. _a._ Power-arm. _b._
Weight-arm.
*Application to the Body.*--In the body the bones serve as levers; the
turning points, or fulcrums, are found at the joints; the muscles supply
the power; and parts of the body, or things to be lifted, serve as
weights. For these levers to _increase_ the motion of the muscles, it is
necessary that the muscles be attached to the bones _near the joints_, and
that the parts to be moved be located at some distance from the joints. In
other words the (muscle) power-arm must be _shorter_ than the (body)
weight-arm.(86)
Examining Fig. 116, it is seen that the distances moved by the power and
weight vary as their respective distances from the fulcrum. That is to
say, if the weight is twice as far from the fulcrum as the power, it will
move through twice the distance, and if three times as far, through three
times the distance. Thus the muscles, by acting through short distances
(on the short arms of levers), are able to move portions of the body
(located on the long arms) through long distances. Can all three classes
of levers be used in this way in the body?
[Fig. 116]
Fig. 116--*Motion producing levers.* Diagrams show relative distances moved
by the power and weight in levers having the power nearer the fulcrum than
is the weight. _F._ Fulcrum. _P, P'._ Power. _W, W'._ Weight.
*Classes of Levers found in the Body.*--Practically all of the levers of
the body belong either to the first class or the third class. In both of
these the muscle power can be applied to the short arm of the lever,
thereby moving the body weight through a longer distance than the muscle
contracts (Fig. 116). In the levers of the second class, however, the
weight occupies this position, being situated _between_ the power and
fulcrum (Fig. 117). The weight, therefore, _cannot_ move farther than the
power in this lever. It must always move a shorter distance. While such a
lever is of great advantage in lifting heavy weights outside of the body,
it cannot be used for increasing the motion of the muscles. For this
reason no well-defined levers of the second class are present in the
body.(87)
[Fig. 117]
Fig. 117--*Weight lifting levers.* Diagrams show relative distances moved
by the power and weight in levers having the weight nearer the fulcrum
than is the power. _F._ Fulcrum. _P, P'._ Power. _W, W'._ Weight.
[Fig. 118]
Fig. 118--*Diagram of the foot lever.* _F._ Fulcrum at ankle joint. _W._
Body weight expressed as pressure against the earth. While the muscle
power acts through the distance _ab_, the fulcrum support (body) is forced
through the distance _FE_.
*Loss of Muscular Force.*--Using a small spring balance for measuring the
power, a light stick for a lever, and a small piece of metal for a weight,
and arranging these to represent some lever of the body (as the forearm),
it is easily shown that the gain in motion causes a corresponding loss in
muscular power. (See Practical Work.) If, for example, the balance is
attached two inches from the fulcrum and the weight twelve inches, the
pull on the balance is found to be six times greater than the weight that
is being lifted. If other positions are tried, it is found that the power
exerted in each case is as many times greater than the weight as the
weight-arm is times longer than the power-arm.
Applying this principle to the levers of the body, it is seen that the
gain in motion is at the expense of muscular force, or, as we say,
_muscular force is exchanged for motion_. This exchange is greatly to the
advantage of the body; for while the ability to lift heavy weights is
important, the ability to move portions of the body rapidly and through
long distances is much more to be desired.
*Important Muscles.*--There are about five hundred separate muscles in the
body. These vary in size, shape, and plan of attachment, to suit their
special work. Some of those that are prominent enough to be felt at the
surface are as follows:
_Of the head_: The _temporal_, in the temple, and the _masseter_, in the
cheek. These muscles are attached to the lower jaw and are the chief
muscles of mastication.
_Of the neck_: The _sterno-mastoids_, which pass between the mastoid
processes, back of the ears, and the upper end of the sternum. They assist
in turning the head and may be felt at the sides of the neck (Fig. 119).
_Of the upper arm_: The _biceps_ on the front side, the _triceps_ behind,
and the _deltoid_ at the upper part of the arm beyond the projection of
the shoulder.
[Fig. 119]
Fig. 119--Back and front views of important muscles.
_Of the forearm_: The _flexors_ of the fingers, on the front side, and the
_extensors_ of the fingers, on the back of the forearm (Fig. 119).
_Of the hand_: The _adductor pollicis_ between the thumb and the palm.
_Of the trunk_: The _pectoralis major_, between the upper front part of
the thorax and the shoulder; the _trapezius_, between the back of the
shoulders and the spine; the _rectus abdominis_, passing over the abdomen
from above downward; and the _erector spinae_, found in the small of the
back.
