In the last two chapters we have talked about the arrangement of the

nervous system and its working as seen in simple reflex actions, and the

special activities of the brain which make up the intellectual processes

and the emotions. Before we can leave the subject of nervous activity,

there are some special kinds of reflexes to be spoken of, and there are

also some additional things to say about the working of the emotions.

When we speak of special reflex actions we have in mind some of the most

useful of our own reflexes. They are not different from the simple

reflexes that are described in Chapter X, except that they involve a

larger number of sense organs and many muscles, and are correspondingly

more complicated. Furthermore, on account of this complication a special

part of the nervous system is devoted to carrying them on. Back in

Chapter VIII, under the heading of motion, the different types of bodily

movement were listed. Chief of these was the act of locomotion; that is,

the means by which we get from one place to another, an act which is, of

course, of prime importance in maintaining life, since it is concerned

both with the securing of food and the escaping of harm. Locomotion is

performed in a number of different ways; in ourselves, walking, running,

leaping, swimming, riding a bicycle are all forms of locomotion. Any of

these acts, as stated in Chapter VIII, requires the accurate timing and

accurate grading of the contractions of a great many different muscles.

As we saw in that same chapter there are special sense organs which

furnish the information upon which this accurate control of the muscles

of locomotion is based. We have now to consider the part played by the

nervous system in carrying it on. Whenever one starts to walk or to

perform any of the other acts of locomotion, nervous disturbances will

be set up in great quantity from the organs of muscle and joint sense in

the parts of the body that are active, and also from the organs of

equilibrium in the ear. It is necessary that the whole volume of nervous

disturbance that is pouring into the central nervous system from these

sources be translated into a series of streams of disturbances passing

out over the motor nerves to cause the muscles to work just as they

should in performing the act. All this is reflex in the strict sense.

That is to say, the muscular action follows the arousing of the sense

organs with no more delay than is necessary for the passage of the

nervous disturbance. The work of transforming the incoming stream into a

suitable outgoing stream is performed by a part of the brain about which

little has been said thus far, namely the _cerebellum_. This is

connected with other parts of the central nervous system in such a way

that pathways from organs of equilibrium and from the organs of muscle

and joint sense lead into it. From it in turn come a great many

connecting nerve cells whose axons pass along the spinal cord and form

nerve junctions with the motor nerve cells; and so nervous disturbances

coming from the cerebellum can arouse the muscles to activity. We all

know that walking or any other locomotor act, after it has once been

learned, is carried on successfully without our performing any

intellectual work in connection with it. What we do have to do is to

start the act, and this is done through a discharge from the cerebrum;

either some sense organ is aroused and sends a disturbance into the

cerebrum, which arouses in us an association which indicates the

necessity for going somewhere, or else a pure memory not stimulated from

outside has the same effect. In either case the nervous disturbance

passes over from the cell where the association resides to the motor

area, and from there down to start the act of walking; but as soon as it

is started, the reflex takes hold and carries it on without any further

thought on our part except as some additional associations enter to

cause us to change or perhaps to stop the action. It is possible that

the complex muscular acts which we group under the head of grasping are

operated in similar fashion. For example, the act of writing is

essentially a modified grasping movement. When we are writing we think

not about the muscular movements but about the sense of what is being

written, and it may be that the actual control of the muscles in this

case is something like that which we know exists in locomotion, although

this is by no means certain. It is of course a very great advantage to

us to have these complicated activities, which we are performing so

constantly, carried on as reflexes without conscious effort on our part.

If we had to think about the movement of every muscle, whenever we

started to walk, our minds would be so taken up with the act itself that

there would be no freedom for anything else; but since the reflex

machinery takes care of these muscular movements for us, our minds are

left free to deal with other matters at the same time as the muscular

actions are going on.

Not only is the simple act of walking carried on for us without mental

effort on our part, but to a very large extent the guidance through the

sense of sight becomes reflex also. Thus, as we go along the street, we

step down from or up on to the curb at the crossings without noticing

particularly what we are doing. The approach of the curb is registered

through the eyes, and the sensory disturbances thus set up are carried

into the brain and intermingled with those from the organs of muscle

sense and equilibrium, so that the motions are adjusted to carry us

safely down or up. This reflex guidance through the eye becomes so

perfect that one who habitually walks over a certain course, as a man in

going from his home to his place of business, ordinarily goes the entire

distance with scarcely any conscious attention to what he is doing; in

fact, persons who move from one locality to another frequently, when

they start to walk home, unless they pay attention, find themselves

going to the old home instead of the new. The reflex machinery which

guided them to their old home works so well that only by conscious

interference with it are they able to direct themselves to the new one.

