gases, partly dissolved in it and partly in chemical union with certain
of its constituents. These are oxygen, carbon dioxid, and nitrogen. The
latter need not be taken into account. The oxygen is the nourishing
material which the tissues require to carry on their work. The carbon
dioxid is a waste substance which the tissues produce by their
activity, and which the blood carries away from them.
As before shown, the blood as it flows through the tissues loses most
of its oxygen, and carbon dioxid takes its place. Now if the blood is
to maintain its efficiency in this respect, it must always be receiving
new supplies of oxygen, and also have some mode of throwing off its
excess of carbon dioxid. This, then, is the double function of the
process of respiration. Again, the blood sent out from the left side of
the heart is of a bright scarlet color. After its work is done, and the
blood returns to the right side of the heart, it is of a dark purple
color. This change in color takes place in the capillaries, and is due
to the fact that there the blood gives up most of its oxygen to the
tissues and receives from them a great deal of carbon dioxid.
In brief, while passing through the capillaries of the lungs the blood
has been changed from the venous to the arterial blood. That is to say,
the blood in its progress through the lungs has rid itself of its
excess of carbon dioxid and obtained a fresh supply of oxygen.[36]
214. Effects of Respiration upon the Air in the Lungs. It is well known
that if two different liquids be placed in a vessel in contact with
each other and left undisturbed, they do not remain separate, but
gradually mix, and in time will be perfectly combined. This is called
diffusion of liquids. The same thing occurs with gases, though the
process is not visible. This is known as the diffusion of gases. It is
also true that two liquids will mingle when separated from each other
by a membrane (sec. 129). In a similar manner two gases, especially if
of different densities, may mingle even when separated from each other
by a membrane.
In a general way this explains the respiratory changes that occur in
the blood in the lungs. Blood containing oxygen and carbon dioxid is
flowing in countless tiny streams through the walls of the air cells of
the lungs. The air cells themselves contain a mixture of the same two
gases. A thin, moist membrane, well adapted to allow gaseous diffusion,
separates the blood from the air. This membrane is the delicate wall of
the capillaries and the epithelium of the air cells. By experiment it
has been found that the pressure of oxygen in the blood is less than
that in the air cells, and that the pressure of carbon dioxid gas in
the blood is greater than that in the air cells. As a result, a
diffusion of gases ensues. The blood gains oxygen and loses carbon
dioxid, while the air cells lose oxygen and gain the latter gas.
Illustration: Fig. 92.—Capillary Network of the Air Cells and Origin of
the Pulmonary Veins.
A, small branch of pulmonary artery;
B, twigs of the pulmonary artery anastomosing to form peripheral
network of the primitive air cells;
C, capillary network around the walls of the air sacs;
D, branches of network converging for form the veinlets of the
pulmonary veins.
The blood thus becomes purified and reinvigorated, and at the same time
is changed in color from purple to scarlet, from venous to arterial. It
is now evident that if this interchange is to continue, the air in the
cells must be constantly renewed, its oxygen restored, and its excess
of carbon dioxid removed. Otherwise the process just described would be
reversed, making the blood still more unfit to nourish the tissues, and
more poisonous to them than before.
215. Change in the Air in Breathing. The air which we exhale during
respiration differs in several important particulars from the air we
inhale. Both contain chiefly the three gases, though in different
quantities, as the following table shows.
Oxygen. Nitrogen. Carbon
Dioxid. Inspired air contains 20.81 79.15 .04
Expired air contains 16.03 79.58 4.38
That is, expired air contains about five per cent less oxygen and five
per cent more carbon dioxid than inspired air.
The temperature of expired air is variable, but generally is higher
than that of inspired air, it having been in contact with the warm air
passages. It is also loaded with aqueous vapor, imparted to it like the
heat, not in the depth of the lungs, but in the upper air passages.
Expired air contains, besides carbon dioxid, various impurities, many
of an unknown nature, and all in small amounts. When the expired air is
condensed in a cold receiver, the aqueous product is found to contain
organic matter, which, from the presence of _micro-organisms_,
introduced in the inspired air, is apt to putrefy rapidly. Some of
these organic substances are probably poisonous, either so in
themselves, as produced in some manner in the breathing apparatus, or
poisonous as being the products of decomposition. For it is known that
various animal substances give rise, by decomposition, to distinct
poisonous products known as _ptomaines_. It is possible that some of
the constituents of the expired air are of an allied nature. See under
“Bacteria” (Chapter XIV).
At all events, these substances have an injurious action, for an
atmosphere containing simply one per cent of pure carbon dioxid has
very little hurtful effect on the animal economy, but an atmosphere in
which the carbon dioxid has been raised one per cent by breathing is
highly injurious.
The quantity of oxygen removed from the air by the breathing of an
adult person at rest amounts daily to about 18 cubic feet. About the
same amount of carbon dioxid is expelled, and this could be represented
by a piece of pure charcoal weighing 9 ounces. The quantity of carbon
dioxid, however, varies with the age, and is increased also by external
cold and by exercise, and is affected by the kind of food. The amount
of water, exhaled as vapor, varies from 6 to 20 ounces daily. The
average daily quantity is about one-half a pint.
216. Modified Respiratory Movements. The respiratory column of air is
often used in a mechanical way to expel bodies from the upper air
passages. There are also, in order to secure special ends, a number of
modified movements not distinctly respiratory. The following peculiar
respiratory acts call for a few words of explanation.
A sigh is a rapid and generally audible expiration, due to the elastic
recoil of the lungs and chest walls. It is often caused by depressing
emotions. Yawning is a deep inspiration with a stretching of the
muscles of the face and mouth, and is usually excited by fatigue or
drowsiness, but often occurs from a sort of contagion.
Hiccough is a sudden jerking inspiration due to the spasmodic
contraction of the diaphragm and of the glottis, causing the air to
rush suddenly through the larynx, and produce this peculiar sound.
Snoring is caused by vibration of the soft palate during sleep, and is
habitual with some, although it occurs with many when the system is
unusually exhausted and relaxed.
Laughing consists of a series of short, rapid, spasmodic expirations
which cause the peculiar sounds, with characteristic movements of the
facial muscles. Crying, caused by emotional states, consists of sudden
jerky expirations with long inspirations, with facial movements
indicative of distress. In sobbing, which often follows long-continued
crying, there is a rapid series of convulsive inspirations, with sudden
involuntary contractions of the diaphragm. Laughter, and sometimes
sobbing, like yawning, may be the result of involuntary imitation.
Experiment 112. _Simple Apparatus to Illustrate the Movements of the
Lungs in the Chest_.—T is a bottle from which the bottom has been
removed; D, a flexible and elastic membrane tied on the bottle, and
capable of being pulled out by the string S, so as to increase the
capacity of the bottle. L is a thin elastic bag representing the lungs.
It communicates with the external air by a glass tube fitted air-tight
through a cork in the neck of the bottle. When D is drawn down, the
pressure of the external air causes L to expand. When the string is let
go, L contracts again, by virtue of its elasticity.
Illustration: Fig. 93.
Coughing is produced by irritation in the upper part of the windpipe
and larynx. A deep breath is drawn, the opening of the windpipe is
closed, and immediately is burst open with a violent effort which sends
a blast of air through the upper air passages. The object is to
dislodge and expel any mucus or foreign matter that is irritating the
air passages.
Sneezing is like coughing; the tongue is raised against the soft
palate, so the air is forced through the nasal passages. It is caused
by an irritation of the nostrils or eyes. In the beginning of a cold in
the head, for instance, the cold air irritates the inflamed mucous
membrane of the nose, and causes repeated attacks of sneezing.
217. How the Atmosphere is Made Impure. The air around us is constantly
being made impure in a great variety of ways. The combustion of fuel,
the respiration of men and animals, the exhalations from their bodies,
the noxious gases and effluvia of the various industries, together with
the changes of fermentation and decomposition to which all organized
matter is liable,—all tend to pollute the atmosphere.
The necessity of external ventilation has been foreseen for us. The
forces of nature,—the winds, sunlight, rain, and growing
vegetation,—all of great power and universal distribution and
application, restore the balance, and purify the air. As to the
principal gases, the air of the city does not differ materially from
that of rural sections. There is, however, a vastly greater quantity of
dust and smoke in the air of towns. The breathing of this dust, to a
greater or less extent laden with bacteria, fungi, and the germs of
disease, is an ever-present and most potent menace to public and
personal health. It is one of the main causes of the excess of
mortality in towns and cities over that of country districts.
This is best shown in the overcrowded streets and houses of great
cities, which are deprived of the purifying influence of sun and air.
The fatal effect of living in vitiated air is especially marked in the
mortality among infants and children living in the squalid and
overcrowded sections of our great cities. The salutary effect of
sunshine is shown by the fact that mortality is usually greater on the
shady side of the street.
218. How the Air is Made Impure by Breathing. It is not the carbon
dioxid alone that causes injurious results to health, it is more
especially the organic matter thrown off in the expired air. The carbon
dioxid which accompanies the organic matter is only the index. In
testing the purity of air it is not difficult to ascertain the amount
of carbon dioxid present, but it is no easy problem to measure the
amount of organic matter. Hence it is the former that is looked for in
factories, churches, schoolrooms, and when it is found to exceed .07
per cent it is known that there is a hurtful amount of organic matter
present.
The air as expelled from the lungs contains, not only a certain amount
of organic matter in the form of vapor, but minute solid particles of
_débris_ and bacterial micro-organisms (Chap. XIV). The air thus
already vitiated, after it leaves the mouth, putrefies very rapidly. It
is at once absorbed by clothing, curtains, carpets, porous walls, and
by many other objects. It is difficult to dislodge these enemies of
health even by free ventilation. The close and disagreeable odor of a
filthy or overcrowded room is due to these organic exhalations from the
lungs, the skin, and the unclean clothing of the occupants.
The necessity of having a proper supply of fresh air in enclosed
places, and the need of removal of impure air are thus evident. If a
man were shut up in a tightly sealed room containing 425 cubic feet of
air, he would be found dead or nearly so at the end of twenty-four
hours. Long before this time he would have suffered from nausea,
headache, dizziness, and other proofs of blood-poisoning. These
symptoms are often felt by those who are confined for an hour or more
in a room where the atmosphere has been polluted by a crowd of people.
The unpleasant effects rapidly disappear on breathing fresh air.
219. The Effect on the Health of Breathing Foul Air. People are often
compelled to remain indoors for many hours, day after day, in shops,
factories, or offices, breathing air perhaps only slightly vitiated,
but still recognized as “stuffy.” Such persons often suffer from ill
health. The exact form of the disturbance of health depends much upon
the hereditary proclivity and physical make-up of the individual. Loss
of appetite, dull headache, fretfulness, persistent weariness,
despondency, followed by a general weakness and an impoverished state
of blood, often result.
Persons in this lowered state of health are much more prone to surfer
from colds, catarrhs, bronchitis, and pneumonia than if they were
living in the open air, or breathing only pure air. Thus, in the
Crimean War, the soldiers who lived in tents in the coldest weather
were far more free from colds and lung troubles than those who lived in
tight and ill-ventilated huts. In the early fall when typhoid fever is
prevalent, the grounds of large hospitals are dotted with canvas tents,
in which patients suffering from this fever do much better than in the
wards.
This tendency to inflammatory diseases of the air passages is
aggravated by the overheated and overdried condition of the air in the
room occupied. This may result from burning gas, from overheated
furnaces and stoves, hot-water pipes, and other causes. Serious lung
diseases, such as consumption, are more common among those who live in
damp, overcrowded, or poorly ventilated homes.
220. The Danger from Pulmonary Infection. The germ of pulmonary
consumption, known as the bacillus tuberculosis, is contained in the
breath and the sputa from the lungs of its victims. It is not difficult
to understand how these bacilli may be conveyed through the air from
the lungs of the sick to those of apparently healthy people. Such
persons may, however, be predisposed, either constitutionally or by
defective hygienic surroundings, to fall victims to this dreaded
disease. Overcrowding, poor ventilation, and dampness all tend to
increase the risk of pulmonary infection.
It must not be supposed that the tubercle bacillus is necessarily
transmitted directly through the air from the lungs of the sick to be
implanted in the lungs of the healthy. The germs may remain for a time
in the dust turn and _débris_ of damp, filthy, and overcrowded houses.
In this congenial soil they retain their vitality for a long time, and
possibly may take on more virulent infective properties than they
possessed when expelled from the diseased lungs.[37]
Illustration: Fig. 94. Example of a Micro-Organism—Bacillus
Tuberculosis in Sputum. (Magnified about 500 diameters.)
221. Ventilation. The question of a practicable and economical system
of ventilation for our homes, schoolrooms, workshops, and public places
presents many difficult and perplexing problems. It is perhaps due to
the complex nature of the subject, that ventilation, as an ordinary
condition of daily health, has been so much neglected. The matter is
practically ignored in building ordinary houses. The continuous renewal
of air receives little if any consideration, compared with the
provision made to furnish our homes with heat, light, and water. When
the windows are closed we usually depend for ventilation upon mere
chance,—on the chimney, the fireplace, and the crevices of doors and
windows. The proper ventilation of a house and its surroundings should
form as prominent a consideration in the plans of builders and
architects as do the grading of the land, the size of the rooms, and
the cost of heating.