_Of the hips_: The _glutens maximus_, fastened between the lower back part
of the hips and the upper part of the femur.
_Of the upper part of the leg_: The _rectus femoris_, the large muscle on
the front of the leg which connects at the lower end with the kneepan.
_Of the lower leg_: The _tibialis anticus_ on the front side, exterior to
the tibia, and the _gastrocnemius_, the large muscle in the calf of the
leg. This is the largest muscle of the body, and is connected with the
heel bone by the _tendon of Achilles_ (Fig. 119).
The use of these muscles is, in most instances, easily determined by
observing the results of their contraction.
HYGIENE OF THE MUSCLES
The hygiene of the muscles is almost expressed by the one word _exercise_.
It is a matter of everyday knowledge that the muscles are developed and
strengthened by use, and that they become weak, soft, and flabby by
disuse. The effects of exercise are, however, not limited to the large
muscles attached to the skeleton, but are apparent also upon the
involuntary muscles, whose work is so closely related to the vital
processes. While it is true that exercise cannot be applied directly to
the involuntary muscles, it is also true that exercise of the voluntary
muscles causes a greater activity on the part of those that are
involuntary and is indirectly a means of exercising them.
*Exercise and Health.*--In addition to its effects upon the muscles
themselves, exercise is recognized as one of the most fundamental factors
in the preservation of the health. Practically every process of the body
is stimulated and the body as a whole invigorated by exercise properly
taken. On the other hand, a lack of exercise has an effect upon the entire
body somewhat similar to that observed upon a single muscle. It becomes
weak, lacks energy, and in many instances actually loses weight when
exercise is omitted. This shows exercise to supply an actual need and to
be in harmony with the nature and plan of the body.
*How Exercise benefits the Body.*--In accounting for the healthful effects
of exercise, it must be borne in mind that the body is essentially a
motion-producing structure. Furthermore, its plan is such that the
movements of its different parts aid indirectly the vital processes. The
student will recall instances of such aid, as, for example, the assistance
rendered by muscular contractions in the circulation of the blood and
lymph, due to the valves in veins and lymph vessels, and the assistance
rendered by abdominal movements in the propulsion of materials through the
food canal. A fact not as yet brought out, however, is that _exercise
stimulates nutritive changes in the cells_, thereby imparting to them new
vigor and vitality. While this effect of exercise cannot be fully
accounted for, two conditions that undoubtedly influence it are the
following:
1. Exercise causes the blood to circulate more rapidly.
2. Exercise increases the movement of the lymph through the lymph vessels.
The increase in the flow of the blood and the lymph causes changes to take
place more rapidly in the liquids around the cells, thereby increasing the
supply of food and oxygen, and hastening the removal of waste.
*One should plan for Exercise.*--Since exercise is demanded by the nature
and plan of the body, to neglect it is a serious matter. People do not
purposely omit exercise, but from lack of time or from its interference
with the daily routine of duties, the needed amount is frequently not
taken. Especially is this true of students and others who follow sedentary
occupations. People of this class should plan for exercise as they plan
for the other great needs of the body--food, sleep, clothing, etc. It is
only by making a sufficient amount of muscular work or play a regular part
of the daily program that the needs of the body for exercise are
adequately supplied.
*Amount and Kind of Exercise.*--The amount of exercise required varies
greatly with different individuals, and definite recommendations cannot be
made. For each individual also the amount should vary with the physical
condition and the other demands made upon the energy. One in health should
exercise sufficiently to keep the muscles firm to the touch and the body
in a vigorous condition.
Of the many forms of exercise from which one may choose, the question is
again one of individual adaptability and convenience. While the different
forms of exercise vary in their effects and may be made to serve different
purposes, the consideration of these is beyond the scope of an elementary
text. As a rule one will not go far wrong by following his inclinations,
observing of course the conditions under which exercise is taken to the
best advantage.
*General Rules for Healthful Exercise.*--That exercise may secure the best
results from the standpoint of health, a number of conditions should be
observed: 1. It should not be excessive or carried to the point of
exhaustion. Severe physical exercise is destructive to both muscular and
nervous tissues. 2. It should, if possible, be of an interesting nature
and taken in the open air. 3. It should be counter-active, that is,
calling into play those parts of the body that have not been used during
the regular work.(88) 4. It should be directed toward the weak rather than
toward the strong parts of the body. 5. When one is already tired from
study, or other work, it should be taken with moderation or omitted for
the time being. (For exercise of the heart muscle and the muscular coat of
the blood vessels see pages 55 and 57.)