Whenever an act of locomotion is learned, it is carried on at first by

nervous disturbances from the _cerebrum_, just as are muscular actions

of all sorts; but, as time goes on, there seems to be a transference of

the control from the _cerebrum_ to the _cerebellum_, and as this happens

the act is more accurately performed. We have an amusing illustration of

this in persons who are learning to ride a bicycle. So long as they are

making movements of the handle bars and at the same time pedaling by

pure acts of the will, they proceed with the very greatest difficulty;

after these actions become habitual, they are carried on easily and

skillfully. The great effort of athletic trainers is to develop what is

called “form.” What this really means is that they try to teach the

athlete the best way of making the desired motions, and then drill him

in that particular way, so that it becomes completely reflex with him.

The more fully this is done, the better will the athlete perform.

All that has been said about the nervous system thus far has applied to

its duty of controlling the activities of skeletal muscles; namely, the

muscles by which the body carries on its principal motions. These, as

already stated in Chapter VII, are not the only kind of muscles in the

body. There are also the smooth muscles and heart muscle. There is also

another kind of functional metabolism besides the metabolism of muscular

motion over which the nerves have control. This is the metabolism of

secretion, or the manufacture of special chemical substances by gland

cells. In later sections of the book we shall hear more about what the

glands do, but here we have to say something about the way they are

controlled through the nervous system. We speak of them along with the

smooth muscles, because it happens that the same nervous arrangements

are found in both cases; the smooth muscles and the glands have a

precisely similar kind of nervous control. In appearance the nerves

which lead from the central nervous system to the smooth muscles and the

glands do not differ much from other nerves. They are composed of nerve

cells, just as are all the other nerve pathways. It happens to be true

that the sheaths which inclose the axons of these particular nerves

usually have little or no white material in them, so that nerve trunks

which are made up of these fibers look gray instead of white, as do the

other nerves. Just why there should be this difference, we do not know.

Of more importance than this difference in appearance is the fact that

the connection from the central nervous system to the smooth muscle cell

and the gland cell in no case is made up of a single nerve cell, but

always of a chain of two. In the case of nerve paths to skeletal muscles

the cell body of the motor nerve cell lies either in the spinal cord or

the brain and its axon extends all the way from there directly to the

muscle fibers. In the case of smooth muscles and glands a connecting

nerve cell extends out from the central nervous system some distance and

makes its nerve junction with the motor nerve cell part way between the

central nervous system and the muscle or gland. These connecting cells

are much branched, as are all connecting cells; also the motor cells

which connect with smooth muscles or glands are very much branched; the

result is that a single nerve cell starting from the spinal cord finally

connects with a large number of smooth muscle cells or gland cells. We

do not have, then, in these organs the fine subdivision of control that

there is in the skeletal muscles, for a single skeletal motor cell in

the spinal cord connects with one or, at the most, a very few muscle

fibers. If we consider the uses to which smooth muscles are put, we

shall see that finely subdivided control is not important, as it is in

the case of the skeletal muscles, where the movements have to be graded

very accurately. Smooth muscles are present in the stomach and

intestines and other abdominal organs which make movements. They are

found also in the walls of the small blood vessels where their

contractions or relaxations can change the caliber of the vessels, and

so affect the amount of blood passing through them. A good example of

this action is seen in the color of the face; when one is pale, it is

because the small blood vessels just under the skin of the face are much

contracted, and very little blood can pass through; when flushed, on the

other hand, the blood vessels are relaxed and there is a large amount of

blood passing. These contractions and relaxations of the blood vessels

are performed by movements of the smooth muscles. Another interesting

place where there are smooth muscles is at the roots of the hairs.

Everybody knows that the hair is supposed to stand on end when one is

frightened. Most of us have thought of this as a picturesque figure of

speech, but as a matter of fact it actually does happen. A human being

has such a small equipment of hair that its rising on end does not make

much of a showing, but in an angry dog or frightened cat we see it in

all its glory. If one looks at the forearm when it is so chilly that

“goose-fleshing” occurs, he sees his hair then standing clearly on end.