The object of ventilation is twofold: First, to provide for the removal
of the impure air; second, for a supply of pure air. This must include
a plan to provide fresh air in such a manner that there shall be no
draughts or exposure of the occupants of the rooms to undue
temperature. Hence, what at first might seem an easy thing to do, is,
in fact, one of the most difficult of sanitary problems.
222. Conditions of Efficient Ventilation. To secure proper ventilation
certain conditions must be observed. The pure air introduced should not
be far below the temperature of the room, or if so, the entering
current should be introduced towards the ceiling, that it may mix with
the warm air.
Draughts must be avoided. If the circuit from entrance to exit is
short, draughts are likely to be produced, and impure air has less
chance of mixing by diffusion with the pure air. The current of air
introduced should be constant, otherwise the balance may occasionally
be in favor of vitiated air. If a mode of ventilation prove successful,
it should not be interfered with by other means of entrance. Thus, an
open door may prevent the incoming air from passing through its proper
channels. It is desirable that the inlet be so arranged that it can be
diminished in size or closed altogether. For instance, when the outer
air is very cold, or the wind blows directly into the inlet, the amount
of cold air entering it may lower the temperature of the room to an
undesirable degree.
In brief, it is necessary to have a thorough mixing of pure and impure
air, so that the combination at different parts of the room may be
fairly uniform. To secure these results, the inlets and outlets should
be arranged upon principles of ventilation generally accepted by
authorities on public health. It seems hardly necessary to say that due
attention must be paid to the source from which the introduced air is
drawn. If it be taken from foul cellars, or from dirty streets, it may
be as impure as that which it is designed to replace.
Animal Heat.
223. Animal or Vital Heat. If a thermometer, made for the purpose, be
placed for five minutes in the armpit, or under the tongue, it will
indicate a temperature of about 98½° F., whether the surrounding
atmosphere be warm or cold. This is the natural heat of a healthy
person, and in health it rarely varies more than a degree or two. But
as the body is constantly losing heat by radiation and conduction, it
is evident that if the standard temperature be maintained, a certain
amount of heat must be generated within the body to make up for the
loss externally. The heat thus produced is known as animal or vital
heat.
This generation of heat is common to all living organisms. When the
mass of the body is large, its heat is readily perceptible to the touch
and by its effect upon the thermometer. In mammals and birds the
heat-production is more active than in fishes and reptiles, and their
temperatures differ in degree even in different species of the same
class, according to the special organization of the animal and the
general activity of its functions. The temperature of the frog may be
85° F. in June and 41° F. in January. The structure of its tissues is
unaltered and their vitality unimpaired by such violent fluctuations.
But in man it is necessary not only for health, but even for life, that
the temperature should vary only within narrow limits around the mean
of 98½° F.
We are ignorant of the precise significance of this constancy of
temperature in warm-blooded animals, which is as important and peculiar
as their average height, Man, undoubtedly, must possess a superior
delicacy of organization, hardly revealed by structure, which makes it
necessary that he should be shielded from the shocks and jars of
varying temperature, that less highly endowed organisms endure with
impunity.
224. Sources of Bodily Heat. The heat of the body is generated by the
chemical changes, generally spoken of as those of oxidation, which are
constantly going on in the tissues. Indeed, whenever protoplasmic
materials are being oxidized (the process referred to in sec. 15 as
katabolism) heat is being set free. These chemical changes are of
various kinds, but the great source of heat is the katabolic process,
known as oxidation.
The vital part of the tissues, built up from the complex classes of
food, is oxidized by means of the oxygen carried by the arterial blood,
and broken down into simpler bodies which at last result in urea,
carbon dioxid, and water. Wherever there is life, this process of
oxidation is going on, but more energetically in some tissues and
organs than in others. In other words, the minutest tissue in the body
is a source of heat in proportion to the activity of its chemical
changes. The more active the changes, the greater is the heat produced,
and the greater the amount of urea, carbon dioxid, and water
eliminated. The waste caused by this oxidation must be made good by a
due supply of food to be built up into protoplasmic material. For the
production of heat, therefore, food is necessary. But the oxidation
process is not as simple and direct as the statement of it might seem
to indicate. Though complicated in its various stages, the ultimate
result is as simple as in ordinary combustion outside of the body, and
the products are the same.
The continual chemical changes, then, chiefly by oxidation of
combustible materials in the tissues, produce an amount of heat which
is efficient to maintain the temperature of the living body at about
98½° F. This process of oxidation provides not only for the heat of the
body, but also for the energy required to carry on the muscular work of
the animal organism.
225. Regulation of the Bodily Temperature. While bodily heat is being
continually produced, it is also as continually being lost by the
lungs, by the skin, and to some extent, by certain excretions. The
blood, in its swiftly flowing current, carries warmth from the tissues
where heat is being rapidly generated, to the tissues or organs in
which it is being lost by radiation, conduction, or evaporation. Were
there no arrangement by which heat could be distributed and regulated,
the temperature of the body would be very unequal in different parts,
and would vary at different times.
The normal temperature is maintained with slight variations throughout
life. Indeed a change of more than a degree above or below the average,
indicates some failure in the organism, or some unusual influence. It
is evident, then, that the mechanisms which regulate the temperature of
the body must be exceedingly sensitive.
The two chief means of regulating the temperature of the body are the
lungs and the skin. As a means of lowering the temperature, the lungs
and air passages are very inferior to the skin; although, by giving
heat to the air we breathe, they stand next to the skin in importance.
As a regulating power they are altogether subordinate to the skin.
Experiment 113. _To show the natural temperature of the body_. Borrow a
physician’s clinical thermometer, and take your own temperature, and
that of several friends, by placing the instrument under the tongue,
closing the mouth, and holding it there for five minutes. It should be
thoroughly cleansed after each use.
226. The Skin as a Heat-regulator. The great regulator of the bodily
temperature is, undoubtedly, the skin, which performs this function by
means of a self-regulating apparatus with a more or less double action.
First, the skin regulates the loss of heat by means of the vaso-motor
mechanism. The more blood passes through the skin, the greater will be
the loss of heat by conduction, radiation, and evaporation. Hence, any
action of the vaso-motor mechanism which causes dilatation of the
cutaneous capillaries, leads to a larger flow of blood through the
skin, and will tend to cool the body. On the other hand, when by the
same mechanism the cutaneous vessels are constricted, there will be a
smaller flow of blood through the skin, which will serve to check the
loss of heat from the body (secs. 195 and 270).
Again, the special nerves of perspiration act directly as regulators of
temperature. They increase the loss of heat when they promote the
secretion of the skin, and diminish the loss when they cease to promote
it.
The practical working of this heat-regulating mechanism is well shown
by exercise. The bodily temperature rarely rises so much as a degree
during vigorous exercise. The respiration is increased, the cutaneous
capillaries become dilated from the quickened circulation, and a larger
amount of blood is circulating through the skin. Besides this, the skin
perspires freely. A large amount of heat is thus lost to the body,
sufficient to offset the addition caused by the muscular contractions.
It is owing to the wonderful elasticity of the sweat-secreting
mechanism, and to the increase in respiratory activity, and the
consequent increase in the amount of watery vapor given off by the
lungs, that men are able to endure for days an atmosphere warmer than
the blood, and even for a short time at a temperature above that of
boiling water. The temperature of a Turkish bath may be as high as 150°
to 175° F. But an atmospheric temperature may be considerably below
this, and yet if long continued becomes dangerous to life. In August,
1896, for instance, hundreds of persons died in this country, within a
few days, from the effects of the excessive heat.
A much higher temperature may be borne in dry air than in humid air, or
that which is saturated with watery vapor. Thus, a shade temperature of
100° F. in the dry air of a high plain may be quite tolerable, while a
temperature of 80° F. in the moisture-laden atmosphere of less elevated
regions, is oppressive. The reason is that in dry air the sweat
evaporates freely, and cools the skin. In saturated air at the bodily
temperature there is little loss of heat by perspiration, or by
evaporation from the bodily surface.
This topic is again discussed in the description of the skin as a
regulator of the bodily temperature (sec. 241).
227. Voluntary Means of Regulating the Temperature. The voluntary
factor, as a means of regulating the heat loss in man, is one of great
importance. Clothing retards the loss of heat by keeping in contact
with it a layer of still air, which is an exceedingly bad conductor.
When a man feels too warm and throws off his coat, he removes one of
the badly conducting layers of air, and increases the heat loss by
radiation and conduction. The vapor next the skin is thus allowed a
freer access to the surface, and the loss of heat by evaporation of the
sweat becomes greater. This voluntary factor by which the equilibrium
is maintained must be regarded as of great importance. This power also
exists in the lower animals, but to a much smaller extent. Thus a dog,
on a hot day, runs out his tongue and stretches his limbs so as to
increase the surface from which heat is radiated and conducted.
The production, like the loss, of heat is to a certain extent under the
control of the will. Work increases the production of heat, and rest,
especially sleep, lessens it. Thus the inhabitants of very hot
countries seek relief during the hottest part of the day by a siesta.
The quantity and quality of food also influence the production of heat.
A larger quantity of food is taken in winter than in summer. Among the
inhabitants of the northern and Arctic regions, the daily consumption
of food is far greater than in temperate and tropical climates.
228. Effect of Alcohol upon the Lungs. It is a well recognized fact
that alcohol when taken into the stomach is carried from that organ to
the liver, where, by the baneful directness of its presence, it
produces a speedy and often disastrous effect. But the trail of its
malign power does not disappear there. From the liver it passes to the
right side of the heart, and thence to the lungs, where its influence
is still for harm.
In the lungs, alcohol tends to check and diminish the breathing
capacity of these organs. This effect follows from the partial
paralyzing influence of the stupefying agent upon the sympathetic
nervous system, diminishing its sensibility to the impulse of healthful
respiration. This diminished capacity for respiration is clearly shown
by the use of the _spirometer_, a simple instrument which accurately
records the cubic measure of the lungs, and proves beyond denial the
decrease of the lung space.
“Most familiar and most dangerous is the drinking man’s inability to
resist lung diseases.”—Dr. Adoph Frick, the eminent German physiologist
of Zurich.
“Alcohol, instead of preventing consumption, as was once believed,
reduces the vitality so much as to render the system unusually
susceptible to that fatal disease.”—R. S. Tracy, M.D., Sanitary
Inspector of the N. Y. City Health Dept.
“In thirty cases in which alcoholic phthisis was present a dense,
fibroid, pigmented change was almost invariably present in some
portion of the lung far more frequently than in other cases of
phthisis.”—_Annual of Medical Sciences_.
“There is no form of consumption so fatal as that from alcohol.
Medicines affect the disease but little, the most judicious diet
fails, and change of air accomplishes but slight real good.... In
plain terms, there is no remedy whatever for alcoholic phthisis. It
may be delayed in its course, but it is never stopped; and not
infrequently, instead of being delayed, it runs on to a fatal
termination more rapidly than is common in any other type of the
disorder.”—Dr. B. W. Richardson in _Diseases of Modern Life_
229. Other Results of Intoxicants upon the Lungs. But a more potent
injury to the lungs comes from another cause. The lungs are the arena
where is carried on the ceaseless interchange of elements that is
necessary to the processes of life. Here the dark venous blood, loaded
with effete material, lays down its carbon burden and, with the
brightening company of oxygen, begins again its circuit. But the enemy
intrudes, and the use of alcohol tends to prevent this benign
interchange.
The continued congestion of the lung tissue results in its becoming
thickened and hardened, thus obstructing the absorption of oxygen, and
the escape of carbon dioxid. Besides this, alcohol destroys the
integrity of the red globules, causing them to shrink and harden, and
impairing their power to receive oxygen. Thus the blood that leaves the
lungs conveys an excess of the poisonous carbon dioxid, and a
deficiency of the needful oxygen. This is plainly shown in the purplish
countenance of the inebriate, crowded with enlarged veins. This
discoloration of the face is in a measure reproduced upon the congested
mucous membrane of the lungs. It is also proved beyond question by the
decreased amount of carbon dioxid thrown off in the expired breath of
any person who has used alcoholics.
The enfeebled respiration explains (though it is only one of the
reasons) why inebriates cannot endure vigorous and prolonged exertion
as can a healthy person. The hurried circulation produced by
intoxicants involves in turn quickened respiration, which means more
rapid exhaustion of the life forces. The use of intoxicants involves a
repeated dilatation of the capillaries, which steadily diminishes their
defensive power, rendering the person more liable to yield to the
invasion of pulmonary diseases.[38]
230. Effect of Alcoholics upon Disease. A theory has prevailed, to a
limited extent, that the use of intoxicants may act as a preventive of
consumption. The records of medical science fail to show any proof
whatever to support this impression. No error could be more serious or
more misleading, for the truth is in precisely the opposite direction.
Instead of preventing, alcohol tends to develop consumption. Many
physicians of large experience record the existence of a distinctly
recognized alcoholic consumption, attacking those constitutions broken
down by dissipation. This form of consumption is steadily progressive,
and always fatal.