*Massage.*--In lieu of exercise taken in the usual way, similar effects are
sometimes obtained by a systematic rubbing, pressing, stroking, or
kneading of the skin and the muscles by one trained in the art. This
process, known as massage, may be gentle or vigorous and is subject to a
variety of modifications. Massage is applied when one is unable to take
exercise, on account of disease or accident, and also in the treatment of
certain bodily disorders. A weak ankle, wrist, or other part of the body,
or even a bruise, may be greatly benefited by massage. The flow of blood
and lymph is stimulated, causing new materials to be passed to the
affected parts and waste materials to be removed. Massage, however, should
never be applied to a boil, or other infected sore. The effect in this
case would be to spread the infection and increase the trouble.
*Summary.*--Motion is provided for in the body mainly through the muscle
cells. These are grouped into working parts, called muscles, which in turn
are attached to the movable parts of the body. The striated muscles, as a
rule, are attached to the mechanical devices found in the skeleton, and
bring about the voluntary, movements. The non-striated muscles surround
the parts on which they act, and produce involuntary movements. Both,
however, are under the control of the nervous system. To bring about the
opposing movements of the body, the striated muscles are arranged in
pairs; and to increase their motion, the bones are used as levers.
Physical exercise is necessary both for the development of the muscles and
for the health and vigor of the entire body.
*Exercises.*--1. Compare the striated and non-striated muscles with
reference to structure, location, and method of work.
2. In what respects is the muscular tissue of the heart like the striated,
and in what respects like the non-striated, muscular tissue?
3. If muscles could push as well as pull, would so many be needed in the
body? Why?
4. Locate muscles that work to some extent against elasticity and gravity.
5. Locate five muscles that act as flexors; five that act as extensors;
two that act as adductors; and two as abductors. Locate sphincter and
radiating muscles.
6. By what means does the nervous system control the muscles?
7. Give proofs of the change of potential into kinetic energy during
muscular contraction.
8. Define the essential properties of muscular tissue and state the
purpose served by each.
9. Describe a lever. For what general purpose are levers used in the body?
What other purpose do they serve outside of the body?
10. Why are levers of the second class not adapted to the work of the
body?
11. Name the class of lever used in bending the elbow; in straightening
the elbow; in raising the knee; in elevating the toes; and in biting. Why
is one able to bite harder with the back teeth than with the front ones
when the same muscles are used in both cases?
12. Measure the distance from the middle of the palm of the hand to the
center of the elbow joint. Find the attachment of the tendon of the biceps
muscle to the radius and measure its distance to the center of the elbow
joint. From these distances calculate the force with which the biceps
contracts in order to support a weight of ten pounds on the palm of the
hand.
13. How does exercise benefit the health? How does a short walk "clear the
brain" and enable one to study to better advantage?
14. When exercisers taken for its effects upon the health, what conditions
should be observed?
PRACTICAL WORK
The reddish muscle found in a piece of beef is a good example of striated
muscle. The clear ring surrounding the intestine of a cat (shown by cross
section) and the outer portion of the preparation from the cow's stomach,
sold at the butcher shop under the name of _tripe_, are good examples of
non-striated muscular tissue. The heart of any animal, of course, shows
the heart muscle.
*To show the Structure of Striated Muscle.*--Boil a tough piece of beef, as
a cut from the neck, until the connective tissue has thoroughly softened.
Then with some pointed instrument, separate the main piece into its fiber
bundles and these in turn into their smallest divisions. The smallest
divisions obtainable are the muscle cells or fibers.
*To show Striated Fibers.*--Place a small muscle from the leg of a frog in
a fifty-per-cent solution of alcohol and leave it there for half a day or
longer. Then cover with water on a glass slide, and with a couple of fine
needles tease out the small muscle threads. Protect with a cover glass and
examine with a microscope, first with a low and then with a high power.
The striations, sarcolemma, and sometimes the nuclei and nerve plates, may
be distinguished in such a preparation.
*To show Non-striated Cells.*--Place a clean section of the small intestine
of a cat in a mixture of one part of nitric acid and four parts of water
and leave for four or five hours. Thoroughly wash out the acid with water
and separate the muscular layer from the mucous membrane. Cover a small
portion of the muscle with water on a glass slide and tease out, with
needles, until it is as finely divided as possible. Examine with a
microscope, first with a low and then with a high power. The cells appear
as very fine, spindle-shaped bodies.