In fact, goose-fleshing is caused by contractions of these smooth

muscles just under the skin. Every hair has a few smooth muscle cells at

its base and they are attached in such a way that, when they contract,

the hair is pulled from the slanting to the erect position. The surface

of the skin acts as the fulcrum on which the motion is made. Besides

these places where smooth muscle is found there are some in the eye. The

muscles of accommodation and also those by which the size of the pupil

is changed are smooth muscles. There are some also in the small

bronchial passages. In persons who suffer from asthma these smooth

muscles become contracted, and the difficulty in breathing experienced

is the result of the partial closing of the air passages caused by this

contraction. Glands which are under the control of the same kind of

nerves as control the smooth muscles are found in many places in the

body; in the head we have the tear glands and the salivary glands; in

the trunk there are the gastric glands, the pancreatic, and various

others about which we shall hear later. With the single exception of the

muscles of accommodation there is no case either among the smooth

muscles or the glands where there is any occasion for a finely

subdivided control. When the muscles of the stomach or intestines become

active, a great many fibers work together. When the hair stands on end,

a great many hairs become affected at once. A flushing or paling of the

skin is also a matter of a large surface. The same is true of gland

secretion. In an active gland there is no picking out of a few cells to

do the secretion, but a large part of the gland, if not the whole of it,

becomes active, whenever any of it is. The advantage of the construction

of the nerve paths from the spinal cord to the smooth muscles or to the

glands is that a single nervous disturbance from the central nervous

system can affect many muscle or gland cells all at once. In the case of

the skeletal muscles, on the other hand, the only way a great many

fibers can be made active at once, is by sending out nervous

disturbances through a great many nerve cells. There is undoubted

economy in having the control of the smooth muscles and glands carried

on by means of comparatively few discharges from the central nervous

system.

A peculiarity of the structures that we are talking about now is that we

have very little voluntary control over them. This contrasts sharply

with the completeness of our control over the activities of our skeletal

muscles. With very few exceptions we can start, stop, or regulate

skeletal muscle activities at pleasure, but the activities of our smooth

muscles and of our glands are not under our control in the same way. We

cannot cause ourselves to turn pale by an act of the will, nor can we

set the stomach to working when we wish. This latter inability is a real

misfortune, for there are many cases of acute indigestion that are due

to stagnation of the stomach, and if we could set the stomach into

operation at will, these would be avoided. Since we have not this

voluntary control, it is of the utmost importance for us to learn how

the control is really carried on, so that the motions necessary to

health can be induced in a normal manner.

To a very large extent the smooth muscles and glands are subject to

reflex control; that is, nervous disturbances coming in from the sense

organs pass over by way of connecting nerve cells to the nerve pathways

leading out to the smooth muscles or to the glands. Examples of this

kind of control are seen in the paling of the skin, when one is chilly,

or its flushing, when one is warm; in the copious secretion of saliva

into the mouth that one can get by chewing a rubber band; or in the

secretion of tears that comes when the front of the eye is hit.

A second kind of control of the smooth muscles and glands is their

control through the emotions. We have already given illustrations of

this in the paling of the skin when one is frightened, or the erection

of the hairs in the tail of an angry cat. For a long time it has been

known that most of our emotions, if not all of them, are accompanied by

some sort of bodily change; laughter and crying are familiar

illustrations. To a certain extent, as in these instances, skeletal

muscles share in the bodily effects of emotions, but their most

important examples are found in the smooth muscles and glands. Every

serious display of emotion is accompanied by a discharge of nervous

disturbances into some of the smooth muscles or some of the glands. This

fact has long been known, but its meaning has become clear only very

recently. Before attempting to explain this, an additional word will be

necessary about the nervous control of smooth muscles. These muscles

differ from skeletal muscles in one important particular, which is this:

when a skeletal muscle is aroused to contraction, it will relax as soon

as the stream of nervous disturbances stops. To keep it in contraction a

continuous stream must come into it through the nervous system. This is

not true of smooth muscle. When a smooth muscle has been caused to

contract, it is very likely to stay contracted without further action on

the part of the nervous system. In fact, in very many cases another kind

of nervous disturbance must be sent into it in order to make it relax.

This means that the smooth muscles, or at least most of them, have two

sets of nerves coming to them; one set which causes them to contract,

the other set which causes them to relax. Both these sets of nerves are

of the type described above as supplying the smooth muscles and glands.

A good illustration of the double action is found in the stomach. There

is one set of nerves whose activity causes the walls of the stomach to

perform the churning motions which go on ordinarily during digestion.

There is another set of nerves whose effect upon the stomach is to cause

all motions in it to stop. This same double arrangement of nerves is

found in many of our most important smooth muscle organs. We said a

moment ago that every emotional display is accompanied by an outflow of

nervous disturbances to the smooth muscle or gland structures. We now

see that what happens in the body is determined by which of the possible

pathways is followed by the nervous disturbance. One turns pale, when

frightened, because in the emotion of fright the nervous discharge is

over the nerves that cause the blood vessels of the face to contract.