The constitutional debility produced by the habit of using alcoholic
beverages tends to render one a prompt victim to the more severe
diseases, as pneumonia, and especially epidemical diseases, which sweep
away vast numbers of victims every year.
231. Effect of Tobacco upon the Respiratory Passages. The effects of
tobacco upon the throat and lungs are frequently very marked and
persistent. The hot smoke must very naturally be an irritant, as the
mouth and nostrils were not made as a chimney for heated and narcotic
vapors. The smoke is an irritant, both by its temperature and from its
destructive ingredients, the carbon soot and the ammonia which it
conveys. It irritates and dries the mucous membrane of the mouth and
throat, producing an unnatural thirst which becomes an enticement to
the use of intoxicating liquors. The inflammation of the mouth and
throat is apt to extend up the Eustachian tube, thus impairing the
sense of hearing.
But even these are not all the bad effects of tobacco. The inhalation
of the poisonous smoke produces unhealthful effects upon the delicate
mucous membrane of the bronchial tubes and of the lungs. Upon the
former the effect is to produce an irritating cough, with short breath
and chronic bronchial catarrh. The pulmonary membrane is congested,
taking cold becomes easy, and recovery from it tedious. Frequently the
respiration is seriously disturbed, thus the blood is imperfectly
aërated, and so in turn the nutrition of the entire system is impaired.
The cigarette is the defiling medium through which these direful
results frequently invade the system, and the easily moulded condition
of youth yields readily to the destructive snare.
“The first effect of a cigar upon any one demonstrates that tobacco can
poison by its smoke and through the lungs.”—London _Lancet_.
“The action of the heart and lungs is impaired by the influence of
the narcotic on the nervous system, but a morbid state of the
larynx, trachea, and lungs results from the direct action of the
smoke.”—Dr. Laycock, Professor of Medicine in the University of
Edinburgh.
Additional Experiments.
Experiment 114. _To illustrate the arrangement of the lungs and the two
pleuræ._ Place a large sponge which will represent the lungs in a thin
paper bag which just fits it; this will represent the pulmonary layer
of the pleura. Place the sponge and paper bag inside a second paper
bag, which will represent the parietal layer of the pleura. Join the
mouths of the two bags. The two surfaces of the bags which are now in
contact will represent the two moistened surfaces of the pleuræ, which
rub together in breathing.
Experiment 115. _To show how the lungs may be filled with air._ Take
one of the lungs saved from Experiment 110. Tie a glass tube six inches
long into the larynx. Attach a piece of rubber to one end of the glass
tube. Now inflate the lung several times, and let it collapse. When
distended, examine every part of it.
Experiment 116. _To take your own bodily temperature or that of a
friend._ If you cannot obtain the use of a physician’s clinical
thermometer, unfasten one of the little thermometers found on so many
calendars and advertising sheets. Hold it for five minutes under the
tongue with the lips closed. Read it while in position or the instant
it is removed. The natural temperature of the mouth is about 98½° F.
Experiment 117. _To show the vocal cords._ Get a pig’s windpipe in
perfect order, from the butcher, to show the vocal cords. Once secured,
it can be kept for an indefinite time in glycerine and water or dilute
alcohol.
Experiment 118. _To show that the air we expire is warm._ Breathe on a
thermometer for a few minutes. The mercury will rise rapidly.
Experiment 119. _To show that expired air is moist_. Breathe on a
mirror, or a knife blade, or any polished metallic surface, and note
the deposit of moisture.
Experiment 120. _To show that the expired air contains carbon dioxid_.
Put a glass tube into a bottle of lime water and breathe through the
tube. The A liquid will soon become cloudy, because the carbon dioxid
of the expired air throws down the lime held in solution.
Experiment 121. “A substitute for a clinical thermometer may be readily
contrived by taking an ordinary house thermometer from its tin case,
and cutting off the lower part of the scale so that the bulb may
project freely. With this instrument the pupils may take their own and
each other’s temperatures, and it will be found that whatever the
season of the year or the temperature of the room, the thermometer in
the mouth will record about 99° F. Care must, of course, be taken to
keep the thermometer in the mouth till it ceases to rise, and to read
while it is still in position.”—Professor H. P. Bowditch.
Experiment 122. _To illustrate the manner in which the movements of
inspiration cause the air to enter the lungs._ Fit up an apparatus, as
represented in Fig. 95, in which a stout glass tube is provided with a
sound cork, B, and also an air-tight piston, D, resembling that of an
ordinary syringe. A short tube, A, passing through the cork, has a
small India-rubber bag, C, tied to it. Fit the cork in the tube while
the piston is near the top. Now, by lowering the piston we increase the
capacity of the cavity containing the bag. The pressure outside the bag
is thus lowered, and air rushes into it through the tube, A, till a
balance is restored. The bag is thus stretched. As soon as we let go
the piston, the elasticity of the bag, being free to act, Movements of
drives out the air just taken in, and the piston returns to its former
place.
Illustration: Fig. 95. Apparatus for Illustrating the Movements of
Respiration.
It will be noticed that in this experiment the elastic bag and its tube
represent the lungs and trachea; and the glass vessel enclosing it, the
thorax.
For additional experiments on the mechanics of respiration, see Chapter
XV.
Chapter IX.
The Skin and the Kidneys.
232. The Elimination of Waste Products. We have traced the food from
the alimentary canal into the blood. We have learned that various food
materials, prepared by the digestive processes, are taken up by the
branches of the portal vein, or by the lymphatics, and carried into the
blood current. The nutritive material thus absorbed is conveyed by the
blood plasma and the lymph to the various tissues to provide them with
nourishment.
We have learned also that oxygen, taken up in the air cells of the
lungs, is being continually carried to the tissues, and that the blood
is purified by being deprived in the lungs of its excess of carbon
dioxid. From this tissue activity, which is mainly oxidation, are
formed certain waste products which, as we have seen, are absorbed by
the capillaries and lymphatics and carried into the venous circulation.
In their passage through the blood and tissues, the albumens, sugars,
starches, and fats are converted into carbon dioxid, water, and urea,
or some closely allied body. Certain articles of food also contain
small amounts of sulphur and phosphorus, which undergo oxidation into
sulphates and phosphates. We speak, then, of carbon dioxid, salts, and
water as waste products of the animal economy. These leave the body by
one of the three main channels,—the lungs, the skin, or the kidneys.
The elimination of these products is brought about by a special
apparatus called organs of excretion. The worn-out substances
themselves are called excretions, as opposed to secretions, which are
elaborated for use in the body. (See note, p. 121.) As already shown,
the lungs are the main channels for the elimination of carbon dioxid,
and of a portion of water as vapor. By the skin the body gets rid of a
small portion of salts, a little carbon dioxid, and a large amount of
water in the form of perspiration. From the kidneys are eliminated
nearly all the urea and allied bodies, the main portion of the salts,
and a large amount of water. In fact, practically all the nitrogenous
waste leaves the body by the kidneys.
Illustration: Fig. 96.—Diagrammatic Scheme to illustrate in a very
General Way Absorption and Excretion.
A, represents the alimentary canal;
L, the pulmonary surface;
K, the surface of the renal epithelium;
S, the skin;
o, oxygen;
h, hydrogen;
n, nitrogen.
233. The Skin. The skin is an important and unique organ of the body.
It is a blood-purifying organ as truly as are the lungs and the
kidneys, while it also performs other and complex duties. It is not
merely a protective covering for the surface of the body. This is
indeed the most apparent, but in some respectes, the lest important, of
its functions. This protective duty is necessary and efficient, as is
proved by the familiar experience of the pain when a portion of the
outer skin has been removed.
The skin, being richly supplied with nerves, is an important organ of
sensibility and touch. In some parts it is closely attached to the
structures beneath, while in others it is less firmly adherent and
rests upon a variable amount of fatty tissue. It thus assists in
relieving the abrupt projections and depressions of the general
surface, and in giving roundness and symmetry to the entire body. The
thickness of the skin varies in different parts of the body. Where
exposed to pressure and friction, as on the soles of the feet and in
the palms of the hands, it is much thickened.
The true skin is 1/12 to ⅛ of an inch in thickness, but in certain
parts, as in the lips and ear passages, it is often not more than 1/100
of an inch thick. At the orifices of the body, as at the mouth, ears,
and nose, the skin gradually passes into mucous membrane, the structure
of the two being practically identical. As the skin is an outside
covering, so is the mucous membrane a more delicate inside lining for
all cavities into which the apertures open, as the alimentary canal and
the lungs.
Illustration: Fig. 97.—A Layer of the Cuticle from the Palm of the
Hand. (Detached by maceration.)
The skin ranks as an important organ of excretion, its product being
sweat, excreted by the sweat glands. The amount of this excretion
evaporated from the general surface is very considerable, and is
modified as becomes necessary from the varied conditions of the
temperature. The skin also plays an important part in regulating the
bodily temperature(sec. 241).
234. The Cutis Vera, or True Skin. The skin is remarkably complex in
its structure, and is divided into two distinct layers, which may be
readily separated: the deeper layer,—the true skin, dermis, or corium;
and the superficial layer, or outer skin,—the epidermis, cuticle, or
scarf skin.
The true skin consists of elastic and white fibrous tissue, the bundles
of which interlace in every direction. Throughout this feltwork
structure which gradually passes into areolar tissue are numerous
muscular fibers, as about the hair-follicles and the oil glands. When
these tiny muscles contract from cold or by mental emotion, the
follicles project upon the surface, producing what is called “goose
flesh.”
The true skin is richly supplied with blood-vessels and nerves, as when
cut it bleeds freely, and is very sensitive. The surface of the true
skin is thrown into a series of minute elevations called the papillæ,
upon which the outer skin is moulded. These abound in blood-vessels,
lymphatics, and peculiar nerve-endings, which will be described in
connection with the organ of touch (sec. 314). The papillæ are large
and numerous in sensitive places, as the palms of the hands, the soles
of the feet, and the fingers. They are arranged in parallel curved
lines, and form the elevated ridges seen on the surface of the outer
skin (Fig. 103).
235. The Epidermis, or Cuticle. Above the true skin is the epidermis.
It is semi-transparent, and under the microscope resembles the scales
of a fish. It is this layer that is raised by a blister.
As the epidermis has neither blood-vessels, nerves, nor lymphatics, it
may be cut without bleeding or pain. Its outer surface is marked with
shallow grooves which correspond to the deep furrows between the
papillæ of the true skin. The inner surface is applied directly to the
papillary layer of the true skin, and follows closely its inequalities.
The outer skin is made up of several layers of cells, which next to the
true skin are soft and active, but gradually become harder towards the
surface, where they are flattened and scale-like. The upper scales are
continually being rubbed off, and are replaced by deeper cells from
beneath. There are new cells continually being produced in the deeper
layer, which push upward the cells already existing, then gradually
become dry, and are cast off as fine, white dust. Rubbing with a coarse
towel after a hot bath removes countless numbers of these dead cells of
the outer skin. During and after an attack of scarlet fever the patient
“peels,” that is, sheds an unusual amount of the seal; cells of the
cuticle.
The deeper and more active layer of the epidermis, the _mucosum_, is
made up of cells some of which contain minute granules of pigment, or
coloring matter, that give color to the skin. The differences in the
tint, as brunette, fair, and blond, are due mainly to the amount of
coloring matter in these pigment cells. In the European this amount is
generally small, while in other peoples the color cells may be brown,
yellow, or even black. The pinkish tint of healthy skin, and the
rosy-red after a bath are due, not to the pigment cells, but to the
pressure of capillaries in the true skin, the color of the blood being
seen through the semi-transparent outer skin.
Illustration: Fig. 98.—Surface of the Palm of the Hand, showing the
Openings of the Sweat Glands and the Grooves between the Papillæ of the
Skin. (Magnified 4 diameters.) [In the smaller figure the same
epidermal surface is shown, as seen with the naked eye.]
Experiment 123. Of course the living skin can be examined only in a
general way. Stretch and pull it, and notice that it is elastic. Note
any liver spots, white scars, moles, warts, etc. Examine the outer skin
carefully with a strong magnifying glass. Study the papillæ on the
palms. Scrape off with a sharp knife a few bits of the scarf skin, and
examine them with the microscope.
236. The Hair. Hairs varying in size cover nearly the entire body,
except a few portions, as the upper eyelids, the palms of the hands,
and the soles of the feet.
The length and diameter of the hairs vary in different persons,
especially in the long, soft hairs of the head and beard. The average
number of hairs upon a square inch of the scalp is about 1000, and the
number upon the entire head is estimated as about 120,000.
Healthy hair is quite elastic, and may be stretched from one-fifth to
one-third more than its original length. An ordinary hair from the head
will support a weight of six to seven ounces. The hair may become
strongly electrified by friction, especially when brushed vigorously in
cold, dry weather. Another peculiarity of the hair is that it readily
absorbs moisture.
237. Structure of the Hair. The hair and the nails are structures
connected with the skin, being modified forms of the epidermis. A hair
is formed by a depression, or furrow, the inner walls of which consist
of the infolded outer skin. This depression takes the form of a sac and
is called the hair-follicle, in which the roots of the hair are
embedded. At the bottom of the follicle there is an upward projection
of the true skin, a papilla, which contains blood-vessels and nerves.