*To illustrate Muscular Stimulus and Contraction.*--Separate the muscles at
the back of the thigh of a frog which has just been killed and draw the
large sciatic nerve to the surface. Cut this as high up as possible and,
with a sharp knife and a small pair of scissors, dissect it out to the
knee. Now cut out entirely the large muscle of the calf of the leg (the
gastrocnemius), but leave attached to it the nerve, the lower tendon, and
the bones of the knee. Mount on an upright support, as shown in Fig. 120,
and fasten the tendon to a lever below by a thread or small wire hook:
[Fig. 120]
Fig. 120--*Apparatus* for demonstrating properties of muscles.
1. Lay the nerve over the ends of the wires from a small battery which are
attached to the support at _A_, and arrange a second break in the circuit
at _B_. At this place the battery circuit is made and broken either by a
telegraph key or by simply touching and separating the wires. Note that
the muscle gives a single contraction, or twitch, both when the current is
made and when it is broken.
2. Remove the current and pinch the end of the nerve, noting the result.
With very fine wires, connect the battery directly to the ends of the
muscle. Stimulate by making and breaking the current as before. In this
experiment the muscle cells are stimulated by the direct action of the
current and not by the current acting on the nerve.
3. With the wires attached to either the muscle or the nerve, make and
break the current in rapid succession. This causes the muscle to enter
into a second contraction before it has relaxed from the first, and if the
shocks follow in rapid succession, to continue in the contracted state.
This condition, which represents the method of contraction of the muscles
in the body, is called _tetanus_.
NOTE.--In these experiments a twitching of the muscle is frequently
observed when no stimulus is being applied. This is due to the drying out
of the nerve and is prevented by keeping it wet with a physiological salt
solution. (See footnote, page 38.)
*To show the Action of Levers.*--With a light but stiff wooden bar, a
spring balance, and a wedge-shaped fulcrum, show:
1. The position of the weight, the fulcrum, and the power in the different
classes of levers, and also the weight-arm and the power-arm in each case.
2. The direction moved by the power and the weight respectively in the use
of the different classes of levers.
3. That when the power-arm and weight-arm are equal, the power equals the
weight and moves through the same distance.
4. That when the power-arm is longer than the weight-arm, the weight is
greater, but moves through a shorter distance than the power.
5. That when the weight-arm is longer than the power-arm, the power is
greater and moves through a shorter distance than the weight.
*To show the Loss of Power in the Use of the Body Levers.*--Construct a
frame similar to, but larger than, that shown in Fig. 120, (about 12
inches high), and hang a small spring balance (250 grams capacity) at the
place where the muscle is attached. Fasten the end of a lever to the
upright piece, at a point on a level with the end of the balance hook.
(The nail or screw used for this purpose must pass loosely through the
lever, and serve as a pivot upon which it can turn.) The lever should
consist of a light piece of wood, and should have a length at least three
times as great as the distance from the hook to the turning point. Connect
the balance hook with the lever by a thread or string, and then hang upon
it a small body of known weight. Note the amount of force exerted at the
balance in order to support the weight at different places on the lever.
At what point is the force just equal to the weight? Where is it twice as
great? Where three times? Show that the force required to support the
weight increases proportionally as the weight-arm and as the distance
through which the weight may be moved by the lever. Apply to the action of
the biceps muscle in lifting weights on the forearm.
*A Study of the Action of the Biceps Muscle.*--Place the fingers upon the
tendon of the biceps where it connects with the radius of the forearm.
With the forearm resting upon the table, note that the tendon is somewhat
loose and flaccid, but that with the slightest effort to raise the forearm
it quickly tightens. Now transfer the fingers to the body of the muscle,
and sweep the forearm through two or three complete movements, noting the
changes in the length and thickness of the muscle. Lay the forearm again
on the table, back of hand down, and place a heavy weight (a flatiron or a
hammer) upon the hand. Note the effort required to raise the weight, and
then shift it along the arm. Observe that the nearer it approaches the
elbow the lighter it seems. Account for the difference in the effort
required to raise the weight at different places. Does the effort vary as
the distance from the tendon?
CHAPTER XVI - THE SKIN
Protective coverings are found at all the exposed surfaces of the body.
These vary considerably at different places, each being adapted to the
conditions under which it serves. The most important ones are the _skin_,
which covers the entire external surface of the body; the _mucous
membrane_, which lines all the cavities that communicate by openings with
the external surface; and the _serous membrane_, which, including the
synovial membranes, lines all the closed cavities of the body. In addition
to the protection which it affords, the skin is one of the means by which
the body is brought into proper relations with its surroundings. It is
because of this function that we take up the study of the skin at this
time.