The mouth waters at the sight of delicious food, because the pleasurable

emotions aroused thereby cause the nerves to the salivary glands to

become active. As was said a moment ago, for a long time there seemed to

be no rhyme or reason about these emotional reactions. Why, for example,

should worry cause the muscles of the stomach to stop completely, thus

bringing on thousands of attacks of acute indigestion? We now have a

clue to the meaning of these facts, which is about as follows: The

emotions are to be looked upon as part of the machinery by which bodily

well-being is promoted. Emotions of pleasure signify that matters are

progressing favorably, and the nervous discharges accompanying these

emotions are those in accord with well-being. For example, the

activities of the alimentary tract in digestion are of a kind that

prosper when the body as a whole is well off. Disagreeable emotions, on

the other hand, are associated with the necessity for self-preservation.

Thus anger or fear arise in the presence of an emergency calling either

for combat or flight. The nervous discharges that accompany these

emotions are found on analysis to bring about bodily changes whose

effect is to prepare the body successfully for one or the other. This is

best shown by actually naming over some of the bodily changes that

occur. We may begin with the top of the head and work down. The hair

stands on end; this is of no importance in human beings, but in many

animals contributes materially to the ferocity of the appearance. The

pupils are dilated, aiding vision; the face is pale, and the mouth is

dry; not because these in themselves are of importance but because they

represent a diversion of energy from unimportant to important parts of

the body. The blood that ordinarily flows through the face is shunted

out of it into the brain and muscles, where it will really help in the

emergency. The activity of the salivary glands is stopped to free the

energy that they would consume for use in the brain or muscles. The

heart beat is quickened; this again, by making the circulation more

efficient, helps the animal to fight effectively. The activity of the

stomach is suspended for the same reason as is that of the salivary

glands. What we have is a marshaling of the bodily forces into the

organs which are of immediate importance in meeting the emergency;

namely, the nervous system and the muscles. The other parts of the body,

which are necessary in the long run, are allowed to stop functioning for

the moment. This behavior of the body in an emergency illustrates better

than anything else could the point that our muscles, nerves, and sense

organs make up the really important parts of us and the rest of the body

has simply the duty of keeping these in good order. In time of

emergency all the energies are devoted to the task of keeping these

parts of us at highest efficiency, however bad may be the effect on the

other less necessary organs; so it is that one who worries himself into

an attack of indigestion is really preparing for an emergency by

sacrificing the less essential for the more essential activities. The

practical difficulty in this is that the worry almost always fails to be

of use, either because the expected emergency does not arise at all or

else arises so far in the future that the immediate bodily changes

caused by the worry do no good. Evidently worry is an emotion to be

combated. The old adage about not crossing the bridge until it is

reached has really a great deal more to do with the preservation of

health than most of us realize.

We have described the control of smooth muscles and glands through

reflexes and in connection with the emotions. We have still to mention

briefly a third way in which nervous disturbances leading to these

organs may be originated. In the last chapter the brain was described as

consisting of three chief parts: the cerebrum, which is the seat of

memory and association, and so of all thought processes; the cerebellum,

which governs the reflex of locomotion; and the brain stem about which

nothing was said except that it is a region through which all nervous

disturbances have to pass on their way into or out from the brain. The

brain stem has a special function of its own in connection with the

control of what are often called the “vital processes.” Since most of

these vital processes are concerned with smooth muscle or gland action

they are of interest to us here. In the brain stem are located groups

of nerve cells to which are given the name of “centers.” From these

extend nervous pathways to various bodily structures. For example, there

are two centers having to do with the activity of the heart, and both of

these have nerve pathways leading to that organ. Over one of them pass

the discharges which cause the heart to beat more rapidly, over the

other the discharges which slow it down. There are also centers for

controlling the caliber of the various small blood vessels and so the

amount of blood that flows through the various organs of the body. Still

another center has control of the secretion of sweat. These are a few of

the centers which are present in the brain stem. It is evident that

these centers must be played upon directly from the sense organs in the

reflex control of the smooth muscles and glands and also from the

cerebrum in their emotional control. The third way in which these

centers can be affected is by a direct action of the blood which

circulates through them. Of course, every center is made up of nerve

cells which are alive and which share with all other living cells the

necessity of being sufficiently nourished. This means that there must be

small blood vessels here and there among the nerve cells, so that

interchanges of material can take place between the blood and the fluid

immediately surrounding the cells. Among the materials which may come

from the blood are some special chemical substances which have the

ability to arouse these cells to activity. For example, the very rapid

beating of the heart following vigorous muscular exercise is due, in

part at least, to the presence of chemical substances that are poured

out into the blood from the laboring muscles and which have the effect

of arousing the center whose action speeds up the heart. In at least

one case, that of the center controlling the sweat glands, the nerve

cells are aroused by an increase in temperature; on a very hot day or

when one exercises briskly the temperature of the blood begins to go up;

this arouses the center which controls the secretion of sweat; nervous

disturbances are poured out to the sweat glands and they become active.