It is covered with epidermic cells which multiply rapidly, thus
accounting for the rapid growth of the hair. Around each papilla is a
bulbous expansion, the hair bulb, from which the hair begins to grow.
Illustration: Fig. 99.—Epidermis of the Foot.
It will be noticed that there are only a few orifices of the sweat
glands in this region. (Magnified 8 diameters.)
The cells on the papillæ are the means by which the hairs grow. As
these are pushed upwards by new ones formed beneath, they are
compressed, and the shape of the follicle determines their cylindrical
growth, the shaft of the hair. So closely are these cells welded to
form the cylinder, that even under a microscope the hair presents only
a fibrous appearance, except in the center, where the cells are larger,
forming the medulla, or pith (Fig. 106).
The medulla of the hair contains the pigment granules or coloring
matter, which may be of any shade between a light yellow and an intense
black. It is this that gives the great variety in color. Generally with
old people the pigment is absent, the cells being occupied by air;
hence the hair becomes gray or white. The thin, flat scales on the
surface of the hair overlap like shingles. Connected with the
hair-follicles are small bundles of muscular fibers, which run
obliquely in the skin and which, on shortening, may cause the hairs to
become more upright, and thus are made to “stand on end.” The bristling
back of an angry cat furnishes a familiar illustration of this muscular
action.
Illustration: Fig. 100.—Hair and Hair-Follicle.
A, root of hair;
B, bulb of the hair;
C, internal root sheath;
D, external root sheath;
E, external membrane of follicle;
F, muscular fibers attached to the follicle;
H, compound sebaceous gland with its duct;
K, L, simple sebaceous gland;
M, opening of the hair-follicle.
Opening into each hair-follicle are usually one or more sebaceous, or
oil, glands. These consist of groups of minute pouches lined with cells
producing an oily material which serves to oil the hair and keep the
skin moist and pliant.
238. The Nails. The nails are also formed of epidermis cells which have
undergone compression, much like those forming the shaft of a hair. In
other words, a nail is simply a thick layer of horny scales built from
the outer part of the scarf skin. The nail lies upon very fine and
closely set papillæ, forming its matrix, or bed. It is covered at its
base with a fold of the true skin, called its root, from beneath which
it seems to grow.
The growth of the nail, like that of the hair and the outer skin, is
effected by the production of new cells at the root and under surface.
The growth of each hair is limited; in time it falls out and is
replaced by a new one. But the nail is kept of proper size simply by
the removal of its free edge.
239. The Sweat Glands. Deep in the substance of the true skin, or in
the fatty tissue beneath it, are the sweat glands. Each gland consists
of a single tube with a blind end, coiled in a sort of ball about 1/60
of an inch in diameter. From this coil the tube passes upwards through
the dermis in a wavy course until it reaches the cuticle, which it
penetrates with a number of spiral turns, at last opening on the
surface. The tubes consist of delicate walls of membrane lined with
cells. The coil of the gland is enveloped by minute blood-vessels. The
cells of the glands are separated from the blood only by a fine
partition, and draw from it whatever supplies they need for their
special work.
Illustration: Fig. 101.—Concave or Adherent Surface of the Nail.
A, border of the root;
B, whitish portion of semilunar shape (the lunula);
C, body of nail. The continuous line around border represents the
free edge.
Illustration: Fig. 102.—Nail in Position.
A, section of cutaneous fold (B) turned back to show the root of the
nail;
B, cutaneous fold covering the root of the nail;
C, semi lunar whitish portion (lunula);
D, free border.
With few exceptions every portion of the skin is provided with sweat
glands, but they are not equally distributed over the body. They are
fewest in the back and neck, where it is estimated they average 400 to
the square inch. They are thickest in the palms of the hands, where
they amount to nearly 3000 to each square inch. These minute openings
occur in the ridges of the skin, and may be easily seen with a hand
lens. The length of a tube when straightened is about 1/4 of an inch.
The total number in the body is estimated at about 2,500,000, thus
making the entire length of the tubes devoted to the secretion of sweat
about 10 miles.
240. Nature and Properties of Sweat. The sweat is a turbid, saltish
fluid with a feeble but characteristic odor due to certain volatile
fatty acids. Urea is always present in small quantities, and its
proportion may be largely increased when there is deficiency of
elimination by the kidneys. Thus it is often observed that the sweat is
more abundant when the kidneys are inactive, and the reverse is true.
This explains the increased excretion of the kidneys in cold weather.
Of the inorganic constituents of sweat, common salt is the largest and
most important. Some carbon dioxid passes out through the skin, but not
more than 1/50 as much as escapes by the lungs.
The sweat ordinarily passes off as vapor. If there is no obvious
perspiration we must not infer that the skin is inactive, since sweat
is continually passing from the surface, though often it may not be
apparent. On an average from 1½ to 4 pounds of sweat are eliminated
daily from the skin in the form of vapor. This is double the amount
excreted by the lungs, and averages about 1/67 of the weight of the
body.
The visible sweat, or sensible perspiration, becomes abundant during
active exercise, after copious drinking of cold water, on taking
certain drugs, and when the body is exposed to excessive warmth.
Forming more rapidly than it evaporates it collects in drops on the
surface. The disagreeable sensations produced by humid weather result
from the fact that the atmosphere is so loaded with vapor that the
moisture of the skin is slowly removed by evaporation.
Experiment 124. Study the openings of the sweat glands with the aid of
a strong magnifying glass. They are conveniently examined on the palms.
A man’s weight may be considerably reduced within a short time by loss
through the perspiration alone. This may explain to some extent the
weakening effect of profuse perspiration, as from night sweats of
consumption, convalescence from typhoid fever, or the artificial
sweating from taking certain drugs.
241. The Skin as a Regulator of the Temperature of the Body. We thus
learn that the skin covers and protects the more delicate structures
beneath it; and that it also serves as an important organ of excretion.
By means of the sweat the skin performs a third and a most important
function, _viz_., that of regulating the temperature of the body.
The blood-vessels of the skin, like those of other parts of the body,
are under the control of the nervous system, which regulates their
diameter. If the nervous control be relaxed, the blood-vessels dilate,
more blood flows through them, and more material is brought to the
glands of the skin to be acted upon. External warmth relaxes the skin
and its blood-vessels. There results an increased flow of blood to the
skin, with increased perspiration. External cold, on the other hand,
contracts the skin and its blood-vessels, producing a diminished supply
of blood and a diminished amount of sweat.
Now, it is a law of physics that the change from liquid to vapor
involves a loss of heat. A few drops of ether or of any volatile liquid
placed on the skin, produce a marked sense of coldness, because the
heat necessary to change the liquid into vapor has been drawn rapidly
from the skin. This principle holds good for every particle of sweat
that reaches the mouth of a sweat gland. As the sweat evaporates, it
absorbs a certain amount of heat, and cools the body to that extent.
242. How the Action of the Skin may be Modified. After profuse sweating
we feel chilly from the evaporation of a large amount of moisture,
which rapidly cools the surface. When the weather is very warm the
evaporation tends to prevent the bodily temperature from rising. On the
other hand, if the weather be cold, much less sweat is produced, the
loss of heat from the body is greatly lessened, and its temperature
prevented from falling. Thus it is plain why medicine is given and
other efforts are made to sweat the fever patient. The increased
activity of the skin helps to reduce the bodily heat.
The sweat glands are under the control of certain nerve fibers
originating in the spinal cord, and are not necessarily excited to
action by an increased flow of blood through the skin. In other words,
the sweat glands may be stimulated to increased action both by an
increased flow of blood, and also by reflex action upon the
vaso-dilator nerves of the parts. These two agencies, while working in
harmony through the vaso-dilators, produce phenomena which are
essentially independent of each other. Thus a strong emotion, like
fear, may cause a profuse sweat to break out, with cold, pallid skin.
During a fever the skin may be hot, and its vessels full of blood, and
yet there may be no perspiration.
Illustration: Fig. 103.—Papillæ of the Skin of the Palm of the Hand.
In each papilla are seen vascular loops (dark lines) running up from
the vascular network below, the tactile corpuscles with their nerve
branches (white lines) which supply the papillæ.
The skin may have important uses with which we are not yet acquainted.
Death ensues when the heat of the body has been reduced to about 70°
F., and suppression of the action of the skin always produces a
lowering of the temperature. Warm-blooded animals usually die when more
than half of the general surface has been varnished. Superficial burns
which involve a large part of the surface of the body, generally have a
fatal result due to shock.
If the skin be covered with some air-tight substance like a coating of
varnish, its functions are completely arrested. The bodily heat falls
very rapidly. Symptoms of blood-poisoning arise, and death soon ensues.
The reason is not clearly known, unless it be from the sudden retention
of poisonous exhalations.
243. The Skin and the Kidneys. There is a close relationship between
the skin and the kidneys, as both excrete organic and saline matter. In
hot weather, or in conditions producing great activity of the skin, the
amount of water excreted by the kidneys is diminished. This is shown in
the case of firemen, stokers, bakers, and others who are exposed to
great heat, and drink heavily and sweat profusely, but do not have a
relative increase in the functions of the kidneys. In cool weather,
when the skin is less active, a large amount of water is excreted by
the kidneys, as is shown by the experience of those who drive a long
distance in severe weather, or who have caught a sudden cold.
Illustration: Fig. 104.—Magnified View of a Sweat Gland with its Duct.
The convoluted gland is seen surrounded with big fat-cells, and may be
traced through the dermis to its outlet in the horny layers of the
epidermis.
244. Absorbent Powers of the Skin. The skin serves to some extent as an
organ for absorption. It is capable of absorbing certain substances to
which it is freely exposed. Ointments rubbed in, are absorbed by the
lymphatics in those parts where the skin is thin, as in the bend of the
elbow or knee, and in the armpits. Physicians use medicated ointments
in this way, when they wish to secure prompt and efficient results.
Feeble infants often grow more vigorous by having their skin rubbed
vigorously daily with olive oil.
A slight amount of water is absorbed in bathing. Sailors deprived of
fresh water have been able to allay partially their intense thirst by
soaking their clothing in salt water. The extent to which absorption
occurs through the healthy skin is, however, quite limited. If the
outer skin be removed from parts of the body, the exposed surface
absorbs rapidly. Various substances may thus be absorbed, and rapidly
passed into the blood. When the physician wishes remedies to act
through the skin, he sometimes raises a small blister, and dusts over
the surface some drug, a fine powder, like morphine.
The part played by the skin as an organ of touch will be considered in
sections 314 and 315.
Experiment 125. _To illustrate the sense of temperature_. Ask the
person to close his eyes. Use two test tubes, one filled with cold and
the other with hot water, or two spoons, one hot and one cold. Apply
each to different parts of the surface, and ask the person whether the
touching body is hot or cold. Test roughly the sensibility of different
parts of the body with cold and warm metallic-pointed rods.
Experiment 126. Touch fur, wood, and metal. The metal feels coldest,
although all the objects are at the same temperature. Why?
Experiment 127. Plunge the hand into water at about 97°F. One
experiences a feeling of heat. Then plunge it into water at about
86°F.; at first it feels cold, because heat is abstracted from the
hand. Plunge the other hand direct into water at 86°F. without
previously placing it in water at 97°F.,—it will feel pleasantly warm.
Experiment 128. _To illustrate warm and cold spots_. With a blunt
metallic point, touch different parts of the skin. Certain points
excite the sensation of warmth, others of cold, although the
temperatures of the skin and of the instrument remain constant.
245. Necessity for Personal Cleanliness. It is evident that the skin,
with its myriads of blood-vessels, nerves, and sweat and oil glands, is
an exceedingly complicated and important structure. The surface is
continually casting off perspiration, oily material, and dead scales.
By friction and regular bathing we get rid of these waste materials. If
this be not thoroughly done, the oily secretion holds the particles of
waste substances to the surface of the body, while dust and dirt
collect, and form a layer upon the skin. When we remember that this
dirt consists of a great variety of dust particles, poisonous matters,
and sometimes germs of disease, we may well be impressed with the
necessity of personal cleanliness.
This layer of foreign matter on the skin is in several ways injurious
to health. It clogs the pores and retards perspiration, thus checking
the proper action of the skin as one of the chief means of getting rid
of the waste matters of the body. Hence additional work is thrown upon
other organs, chiefly the lungs and the kidneys, which already have
enough to do. This extra work they can do for only a short time. Sooner
or later they become disordered, and illness follows. Moreover, as this
unwholesome layer is a fertile soil in which bacteria may develop, many
skin diseases may result from this neglect. It is also highly probable
that germs of disease thus adherent to the skin may then be absorbed
into the system. Parasitic skin diseases are thus greatly favored by
the presence of an unclean skin. It is also a fact that uncleanly
people are more liable to take cold than those who bathe often.
The importance of cleanliness would thus seem too apparent to need
special mention, were it not that the habit is so much neglected. The
old and excellent definition that dirt is suitable matter, but in the
wrong place, suggests that the place should be changed. This can be
done only by regular habits of personal cleanliness, not only of the
skin, the hair, the teeth, the nails, and the clothing, but also by the
rigid observance of a proper system in daily living.