*The Skin* is one of the most complex structures of the body, and serves
several distinct purposes. It is estimated to have an area of from 14 to
16 square feet, and to have a thickness which varies from less than one
eighth to more than one fourth of an inch. It is thickest on the palms of
the hands and the soles of the feet, the places where it is most subject
to wear. It is made up of two distinct layers--an outer layer called the
_epidermis_, or cuticle, and an inner layer called the _dermis_, or cutis
vera (Fig. 121).
*The Dermis.*--This is the thicker and heavier of the two layers, and is
made up chiefly of connective tissue. The network of tough fibers which
this tissue supplies, forms the essential body of the dermis and gives to
it its power of resistance. It is on account of the connective tissue that
the skins of animals can be converted into leather by tanning. A variety
of structures, including blood and lymph vessels, oil and perspiratory
glands, hair follicles, and nerves, are found embedded in the connective
tissue (Fig. 122). These aid in different ways in the work of the skin.
[Fig. 121]
Fig. 121--*Section of skin* magnified, _a, b._ Epidermis, _b._ Pigment
layer. _c._ Papillae, _d._ Dermis. _e._ Fatty tissue. _f, g, h._ Sweat
gland and duct. _i, k._ Hair and follicle. _l._ Oil gland.
On the outer surface of the dermis are numerous elevations, called
_papillae_. These average about one one-hundredth of an inch in height, and
one two hundred and fiftieth of an inch in diameter. They are most
numerous on the palms of the hands, the soles of the feet, and the under
surfaces of the fingers and toes. At these places they are larger than in
other parts of the body, and are closely grouped, forming the parallel
curved ridges which cover the surfaces. Each papilla contains a loop of
capillaries and a small nerve, and many of them are crowned with touch
corpuscles (page 342).
[Fig. 122]
Fig. 122--*Diagram* of section of skin showing its different structures.
*The Epidermis* is much thinner than the dermis. It is made up of several
layers of cells which are flat and scale-like at the surface, but are
rounded in form where the epidermis joins the dermis. The epidermis has
the appearance of being _moulded onto_ the dermis, filling up the
depressions between the papillae and having corresponding irregularities
(Fig. 121). No blood vessels are found in the epidermis, its nourishment
being derived from the lymph which reaches it from the dermis. Only the
part next to the dermis is made up of _living_ cells. These are active,
however, in the formation of new cells, which take the place of those that
are worn off at the surface. Some of the cells belonging to the inner
layer of epidermis contain _pigment granules_, which give the skin its
color (Fig. 121). The epidermis contains no nerves and is therefore
non-sensitive. The hair and the nails are important modifications of the
epidermis.
*A Hair* is a slender cylinder, formed by the union of epidermal cells,
which grows from a kind of pit in the dermis, called the _hair follicle_.
The oval and somewhat enlarged part of the hair within the follicle is
called the _root_, or _bulb_, and the uniform cylinder beyond the follicle
is called the _shaft_. Connected with the sides of the follicles are the
_oil_, or _sebaceous, glands_ (Figs. 121 and 122). These secrete an oily
liquid which keeps the hair and cuticle soft and pliable. Attached to the
inner ends of the follicles are small, involuntary muscles whose
contractions cause the roughened condition of the skin that occurs on
exposure to cold.
*A Nail* is a tough and rather horny plate of epidermal tissue which grows
from a depression in the dermis, called the _matrix_. The back part of the
nail is known as the _root_, the middle convex portion as the _body_, and
the front margin as _the free edge_ (Fig. 123). Material for the growth of
the nail is derived from the matrix, which is lined with active epidermal
cells and is richly supplied with blood vessels. Cells added to the root
cause the nail to grow in length (forward) and cells added to the under
surface cause it to grow in thickness. The cuticle adheres to the nail
around its entire circumference so that the covering over the dermis is
complete.
[Fig. 123]
Fig. 123--*Section of end of finger* showing nail in position.
*Functions of the Skin.*--The chief function of the skin is that of
protection. It is able to protect the body on account of the tough
connective tissue in the dermis, the non-sensitive cells of the epidermis,
and also by the touch corpuscles and their connecting nerve fibers. This
protection is of at least three kinds, as follows:
1. _From mechanical injuries_ such as might result from contact with hard,
rough, or sharp objects. The main quality needed for resisting mechanical
injuries is _toughness_, and this is supplied both by the epidermis and by
the connective tissue of the dermis.