246. Baths and Bathing. In bathing we have two distinct objects in
view,—to keep the skin clean and to impart vigor. These are closely
related, for to remove from the body worn-out material, which tends to
injure it, is a direct means of giving vigor to all the tissues. Thus a
cold bath acts upon the nervous system, and calls out, in response to
the temporary abstraction of heat, a freer play of the general vital
powers. Bathing is so useful, both locally and constitutionally, that
it should be practiced to such an extent as experience proves to be
beneficial. For the general surface, the use of hot water once a week
fulfills the demands of cleanliness, unless in special occupations.
Whether we should bathe in hot or cold water depends upon
circumstances. Most persons, especially the young and vigorous, soon
become accustomed to cool, and even cold water baths, at all seasons of
the year.
The hot bath should be taken at night before going to bed, as in the
morning there is usually more risk of taking cold. The body is readily
chilled, if exposed to cold when the blood-vessels of the skin have
been relaxed by heat. Hot baths, besides their use for the purposes of
cleanliness, have a sedative influence upon the nervous system, tending
to allay restlessness and weariness. They are excellent after severe
physical or mental work, and give a feeling of restful comfort like
that of sleep.
Illustration: Fig. 105.—Epithelial Cells from the Sweat Glands. The
cells are very distinct, with nuclei enclosing pigmentary granulations
(Magnified 350 times)
Cold baths are less cleansing than hot, but serve as an excellent tonic
and stimulant to the bodily functions. The best and most convenient
time for a cold bath is in the morning, immediately after rising. To
the healthy and vigorous, it is, if taken at this time, with proper
precautions, a most agreeable and healthful luxury. The sensation of
chilliness first felt is caused by the contraction of the skin and its
blood-vessels, so that the blood is forced back, as it were, into the
deeper parts of the body. This stimulates the nervous system, the
breathing becomes quicker and deeper, the heart beats more vigorously,
and, as a consequence, the warm blood is sent back to the skin with
increased force. This is known as the stage of reaction, which is best
increased by friction with a rough towel. This should produce the
pleasant feeling of a warm glow all over the body.
A cold bath which is not followed by reaction is likely to do more harm
than good. The lack of this reaction may be due to the water being too
cold, the bath too prolonged, or to the bather being in a low condition
of health. In brief, the ruddy glow which follows a cold bath is the
main secret of its favorable influence.
The temperature of the water should be adapted to the age and strength
of the bather. The young and robust can safely endure cold baths, that
would be of no benefit but indeed an injury to those of greater age or
of less vigorous conditions of health. After taking a bath the skin
should be rapidly and vigorously rubbed dry with a rough towel, and the
clothing at once put on.
247. Rules and Precautions in Bathing. Bathing in cold water should not
be indulged in after severe exercise or great fatigue, whether we are
heated or not. Serious results have ensued from cold baths when the
body is in a state of exhaustion or of profuse perspiration. A daily
cold bath when the body is comfortably warm, is a safe tonic for almost
all persons during the summer months, and tends especially to restore
the appetite. Cold baths, taken regularly, render persons who are
susceptible to colds much less liable to them, and less likely to be
disturbed by sudden changes of temperature. Persons suffering from
heart disease or from chronic disease of an important organ should not
indulge in frequent cold bathing except by medical advice. Owing to the
relaxing nature of hot baths, persons with weak hearts or suffering
from debility may faint while taking them.
Outdoor bathing should not be taken for at least an hour after a full
meal, and except for the robust it is not prudent to bathe with the
stomach empty, especially before breakfast. It is a wise rule, in
outdoor or sea bathing, to come out of the water as soon as the glow of
reaction is felt. It is often advisable not to apply cold water very
freely to the head. Tepid or even hot water is preferable, especially
by those subject to severe mental strain. But it is often a source of
great relief during mental strain to bathe the face, neck, and chest
freely at bedtime with cold water. It often proves efficient at night
in calming the sleeplessness which results from mental labor.
Hot baths, if taken at bedtime, are often serviceable in preventing a
threatened cold or cutting it short, the patient going immediately to
bed, with extra clothing and hot drinks. The free perspiration induced
helps to break up the cold.
Salt water acts more as a stimulant to the skin than fresh water.
Salt-water bathing is refreshing and invigorating for those who are
healthy, but the bather should come out of the water the moment there
is the slightest feeling of chilliness. The practice of bathing in salt
water more than once a day is unhealthful, and even dangerous. Only the
strongest can sustain so severe a tax on their power of endurance. Sea
bathing is beneficial in many ways for children, as their skin reacts
well after it. In all cases, brisk rubbing with a rough towel should be
had afterwards.
Illustration: Fig. 106.—Magnified Section of the Lower Portion of a
Hair and Hair-Follicle.
A, membrane of the hair-follicle, cells with nuclei and pigmentary
granules;
B, external lining of the root sheath;
C, internal lining of the root sheath;
D, cortical or fibrous portion of the hair shaft;
E, medullary portion (pith) of shaft;
F, hair-bulb, showing its development from cells from A.
The golden rule of all bathing is that it must never be followed by a
chill. If even a chilliness occur after bathing, it must immediately be
broken up by some appropriate methods, as lively exercise, brisk
friction, hot drinks, and the application of heat.
Swimming is a most valuable accomplishment, combining bathing and
exercise. Bathing of the feet should never be neglected. Cleanliness of
the hair is also another matter requiring strict attention, especially
in children.
248. Care of the Hair and Nails. The hair brush should not be too
stiff, as this increases the tendency towards scurfiness of the head.
If, however, the hair is brushed too long or too hard, the scalp is
greatly stimulated, and an increased production of scurf may result. If
the head be washed too often with soap its natural secretion is
checked, and the scalp becomes dry and scaly. The various hair pomades
are as a rule undesirable and unnecessary.
The nails should be kept in proper condition, else they are not only
unsightly, but may serve as carriers of germs of disease. The nails are
often injured by too much interference, and should never be trimmed to
the quick. The upper surfaces should on no account be scraped. The
nail-brush is sufficient to cleanse them without impairing their smooth
and polished surfaces.
Illustration: Fig. 107.—Longitudinal Section of a Finger-Nail.
A, last phalanx of the fingers;
B, true skin on the dorsal surface of the finger;
C, epidermis;
D, true skin;
E, bed of the nail;
F, superficial layer of the nail;
H, true skin of the pulp of the finger.
249. Use of Clothing. The chief use of clothing, from a hygienic point
of view, is to assist in keeping the body at a uniform temperature. It
also serves for protection against injury, and for personal adornment.
The heat of the body, as we have learned, is normally about 98½° F.
This varies but slightly in health. A rise of temperature of more than
one degree is a symptom of disturbance. The normal temperature does not
vary with the season. In summer it is kept down by the perspiration and
its rapid evaporation. In winter it is maintained by more active
oxidation, by extra clothing, and by artificial heat.
The whole matter of clothing is modified to a great extent by climatic
conditions and local environments,—topics which do not come within the
scope of this book.
250. Material Used for Clothing. It is evident that if clothing is to
do double duty in preventing the loss of heat by radiation, and in
protecting us from the hot rays of the sun, some material must be used
that will allow the passage of heat in either direction. The ideal
clothing should be both a bad conductor and a radiator of heat. At the
same time it must not interfere with the free evaporation of the
perspiration, otherwise chills may result from the accumulation of
moisture on the surface of the body.
Wool is a bad conductor, and should be worn next the skin, both in
summer and winter, especially in variable climates. It prevents, better
than any other material, the loss of heat from the body, and allows
free ventilation and evaporation. Its fibers are so lightly woven that
they make innumerable meshes enclosing air, which is one of the best of
non-conductors.
Silk ranks next to wool in warmth and porosity. It is much softer and
less irritating than flannel or merino, and is very useful for summer
wear. The practical objection to its general use is the expense. Fur
ranks with wool as a bad conductor of heat. It does not, however, like
wool, allow of free evaporation. Its use in cold countries is
universal, but in milder climates it is not much worn.
Cotton and linen are good conductors of heat, but are not absorbents of
moisture, and should not be worn next the skin. They are, however, very
durable and easily cleansed. As an intermediate clothing they may be
worn at all seasons, especially over wool or silk. Waterproof clothing
is also useful as a protection, but should not be worn a longer time
than necessary, as it shuts in the perspiration, and causes a sense of
great heat and discomfort.
The color of clothing is of some importance, especially if exposed
directly to the sun’s rays. The best reflectors, such as white and
light gray clothing, absorb comparatively little heat and are the
coolest, while black or dark-colored materials, being poor reflectors
and good absorbents, become very warm.
251. Suggestions for the Use of Clothing. Prudence and good sense
should guide us in the spring, in changing winter flannels or clothing
for fabrics of lighter weight. With the fickle climate in most sections
of this country, there are great risks of severe colds, pneumonia, and
other pulmonary diseases from carelessness or neglect in this matter. A
change from heavy to lighter clothing should be made first in the outer
garments, the underclothing being changed very cautiously.
The two essentials of healthful clothing are cleanliness and dryness.
To wear garments that are daily being soiled by perspiration and other
cutaneous excretions, is a most uncleanly and unhealthful practice.
Clothing, especially woolen underclothing, should be frequently
changed. One of the objections to the use of this clothing is that it
does not show soiling to the same extent as do cotton and linen.
Infectious and contagious diseases may be conveyed by the clothing.
Hence, special care must be taken that all clothing in contact with
sick people is burned or properly disinfected. Children especially are
susceptible to scarlet fever, diphtheria, and measles, and the greatest
care must be exercised to prevent their exposure to infection through
the clothing.
We should never sleep in a damp bed, or between damp sheets. The vital
powers are enfeebled during sleep, and there is always risk of
pneumonia or rheumatism. The practice of sitting with wet feet and damp
clothing is highly injurious to health. The surface of the body thus
chilled may be small, yet there is a grave risk of serious, if not of
fatal, disease. No harm may be done, even with clothing wet with water
or damp with perspiration, so long as exercise is maintained, but the
failure or inability to change into dry garments as soon as the body is
at rest is fraught with danger.
Woolen comforters, scarfs, and fur mufflers, so commonly worn around
the neck, are more likely to produce throat troubles and local chill
than to have any useful effect. Harm ensues from the fact that the
extra covering induces local perspiration, which enfeebles the natural
defensive power of the parts; and when the warmer covering is removed,
the perspiring surface is readily chilled. Those who never bundle their
throats are least liable to suffer from throat ailments.
252. Ill Effects of Wearing Tightly Fitting Clothing. The injury to
health caused by tight lacing, when carried to an extreme, is due to
the compression and displacement of various organs by the pressure
exerted on them. Thus the lungs and the heart may be compressed,
causing short breath on exertion, palpitation of the heart, and other
painful and dangerous symptoms. The stomach, the liver, and other
abdominal organs are often displaced, causing dyspepsia and all its
attendant evils. The improper use of corsets, especially by young
women, is injurious, as they interfere with the proper development of
the chest and abdominal organs. The use of tight elastics below the
knee is often injurious. They obstruct the local venous circulation and
are a fruitful source of cold feet and of enlarged or varicose veins.
Tightly fitting boots and shoes often cause corns, bunions, and
ingrowing nails; on the other hand, if too loosely worn, they cause
corns from friction. Boots too narrow in front crowd the toes together,
make them overlap, and render walking difficult and painful.
High-heeled boots throw the weight of the body forwards, so that the
body rests too much on the toes instead of on the heels, as it should,
thus placing an undue strain upon certain groups of muscles of the leg,
in order to maintain the balance, while other groups are not
sufficiently exercised. Locomotion is never easy and graceful, and a
firm, even tread cannot be expected.
The compression of the scalp by a tight-fitting hat interferes with the
local circulation, and may cause headaches, neuralgia, or baldness, the
nutrition of the hair-follicles being diminished by the impaired
circulation. The compression of the chest and abdomen by a tight belt
and various binders interferes with the action of the diaphragm,—the
most important muscle of respiration.
253. Miscellaneous Hints on the Use of Clothing. Children and old
people are less able to resist the extreme changes of temperature than
are adults of an average age. Special care should be taken to provide
children with woolen underclothing, and to keep them warm and in
well-ventilated rooms. Neither the chest nor limbs of young children
should be unduly exposed, as is often done, to the cold blasts of
winter or the fickle weather of early spring. Very young children
should not be taken out in extremely cold weather, unless quite warmly
clad and able to run about. The absurd notion is often entertained that
children should be hardened by exposure to the cold. Judicious
“hardening” means ample exposure of well-fed and well-clothed children.
Exposure of children not thus cared for is simple cruelty. The many
sicknesses of children, especially diseases of the throat and lungs,
may often be traced directly to gross carelessness, ignorance, or
neglect with reference to undue exposure. The delicate feet of children
should not be injured by wearing ill-fitting or clumsy boots or shoes.
Many deformities of the feet, which cause much vexation and trouble in
after years, are acquired in early life.
No one should sleep in any of the clothes worn during the day, not even
in the same underclothing. All bed clothing should be properly aired,
by free exposure to the light and air every morning. Never wear wet or
damp clothing one moment longer than necessary. After it is removed rub
the body thoroughly, put on at once dry, warm clothing, and then
exercise vigorously for a few minutes, until a genial glow is felt.
Neglect of these precautions often results in rheumatism, neuralgia,
and diseases of the chest, especially among delicate people and young
women.
Pupils should not be allowed to sit in the schoolroom with any outer
garments on. A person who has become heated in a warm room should not
expose himself to cold without extra clothing. We must not be in a
hurry to put on heavy clothes for winter, but having once worn them,
they must not be left off until milder weather renders the change safe.
The cheaper articles of clothing are often dyed with lead or arsenic.
Hence such garments, like stockings and colored underclothing, worn
next the skin have been known to produce severe symptoms of poisoning.
As a precaution, all such articles should be carefully washed and
thoroughly rinsed before they are worn.
The Kidneys.
254. The Kidneys. The kidneys are two important organs in the abdomen,
one on each side of the spine. They are of a reddish-brown color, and
are enveloped by a transparent capsule made up of a fold of the
peritoneum. Embedded in fat, the kidneys lie between the upper lumbar
vertebræ, and the crest of the hip bone. The liver is above the right
kidney, and the spleen above the left, while both lie close against the
rear wall of the abdomen, with the intestines in front of them. The
human kidneys, though somewhat larger, are exactly of the same shape,
color, and general appearance as those of the sheep, so commonly seen
in the markets.
The kidneys are about four inches long, two inches across, one inch
thick, and weigh from 41/2 to 51/2 ounces each. The hollow or concave
side of the kidneys is turned inwards, and the deep fissure of this
side, known as the hilus, widens out to form the pelvis. Through the
hilus the renal artery passes into each kidney, and from each hilus
passes outwards the renal vein, a branch of the inferior vena cava.
A tube, called the ureter, passes out from the concave border of each
kidney, turns downwards, and enters the bladder in the basin of the
pelvis. This tube is from 12 to 14 inches long, about as large as a
goose quill, and conveys the secretion of the kidneys to the bladder.
255. Structure of the Kidneys. The pelvis is surrounded by reddish
cones, about twelve in number, projecting into it, called the pyramids
of Malpighi. The apices of these cones, known as the _papillæ_, are
crowded with minute openings, the mouths of the uriniferous tubules,
which form the substance of the kidney. These lie parallel in the
medullary or central structure, but On reaching the cortical or outer
layer, they wind about and interlace, ending, at last, in dilated
closed sacs called Malpighian capsules.
Illustration: Fig. 108.—Vertical Section of the Kidney.
A, pyramids of Malpighi;
B, apices, or papillæ, of the pyramids, surrounded by subdivisions of
the pelvis known as cups or calices;
C, pelvis of the kidney;
D, upper end of ureter.
256. Function of the Kidneys. The Malpighian capsules are really the
beginning of the tubules, for here the work of excretion begins. The
thin wall of the capillaries within each capsule separates the blood
from the cavity of the tubule. The blood-pressure on the delicate
capillary walls causes the exudation of the watery portions of the
blood through the cell walls into the capsule. The epithelial cell
membrane allows the water of the blood with certain salts in solution
to pass, but rejects the albumen. From the capsules, the excretion
passes through the tubules into the pelvis, and on through the ureters
to the bladder. But the delicate epithelial walls of the tubules
through which it passes permit the inflow of urea and other waste
products from the surrounding capillaries. By this twofold process are
separated from the blood the fluid portions of the renal secretion with
soluble salts, and the urea with other waste material.
257. How the Action of the Kidneys may be Modified. The action of the
kidneys is subject to very marked and sudden modifications, especially
those operating through the nervous system. Thus whatever raises the
blood-pressure in the capillaries of the capsules, will increase the
quantity of fluid filtering through them. That is, the watery portion
of the secretion will be increased without necessarily adding to its
solids. So anything which lowers the blood-pressure will diminish the
watery portion of the secretion, that is, the secretion will be scanty,
but concentrated.
The Renal Secretion.—The function of the kidneys is to secrete a fluid
commonly known as the urine. The average quantity passed in 24 hours by
an adult varies from 40 to 60 fluid ounces. Normal urine consists of
about 96 per cent of water and 4 per cent of solids. The latter consist
chiefly of certain nitrogenous substances known as urea and uric acid,
a considerable quantity of mineral salts, and some coloring matter.
Urea, the most important and most abundant constituent of urine,
contains the four elements, but nitrogen forms one-half its weight.
While, therefore, the lungs expel carbon dioxid chiefly, the kidneys
expel nitrogen. Both of these substances express the result of
oxidations going on in the body. The urea and uric acids represent the
final result of the breaking down in the body of nitrogenous
substances, of which albumen is the type.
Unusual constituents of the urine are _albumen, sugar_, and _bile_.
When albumen is present in urine, it often indicates some disease of
the kidneys, to which the term _albuminuria_ or Bright’s Disease is
applied. The presence of grape sugar or glucose indicates the disease
known as diabetes. Bile is another unusual constituent of the urine,
appearing in _jaundice_.
The bladder is situated in the pelvic cavity or in the lowest part of
the abdomen. When full, the bladder is pear-shaped; when empty, it is
collapsed and lies low in the pelvis. The functions of the bladder are
to collect and retain the urine, which has reached it drop by drop from
the kidneys through the ureters, until a certain quantity accumulates,
and then to expel it from the body.
Illustration: Fig. 109.—Vertical Section of the Back. (Showing kidneys
_in situ_ and the relative position of adjacent organs and vessels.)
[Posterior view.]
A, 12th dorsal vertebra;
B, diaphragm;
C, receptaculum chyli;
D, small intestines
In the kidneys, as elsewhere, the vaso-motor nerves are distributed to
the walls of the blood-vessels, and modify the quantity and the
pressure of blood in these organs. Thus, some strong emotion, like fear
or undue anxiety, increases the blood-pressure, drives more blood to
the kidneys, and causes a larger flow of watery secretion. When the
atmosphere is hot, there is a relaxation of the vessels of the skin,
with a more than ordinary flow of blood, which is thus withdrawn from
the deeper organs. The blood-pressure in the kidneys is not only
diminished, but the total quantity passing through them in a given time
is much lessened. As a result, the secretion of the kidneys is scanty,
but it contains an unusual percentage of solids.
When the atmosphere is cold, the reverse is true. The cutaneous vessels
contract, the blood is driven to the deeper organs with increased
pressure, and there is a less amount of sweat, but an increased renal
secretion, containing a smaller proportion of solids. Certain drugs
have the power of increasing or diminishing the renal secretion. As the
waste matters eliminated by the kidneys are being constantly produced
in the tissues, the action of the renal organs is continuous, in marked
contrast with the intermittent flow of most of the secretions proper,
as distinguished from the excretions.
258. Effects of Alcoholic Drinks upon the Kidneys. The kidneys differ
from some of the other organs in this: those can rest a while without
any harm to themselves, or to the body. We can keep the eyes closed for
a few days, if necessary, without injury, and in fact often with
benefit; or, we can abstain from food for some days, if need be, and
let the stomach rest. But the kidneys cannot, with safety, cease their
work. Their duty in ridding the blood of waste products, and of any
foreign or poisonous material introduced, must be done not only
faithfully, but continually, or the whole body at once suffers from the
evil effects of the retained waste matters.
This vital fact is the key to the injurious results developed in the
kidneys by the use of alcoholic drinks. These two organs have large
blood-vessels conveying full amounts of blood to and from their
structures, and they feel very quickly the presence of alcohol.
Alcoholic liquors excite and irritate the delicate renal membranes, and
speedily disturb and eventually destroy their capacity to excrete the
proper materials from the blood.
The continued congestion of the minute structure of the kidney cuts off
the needed nutrition of the organ, and forms the primary step in the
series of disasters. Sometimes from this continued irritation, with the
resulting inflammation, and sometimes from change of structure of the
kidney by fatty degeneration, comes the failure to perform its proper
function. Then, with this two-edged sword of disaster, the urea, which
becomes a poisonous element, and should be removed, is retained in the
system, while the albumen, which is essential to healthy blood, is
filtered away through the diseased kidney.
259. Alcoholic Liquors as a Cause of Bright’s Disease. The unfortunate
presence of albumen in the urine is often a symptom of that insidious
and fatal malady known as _albuminuria_ or Bright’s disease, often
accompanied with dropsy and convulsions. One of the most constant
causes of this disease is the use of intoxicants. It is not at all
necessary to this fatal result that a person be a heavy drinker.
Steady, moderate drinking will often accomplish the work. Kidney
diseases produced by alcoholic drinks, are less responsive to medical
treatment and more fatal than those arising from any other known
cause.[39]
Experiment 129. Obtain a sheep’s kidney in good order. Observe that its
shape is something like that of a bean, and note that the concave part
(hilus), when in its normal position, is turned towards the backbone.
Notice that all the vessels leave and enter the kidney at the hilus.
Observe a small thick-walled vessel with open mouth from which may be
pressed a few drops of blood. This is the renal artery. Pass a bristle
down it. With the forceps, or even with a penknife, lift from the
kidney the fine membrane enclosing it. This is the kidney capsule.
Divide the kidney in halves by a section from its outer to near its
inner border. Do not cut directly through the hilus. Note on the cut
surfaces, on the outer side, the darker cortical portion, and on the
inner side, the smooth, pale, medullary portion. Note also the pyramids
of Malpighi.
Chapter X.
The Nervous System.
260. General View of the Nervous System. Thus far we have learned
something of the various organs and the manner in which they do their
work. Regarding our bodily structure as a kind of living machine, we
have studied its various parts, and found that each is designed to
perform some special work essential to the well-being of the whole. As
yet we have learned of no means by which these organs are enabled to
adjust their activities to the needs of other tissues and other organs.
We are now prepared to study a higher, a more wonderful and complex
agency,—the nervous system, the master tissue, which controls,
regulates, and directs every other tissue of the human body.
The nervous system, in its properties and mode of action, is distinct
from all the other systems and organs, and it shares with no other
organ or tissue the power to do its special work. It is the medium
through which all impressions are received. It connects all the parts
of the body into an organism in which each acts in harmony with every
other part for the good of the whole. It animates and governs all
movements, voluntary or involuntary,—secretion, excretion, nutrition;
in fact all the processes of organic life are subject to its regulating
power. The different organs of the body are united by a common sympathy
which regulates their action: this harmonious result is secured by
means of the nervous system.
This system, in certain of its parts, receives impressions, and
generates a force peculiar to itself. We shall learn that there can be
no physical communication between or coördination of the various parts
of organs, or harmonious acts for a desire result, without the nerves.
General impressions, as in ordinary sensation, or special impressions,
as in sight, smell, taste, or hearing,—every instinct, every act of the
will, and every thought are possible only through the action of the
nerve centers.
261. Nerve Cells. However complicated the structure of nerve tissue in
man seems to be, it is found to consist of only two different elements,
nerve cells and nerve fibers. These are associated and combined in many
ways. They are arranged in distinct masses called nerve centers, or in
the form of cords known as nerves. The former are made up of nerve
fibers; the latter of both cells and fibers.
Illustration: Fig. 110.—Nerve Cells from the Spinal Cord.
Nerve cells, which may be regarded as the central organs of the nerve
fibers, consist of masses of cell protoplasm, with a large _nucleus_
and _nucleolus_. They bear a general resemblance to other cells, but
vary much in size and shape. Nerve cells grow, become active, and die,
as do other cells. A number of processes branch off from them, some
cells giving one or two, others many. The various kinds of nerve cells
differ much in the shape and number of processes. One of the processes
is a strand which becomes continuous with the axis cylinder of the
nerve fibers; that is, the axis cylinders of all nerve fibers are
joined in one place or another with at least one cell.
Each part of this system has its own characteristic cell. Thus we have
in the spinal cord the large, irregular cells with many processes, and
in the brain proper the three-sided cells with a process jutting out
from each corner. So characteristic are these forms of cells, that any
particular part of nerve structure may be identified by the kind of
cells seen under the microscope. Nerve cells and nerve fibers are often
arranged in groups, the various cells of the groups communicating with
one another. This clustered arrangement is called a nerve center.
262. Nerve Fibers. The nerve fibers, the essential elements of the
nerves, somewhat resemble tubes filled with a clear, jelly-like
substance. They consist of a rod, or central core, continuous
throughout the whole length of the nerve, called the axis cylinder.
This core is surrounded by the white substance of Schwann, or medullary
sheath, which gives the nerve its characteristic ivory-white
appearance. The whole is enclosed in a thin, delicate sheath, known as
neurilemma.
Illustration: Fig. 111.—Nerve Cells from the Gray Matter of the Brain.
The axis cylinder generally passes without any break from the nerve
centers to the end of the fibers.[40] The outer sheath (neurilemma) is
also continuous throughout the length of the fibers. The medullary
sheath, on the other hand, is broken at intervals of about 1/25 of an
inch, and at the same intervals nuclei are found along the fiber,
around each of which is a minute protoplasmic mass. Between each pair
of nuclei the sheath is interrupted. This point is known as the _node
of Ranvier_.