2. _From chemical injuries_ caused by contact with various chemical
agents, as acids, alkalies, and the oxygen of the air. The epidermis,
being of such a nature as to resist to a considerable extent the action of
chemical agents, affords protection from these substances. (89)
3. _From disease germs_ which are everywhere present. The epidermis is the
main protective agent against attacks of germs, but should the epidermis
be broken, they meet with further resistance from the fluids of the dermis
and the white corpuscles of the blood.
4. _From an excessive evaporation of liquid from the surface of the body_.
In the performance of this function, the skin is an important means of
keeping the tissues soft and the blood and lymph from becoming too
concentrated.
*Other Functions of the Skin.*--Through the perspiratory glands the skin is
an _organ of excretion_. While the secretion from a single gland is small,
the waste that leaves the body through all of the perspiratory glands is
considerable (90) (page 206). By means of the nerves terminating in the
touch corpuscles, the skin serves as the _organ of touch_, or feeling
(Chapter XX). To a slight extent also the skin may absorb liquid
substances, these being taken up by the blood and lymph vessels, and
perform a respiratory function, throwing off carbon dioxide. But the most
important function of the skin, in addition to protection, is that of
serving as
*An Organ of Adaptation.*--Forming, as it does, the boundary between the
body and its physical environment, the skin is perhaps the most important
agent through which the body is adapted to its immediate surroundings.
Evidence of this is found in the great variety of influences which are
able to affect the body through their action upon the nerves in the skin,
and in the changes which the epidermis undergoes on exposure. The latter
function is especially marked in the lower animals, the coverings of
epidermal tissue (hair, scales, feathers, etc.) adapting each species to
the physical conditions under which it lives. In man the most striking
example of adaptation through the skin is seen in the variations in the
quantity of blood circulating through it, corresponding to the changes in
the temperature outside of the body. These variations are of great
importance, having to do with the
*Maintenance of the Normal Temperature.*--It is necessary to the
continuance of life that the temperature of the body be kept at a nearly
uniform degree, called the _normal temperature_, which is about 98.6 deg. F.
The maintenance of the normal temperature depends mainly upon four
conditions: the chemical changes at the cells, the circulation of the
blood, the nervous system, and _the skin_. The chemical changes produce
the heat, the blood in its circulation distributes the heat over the body,
and the nervous system controls the heat-producing and distributing
processes (page 320). The skin is the chief means by which the body gets
rid of an excess of heat and, by so doing, avoids overheating. (91)
*How the Skin cools the Body.*--The skin is a means of ridding the body of
an excess of heat in at least two ways:
1. _By the conduction and radiation of heat from its surface_ as from a
stove. This goes on all the time, but varies with the amount of heat
brought to the surface by the blood.
2. _By the evaporation of the perspiration._ It is a well-established and
easily demonstrated principle that liquids in evaporating use up heat.(See
Practical Work.) It is also a matter of everyday experience that the
perspiration has a cooling effect upon the body and that its flow
increases with the amount of heat to be gotten rid of. The quantity of
perspiration secreted, and of heat disposed of through its evaporation,
also varies with the amount of blood circulating through the skin.
*Temperature Regulation by the Skin.*--Variations in the quantity of blood
circulating through the skin enable this organ to throw off just the right
amount of heat for keeping the body at the normal temperature. If it is
necessary for the body to rid itself of an excess of heat, the quantity of
blood circulating in the skin is increased. This brings the blood near the
surface, where more heat can be radiated and where it may cause an
increase in the perspiration. On the other hand, if the body is in danger
of losing too much heat, the circulation diminishes in the skin and
increases in the internal organs. This stops the rapid loss of heat from
the surface. The skin in this work is of course made to cooperate with
other parts of the body. That it is not the only organ concerned in
regulating the escape of heat is seen in the results that follow
sensations either of chilliness or of heat at the surface.
*Effects of Heat and Cold Sensations.*--Sensations, or feelings, of heat
and cold are made possible through the nerves which connect the brain with
the _temperature corpuscles_, found in the skin (page 343). As the warm
blood recedes from the skin, a sensation of cold is felt, but when the
blood returns, there is again the feeling of warmth. The sensation of cold
prompts one to seek a warmer place, or to put on more clothing; while the
sensation of heat, if it be oppressive, leads to activities of an opposite
kind. Prompted in this way by the sensations from the skin, one
voluntarily supplies the external conditions, such as clothing and heat,
that affect the body temperature.