Some nerve fibers have no inner sheath (medullary), the outer alone
protecting the axis cylinder. These are known as the non-medullary
fibers. They are gray, while the ordinary medullary fibers are white in
appearance. The white nerve fibers form the white part of the brain and
of the spinal cord, and the greater part of the cerebro-spinal nerves.
The gray fibers occur chiefly in branches from the sympathetic ganglia,
though found to some extent in the nerves of the cerebro-spinal system.
In a general way, the nerve fibers resemble an electric cable wire with
its central rod of copper, and its outer non-conducting layer of silk
or gutta percha. Like the copper rod, the axis cylinder along which the
nerve impulse travels is the essential part of a nerve fiber. In a cut
nerve this cylinder projects like the wick of a candle. It is really
the continuation of a process of a nerve cell. Thus the nerve cells and
nerve fibers are related, in that the process of one is the axis
cylinder and essential part of the other.
The separate microscopic threads or fibers, bound together in cords of
variable size, form the nerves. Each strand or cord is surrounded and
protected by its own sheath of connective tissue, made up of nerves.
According to its size a nerve may have one or many of these strands.
The whole nerve, not unlike a minute tendon in appearance, is covered
by a dense sheath of fibrous tissue, in which the blood-vessels and
lymphatics are distributed to the nerve fibers.
Illustration: Fig. 112.—Medullated Nerve Fibers.
A, a medullated nerve fiber, showing the subdivision of the medullary
sheath into cylindrical sections imbricated with their ends, a nerve
corpuscle with an oval nucleus is seen between the neurilemma and the
medullary sheath;
B, a medullated nerve fiber at a node or constriction of Ranvier, the
axis cylinder passes uninterruptedly from one segment into the other,
but the medullary sheath is interrupted.
263. The Functions of the Nerve Cells and Nerve Fibers. The nerve cells
are a highly active mass of living material. They find their
nourishment in the blood, which is supplied to them in abundance. The
blood not only serves as nourishment, but also supplies new material,
as it were, for the cells to work over for their own force or energy.
Thus we may think of the nerve cells as a sort of a miniature
manufactory, deriving their material from the blood, and developing
from it nervous energy.
The nerve fibers, on the other hand, are conductors of nervous energy.
They furnish a pathway along which the nerve energy generated by the
cells may travel. Made up as they are of living nerve substance, the
fibers can also generate energy, yet it is their special function to
conduct influences to and from the cells.
Illustration: Fig. 113.—Non-Medullated Fibers.
Two nerve fibers, showing the nodes or constrictions of Ranvier and the
axis cylinder. The medullary sheath has been dissolved away. The deeply
stained oblong nuclei indicate the nerve corpuscles within the
neurilemma.
264. The Nervous System Compared to a Telegraphic System. In men and
other highly organized animals, nerves are found in nearly every tissue
and organ of the body. They penetrate the most minute muscular fibers;
they are closely connected with the cells of the glands, and are found
in the coats of even the smallest blood-vessels. They are among the
chief factors of the structure of the sense organs, and ramify through
the skin. Thus the nervous system is the system of organs through the
functions of which we are brought into relation with the world around
us. When we hear, our ears are bringing us into relation with the outer
world. So sight opens up to us another gateway of knowledge.
It will help us the better to understand the complicated functions of
the nervous system, if we compare it to a telegraph line. The brain is
the main office, and the multitudes of nerve fibers branching off to
all parts of the body are the wires. By means of these, nerve messages
are constantly being sent to the brain to inform it of what is going on
in various parts of the body, and asking what is to be done in each
case. The brain, on receiving the intelligence, at once sends back the
required instructions. Countless messages are sent to and fro with
unerring accuracy and marvelous rapidity.
Thus, when we accidentally pick up something hot, it is instantly
dropped. A nerve impulse passes from the nerves of touch in the fingers
to the brain, which at once hurries off its order along another set of
nerves for the hand to drop the burning object. These examples, so
common in daily life, may be multiplied to any extent. Almost every
voluntary act we perform is executed under the direction of the nervous
system, although the time occupied is so small that it is beyond our
power to estimate it. The very frequency with which the nerves act
tends to make us forget their beneficent work.
265. Divisions of the Nervous System. This system in man consists of
two great divisions. The first is the great nerve center of the body,
the cerebro-spinal system, which rules the organs of animal life. This
includes the brain, the spinal cord, and the cerebro-spinal nerves.
Nerves are given off from the brain and the cord, and form the mediums
of communication between the external parts of the body, the muscles or
the sense organs, and the brain.
The second part is the sympathetic system, which regulates the organic
life. This consists of numerous small nerve centers arranged in oval
masses varying greatly in size, called ganglia or knots. These are
either scattered irregularly through the body, or arranged in a double
chain of knots lying on the front of the spine, within the chest and
abdomen. From this chain large numbers of nerves are given off, which
end chiefly in the organs of digestion, circulation, and respiration.
The sympathetic system serves to bring all portions of the animal
economy into direct sympathy with one another.
266. The Brain as a Whole. The brain is the seat of the intellect, the
will, the affections, the emotions, the memory, and sensation. It has
also many other and complex functions. In it are established many
reflex, automatic, and coordinating centers, which are as independent
of consciousness as are those of the spinal cord.
The brain is the largest and most complex mass of nerve tissue in the
body, made up of an enormous collection of gray cells and nerve fibers.
This organ consists of a vast number of distinct ganglia, or separate
masses of nerve matter, each capable of performing separate functions,
but united through the cerebral action into a harmonious whole.
Illustration: Fig. 114.—The Upper Surface of the Cerebrum. (Showing its
division into two hemispheres, and also the convolutions)
The average weight of the adult human brain is about 50 ounces for men
and 45 ounces for women. Other things being equal, the size and weight
of the brain bear a general relation to the mental power of the
individual. As a rule, a large, healthy brain stands for a vigorous and
superior intellect. The brains of many eminent men have been found to
be 8 to 12 ounces above the average weight, but there are notable
exceptions. The brains of idiots are small; indeed, any weight under a
certain size, about 30 ounces, seems to be invariably associated with
an imbecile mind.
The human brain is absolutely heavier than that of any other animal,
except the whale and elephant. Comparing the size of these animals with
that of man, it is instructive to notice how much larger in proportion
to the body is man’s brain. The average proportion of the weight of the
brain to the weight of the body is greater in man than in most animals,
being about 1 to 36. In some small birds, in the smaller monkeys, and
in some rodents, the proportional weight of the brain to that of the
body is even greater than in man.
267. The Cerebrum. The three principal masses which make up the brain
when viewed as a whole are:
The cerebrum, or brain proper.
The cerebellum, or lesser brain.
The medulla oblongata.
The cerebrum comprises nearly seven-eighths of the entire mass, and
fills the upper part of the skull. It consists of two halves, the right
and left cerebral hemispheres. These are almost separated from each
other by a deep median fissure. The hemispheres are united at the
bottom of the fissure by a mass of white fibers passing from side to
side. Each of these hemispheres is subdivided into three lobes, so that
the entire cerebrum is made up of six distinct lobes.
The cerebrum has a peculiar convoluted appearance, its deep folds being
separated by fissures, some of them nearly an inch in depth.
It is composed of both white and gray matter. The former comprises the
greater part of the mass, while the latter is spread over the surface
in a layer of about ⅛ of an inch thick. The gray matter is the portion
having the highest functions, and its apparent quantity is largely
increased by being formed in convolutions.
The convolutions of the cerebrum are without doubt associated with all
those higher actions which distinguish man’s life; but all the
convolutions are not of equal importance. Thus it is probable that only
the frontal part of the brain is the intellectual region, while certain
convolutions are devoted to the service of the senses.
The cerebrum is the chief seat of the sensations, the intellect, the
will, and the emotions. A study of cerebral injuries and diseases, and
experiments upon the lower animals, prove that the hemispheres, and
more especially the gray matter, are connected with mental states. The
convolutions in the human brain are more prominent than in that of the
higher animals, most nearly allied to man, although some species of
animals, not especially intelligent, have marked cerebral convolutions.
The higher races of men have more marked convolutions than those less
civilized.
A view of the under surface of the brain, which rests on the floor of
the skull, shows the origin of important nerves, called the cranial
nerves, the cerebellum, the structure connecting the optic nerves
(optic commissure), the bridge of nervous matter (pons Varolii)
connecting the two hemispheres of the cerebellum, and lastly numerous
and well-marked convolutions.
268. The Cerebellum. The cerebellum, or lesser brain, lies in the back
of the cranium, and is covered over in man by the posterior lobe of the
cerebrum. It is, at it were, astride of the back of the cerebro-spinal
axis, and consists of two hemispheres joined by a central mass. On its
under surface is a depression which receives the medulla oblongata. The
cerebellum is separated from the cerebrum by a horizontal partition of
membrane, a portion of the dura mater. In some animals, as in the cat,
this partition is partly bone.
The cerebellum is connected with other parts of the nervous system by
strands of white matter on each side, radiating from the center and
divided into numerous branches. Around these branches the gray matter
is arranged in a beautiful manner, suggesting the leaves of a tree:
hence its name, arbor vitæ, or the tree of life.
The functions of the cerebellum are not certainly known. It appears to
influence the muscles of the body so as to regulate their movements;
that is, it serves to bring the various muscular movements into
harmonious action. The mechanism by which it does this has not yet been
clearly explained. In an animal from which the cerebellum has been
removed, the functions of life do not appear to be destroyed, but all
power of either walking or flying straight is lost.
Illustration: Fig. 115.—A Vertical Section of the Brain.
A, frontal lobe of the cerebrum;
B, parietal lobe;
C, parieto occipital lobe with fissure between this lobe and
D, the occipital lobe;
E, cerebellum;
F, arbor vitæ;
H, pons Varolu;
K, medulla oblongata;
L, portion of lobe on the opposite side of brain.
The white curved band above H represents the corpus callosum.
Disease or injury of the cerebellum usually produces blindness,
giddiness, a tendency to move backwards, a staggering, irregular gait,
and a feeling of insecurity in maintaining various positions. There is
no loss of consciousness, or other disturbance of the mental functions.
269. The Membranes of the Brain. The brain and spinal cord are
protected by three important membranes, known as the meninges,—the dura
mater, the arachnoid, and the pia mater.
The outer membrane, the dura mater, is much thicker and stronger than
the others, and is composed of white fibrous and elastic connective
tissue. It closely lines the inner surface of the skull, and forms a
protective covering for the brain. Folds of it pass between the several
divisions of the brain and serve to protect them.
The arachnoid is a thin membrane which lies beneath the dura mater. It
secretes a serous fluid which keeps the inner surfaces moist.
The pia mater is a very delicate, vascular membrane which covers the
convolutions, dips into all the fissures, and even penetrates into the
interior of the brain. It is crowded with blood-vessels, which divide
and subdivide very minutely before they penetrate the brain. The
membranes of the brain are sometimes the seat of inflammation, a
serious and painful disease, commonly known as brain fever.
270. The Medulla Oblongata. This is the thick upper part of the spinal
cord, lying within the cavity of the skull. It is immediately under the
cerebellum, and forms the connecting link between the brain and the
spinal cord. It is about an inch and a quarter long, and from one-half
to three-fourths of an inch wide at its upper part. The medulla
oblongata consists, like the spinal cord, of columns of white fibers
and masses of gray matter, but differently arranged. The gray matter is
broken up into masses which serve as centers of origin for various
nerves. The functions of the medulla oblongata are closely connected
with the vital processes. It is a great nerve tract for transmitting
sensory and motor impressions, and also the seat of a number of centers
for reflex actions of the highest importance to life. Through the
posterior part of the medulla the sensory impressions pass, that is,
impressions from below upwards to the brain resulting in sensation or
feeling. In the anterior part of the medulla, pass the nerves for motor
transmission, that is, nerve influences from above downwards that shall
result in muscular contractions in some part of the body.
The medulla is also the seat of a number of reflex centers connected
with the influence of the nervous system on the blood-vessels, the
movements of the heart, of respiration, and of swallowing, and on the
secretion of saliva. This spot has been called the “vital knot.” In the
medulla also are centers for coughing, vomiting, swallowing, and the
dilatation of the pupil of the eye. It is also in part the deep origin
of many of the important cranial nerves.
Illustration: Fig. 116.—Illustrating the General Arrangement of the
Nervous System. (Posterior view.)
271. The Cranial Nerves. The cranial or cerebral nerves consist of
twelve pairs of nerves which pass from the brain through different
openings in the base of the skull, and are distributed over the head
and face, also to some parts of the trunk and certain internal organs.
These nerves proceed in pairs from the corresponding parts of each side
of the brain, chiefly to the organs of smell, taste, hearing, and
sight.
The cranial nerves are of three kinds: sensory, motor, and both
combined, _viz_., mixed.
Distribution and Functions of the Cranial Nerves. The cranial nerves
are thus arranged in pairs:
The first pair are the olfactory nerves, which pass down through the
ethmoid bone into the nasal cavities, and are spread over the inner
surface of the nose. They are sensory, and are the special nerves of
smell.
The second pair are the optic nerves, which, under the name of the
_optic tracts_, run down to the base of the brain, from which an optic
nerve passes to each eyeball. These are sensory nerves, and are devoted
to sight.
The third, fourth, and sixth pairs proceed to the muscles of the eyes
and control their movements. These are motor nerves, the movers of the
eye.
Each of the fifth pair of nerves is in three branches, and proceeds
mainly to the face. They are called tri-facial, and are mixed nerves,
partly sensory and partly motor. The first branch is purely sensory,
and gives sensibility to the eyeball. The second gives sensibility to
the nose, gums, and cheeks. The third (mixed) gives the special
sensation of taste on the front part of the tongue, and ordinary
sensation on the inner side of the cheek, on the teeth, and also on the
scalp in front of the ear. The motor branches supply the chewing
muscles.
The seventh pair, the facial, proceed to the face, where they spread
over the facial muscles and control their movements. The eighth pair
are the auditory, or nerves of hearing, and are distributed to the
special organs of hearing.
The next three pairs of nerves all arise from the medulla, and escape
from the cavity of the skull through the same foramen. They are
sometimes described as one pair, namely, the eighth, but it is more
convenient to consider them separately.
The ninth pair, the glosso-pharyngeal, are partly sensory and partly
motor. Each nerve contains two roots: one a nerve of taste, which
spreads over the back part of the tongue; the other a motor nerve,
which controls the muscles engaged in swallowing.
The tenth pair, the pneumogastric, also known as the vagus or wandering
nerves, are the longest and most complex of all the cranial nerves.
They are both motor and sensory, and are some of the most important
nerves in the body. Passing from the medulla they descend near the
œsophagus to the stomach, sending off, on their way, branches to the
throat, the larynx, the lungs, and the heart. Some of their branches
restrain the movements of the heart, others convey impressions to the
brain, which result in quickening or slowing the movements of
breathing. Other branches pass to the stomach, and convey to the brain
impressions which inform us of the condition of that organ. These are
the nerves by which we experience the feelings of pain in the stomach,
hunger, nausea, and many other vague impressions which we often
associate with that organ.
Illustration: Fig. 117.—Anterior View of the Medulla Oblongata.
A, chiasm of the optic nerves;
B, optic tracts;
C, motor oculi communis;
D, fifth nerve;
E, motor oculi externus;
F, facial nerve;
H, auditory nerve;
I, glosso-pharyngeal nerve;
K, pneumogastric;
L, spinal accessory;
M, cervical nerves;
N, upper extremity of spinal cord;
O, decussation of the anterior pyramids;
R, anterior pyramids of the medulla oblongata;
S, pons Varolii.
The eleventh pair, the spinal accessory, are strictly motor, and supply
the muscles of the neck and the back.
The twelfth pair, the hypoglossal, are also motor, pass to the muscles
of the tongue, and help control the delicate movements in the act of
speech.
272. The Spinal Cord. This is a long, rod-like mass of white nerve
fibers, surrounding a central mass of gray matter. It is a continuation
of the medulla oblongata, and is lodged in the canal of the spinal
column. It extends from the base of the skull to the lower border of
the first lumbar vertebra, where it narrows off to a slender filament
of gray substance.
The spinal cord is from 16 to 18 inches long, and has about the
thickness of one’s little finger, weighing about 1½ ounces. Like the
brain, it is enclosed in three membranes, which in fact are the
continuation of those within the skull. They protect the delicate cord,
and convey vessels for its nourishment. The space between the two inner
membranes contains a small quantity of fluid, supporting the cord, as
it were in a water-bath. It is thus guarded against shocks.
The cord is suspended and kept in position in the canal by delicate
ligaments at regular intervals between the inner and outer membranes.
Finally, between the canal, enclosed by its three membranes, and the
bony walls of the spinal canal, there is considerable fatty tissue, a
sort of packing material, imbedded in which are some large
blood-vessels.
273. Structure of the Spinal Cord. The arrangement of the parts of the
spinal cord is best understood by a transverse section. Two fissures,
one behind, the other in front, penetrate deeply into the cord, very
nearly dividing it into lateral halves. In the middle of the isthmus
which joins the two halves, is a very minute opening, the _central
canal_ of the cord. This tiny channel, just visible to the naked eye,
is connected with one of the openings of the medulla oblongata, and
extends, as do the anterior and posterior fissures, the entire length
of the cord.
The spinal cord, like the brain, consists of gray and white matter, but
the arrangement differs. In the brain the white matter is within, and
the gray matter is on the surface. In the cord the gray matter is
arranged in two half-moon-shaped masses, the backs of which are
connected at the central part. The white matter, consisting mainly of
fibers, running for the most part in the direction of the length of the
cord, is outside of and surrounds the gray crescents. Thus each half or
side of the cord has its own gray crescent, the horns of which point
one forwards and the other backwards, called respectively the anterior
and posterior cornua or horns.
It will also be seen that the white substance itself, in each half of
the cord, is divided by the horns of the gray matter and by fibers
passing from them into three parts, which are known as the anterior,
posterior, and lateral columns.
Experiment 130. Procure at the market an uninjured piece of the spinal
cord from the loin of mutton or the sirloin or the rib of beef. After
noting its general character while fresh, put it to soak in dilute
alcohol, until it is sufficiently hard to be cut in sections.
274. The Spinal Nerves. From the gray matter on each side of the spinal
cord 31 spinal nerves are given off and distributed chiefly to the
muscles and the skin. They pass out at regular intervals on each side
of the canal, by small openings between the vertebræ. Having escaped
from the spine, they pass backwards and forwards, ramifying in the soft
parts of the body. The first pair pass out between the skull and the
atlas, the next between the atlas and the axis, and so on down the
canal. The eighth pair, called _cervical_, pass out in the region of
the neck; twelve, called _dorsal_, in the region of the ribs; five are
_lumbar_, and five _sacral_, while the last pair leave the cord near
the coccyx.
Each spinal nerve has two roots, one from the anterior, the other from
the posterior portion of the cord. These unite and run side by side,
forming as they pass between the vertebræ one silvery thread, or nerve
trunk. Although bound up in one bundle, the nerve fibers of the two
roots remain quite distinct, and perform two entirely different
functions.
After leaving the spinal cord, each nerve divides again and again into
finer and finer threads. These minute branches are distributed through
the muscles, and terminate on the surface of the body. The anterior
roots become motor nerves, their branches being distributed to certain
muscles of the body, to control their movements. The posterior roots
develop into sensory nerves, their branches being distributed through
the skin and over the surface of the body to become nerves of touch. In
brief, the spinal nerves divide and subdivide, to reach with their
twigs all parts of the body, and provide every tissue with a nerve
center, a station from which messages may be sent to the brain.
Illustration: Fig. 118.—Side View of the Spinal Cord. (Showing the
fissures and columns.)
A, anterior median fissure;
B, posterior median fissure;
C, anterior lateral fissure;
D, posterior lateral fissure;
E, lateral column;
F, anterior column;
G, posterior column;
H, posterior median column;
K, anterior root;
L, posterior root;
M, ganglion of
N, a spinal nerve.
275. The Functions of the Spinal Nerves. The messages which pass along
the spinal nerves to and from the brain are transmitted mostly through
the gray matter of the cord, but some pass along the white matter on
the outer part. As in the brain, however, all the active powers of the
cord are confined to the gray matter. The spinal nerves themselves have
nothing to do with sensation or will. They are merely conductors to
carry messages to and fro. They neither issue commands nor feel a
sensation. Hence, they consist entirely of white matter.
276. Functions of the Spinal Cord. The spinal cord is the principal
channel through which all impulses from the trunk and extremities pass
to the brain, and all impulses to the trunk and extremities pass from
the brain. That is, the spinal cord receives from various parts of the
body by means of its sensory nerves certain impressions, and conveys
them to the brain, where they are interpreted.
The cord also transmits by means of its motor nerves the commands of
the brain to the voluntary muscles, and so causes movement. Thus, when
the cord is divided at any point, compressed, as by a tumor or broken
bone, or disorganized by disease, the result is a complete loss of
sensation and voluntary movement below the point of injury. If by
accident a man has his spinal cord injured at some point, he finds he
has lost all sensation and power of motion below that spot. The impulse
to movement started in his brain by the will does not reach the muscles
he wishes to move, because traveling _down_ the spinal cord, it cannot
pass the seat of injury.
So the impression produced by pricking the leg with a pin, which,
before pain can be felt, must travel up the spinal cord to the brain,
cannot reach the brain because the injury obstructs the path. The
telegraph wire has been cut, and the current can no longer pass.
277. The Spinal Cord as a Conductor of Impulses. The identity in
structure of the spinal nerves, whether motor or sensory, and the vast
number of nerves in the cord make it impossible to trace for any
distance with the eye, even aided by the microscope and the most
skillful dissection, the course of nerve fibers. The paths by which the
motor impulses travel down the cord are fairly well known. These
impulses originate in the brain, and passing down keep to the same side
of the cord, and go out by nerves to the same side of the body.
The sensory impulses, however, soon after they enter the cord by the
nerve of one side, cross in the cord to the opposite side, up which
they travel to the brain. Thus the destruction of one lateral half of
the cord causes paralysis of motion on the _same side_ as the injury,
but loss of sensation on the _opposite side_, because the posterior
portion destroyed consists of fibers which have crossed from the
opposite side.
Experiment proves that if both roots of a spinal nerve be cut, all
those parts of the body to which they send branches become paralyzed,
and have neither sense of pain nor power of voluntary movement. The
parts might even be cut or burned without pain. It is precisely like
cutting a telegraph wire and stopping the current.
Illustration: Fig. 119.—The Base of the Brain.
A, anterior lobe of the cerebrum;
B, olfactory nerve;
C, sphenoid portion of the posterior lobe;
D, optic chiasm;
E, optic tract;
F, abducens;
H, M, hemispheres of the cerebellum;
K, occipital portion of the occipital lobe;
L, fissure separating the hemispheres;
N, medulla oblongata;
O, olivary body;
P, antenor pyramids;
R, pons Valoru;
S, section of olfactory nerve, with the trunk removed to show sulcus
in which it is lodged;
T, anterior extremity of median fissure
Experiment also proves that if only the posterior root of a spinal
nerve be cut, all sensation is lost in the parts to which the nerve
passes, but the power of moving these parts is retained. But if the
anterior root alone be divided, all power of motion in the parts
supplied by that nerve is lost, but sensation remains. From these and
many other experiments, it is evident that those fibers of a nerve
which are derived from the anterior root are motor, and those from the
posterior root sensory, fibers. Impulses sent _from_ the brain and
spinal cord to muscles will, therefore, pass along the anterior roots
through those fibers of the nerves which are derived from these (motor)
roots. On the other hand, impressions or sensations passing _to_ the
brain will enter the spinal cord and reach the brain through the
posterior or sensory roots.
278. The Spinal Cord as a Reflex Center. Besides this function of the
spinal cord as a great nerve conductor to carry sensations to the
brain, and bring back its orders, it is also an independent center for
what is called reflex action. By means of its sensory nerves it
receives impressions from certain parts of the body, and on its own
authority sends back instructions to the muscles by its motor nerves,
without consulting the brain. This constitutes reflex action, so called
because the impulse sent to the spinal cord by certain sensory nerves
is at once reflected or sent back as a motor impulse to the muscles.
This reflex action is a most important function of the spinal cord.
This power is possessed only by the gray matter of the cord, the white
substance being simply a conductor.
The cells of gray matter are found all along the cord, but are grouped
together in certain parts, notably in the cervical and lumbar regions.
The cells of the anterior horns are in relation with the muscles by
means of nerve fibers, and are also brought into connection with the
skin and other sensory surfaces, by means of nerve fibers running in
the posterior part of the cord. Thus there is established in the spinal
cord, without reference to the brain at all, a reflex mechanism.
279. Reflex Centers. For the purpose of illustration, we might consider
the body as made up of so many segments piled one on another, each
segment presided over by a similar segment of spinal cord. Each bodily
segment would have sensory and motor nerves corresponding to its
connection with the spinal cord. The group of cells in each spinal
segment is intimately connected with the cells of the segments above
and below. Thus an impression reaching the cells of one spinal segment
might be so strong as to overflow into the cells of other segments, and
thus cause other parts of the body to be affected.
Take as an example the case of a child who has eaten improper food,
which irritates its bowels. Sensory nerves of the bowels are disturbed,
and powerful impressions are carried up to a center in the spinal cord.
These impressions may now overflow into other centers, from which
spasmodic discharges of nerve energy may be liberated, which passing to
the muscles, throw them into violent and spasmodic contraction. In
other words, the child has a fit, or convulsion. All this disturbance
being the result of reflex action (the spasmodic motions being quite
involuntary, as the brain takes no part in them), the child meanwhile
is, of course, entirely unconscious and, however it may seem to be
distressed, really suffers no pain.
Scattered along the entire length of the spinal cord, especially in the
upper part, are groups of nerve cells which preside over certain
specific functions of animal life; that is, definite collections of
cells which control definite functions. Thus there are certain centers
for maintaining the action of the heart, and the movements of
breathing; and low down in the cord, in the lumbar regions, are centers
for the control of the various abdominal organs.