The effect of their activity is to remove heat from the body, and so

help to lower its temperature. Here we have an example of an automatic

regulating device in which nerve cells are controlled through the blood.

Except for this one instance of the effect of blood temperature the

actions of this class are thought to be wholly by means of chemical

substances, and for that reason we speak commonly of the third method of

control of the smooth muscles and glands as chemical control. We have

then altogether three methods of control; through reflexes, through the

emotions, and through chemical substances in the blood.

Since we have spoken of chemical control it will be appropriate to close

this chapter with just a further word about it. In addition to the

regulation of smooth muscle and gland activity there are a number of

other bodily processes which are affected more or less through chemical

agents brought to them by the blood. Many of these processes are of a

kind that cannot well be controlled through the nervous system. For

example, growth is something which goes on very slowly and yet which

distinctly requires some kind of control. It has not perhaps occurred to

us that it is more than chance that our ears are substantially the same

size, our arms about the same length, our legs the same length, our

feet about the same size, and so on. Yet if we pause a moment we realize

that as the ears grow there would seem to be no particular reason why

they should both stop when they have reached the same size, unless there

is some definite regulation. This regulation is present in the form of

chemical substances. Of late years a very large amount of study has been

given to the materials which have the power of acting as chemical

regulators, and a good deal has been learned about them, although there

is every reason to believe that a great deal more remains to be found

out. It has been necessary to invent a name for substances which act as

chemical regulators, because, without such a name, talking about them

becomes too cumbersome. The name that has been adopted is _hormone_,

from a Greek word meaning to arouse or to stimulate, referring to the

ability these substances have of acting upon living cells. Thus far we

have spoken of hormones only in connection with their ability to excite

those nerve cells by which smooth muscles and glands are controlled. As

a matter of fact, their action is much wider than that. They have the

power of affecting the metabolism of very many body cells. Some of them

appear to have the special function of regulating the rate of

metabolism. These we shall talk about in detail, when we return to the

study of metabolism in a later chapter. One of the hormones is

interesting in connection with the relation of emotions to smooth muscle

and gland activity. This is a secretion manufactured by a pair of small

glands located in the abdomen near the kidneys. They are known as the

_adrenal bodies_. These glands secrete a substance to which has been

given the name of adrenalin, which has been shown to be a very

efficient chemical excitant to a number of bodily processes. We are

particularly interested here in a property it has of arousing smooth

muscles and glands in precisely the same way as they are aroused in

connection with the disagreeable emotions, such as fright or anger. For

example the injection of a little of this adrenalin into the veins of a

person or animal will cause his heart beat to be quickened, the blood

flow into the brain and the muscles to be increased at the expense of

the flow through the skin and abdominal organs, and the other effects to

occur that were described above as accompanying these emotions. These

facts about the effects of adrenalin had been known for some time before

it was realized that they fit in with the reaction by which the body

prepares itself to operate efficiently in time of stress. Now we have

learned to look upon adrenalin as the “emergency hormone,” meaning that

it is a chemical substance which does the same things to the body that

are done through the emotions in time of emergency. We look upon this as

a reenforcement of the nervous action, indicating the efficiency with

which the fitting of the body for time of stress works out. The adrenal

bodies are themselves glands and are acted upon like other glands

through the part of the nervous system that we have just been

describing. In time of emergency a stream of nervous discharges pours

into them; they secrete an abundant supply of adrenalin which passes out

into the blood and is carried by the blood to all parts of the body,

exerting its special functions wherever it comes. There is no doubt that

the ability of the body to care for itself in time of need is helped by

the outpouring of adrenalin. In one respect, however, this arrangement

does not work out perfectly. This is because chemical substances that

are poured out into the blood cannot be gotten rid of instantly, and are

likely to linger for a shorter or longer time after the actual emergency

subsides, so the bodily effects persist for a time after the occasion

for them has disappeared. We are all familiar with the fact, for

example, that the heart goes on thumping for a long time after a sudden

shock. It may not have occurred to us to wonder what kept it going at

that rate after the immediate disturbance was passed, but we now see

that it must do so until the adrenalin which was poured out in

connection with the shock has been gotten rid of, which is a more or

less gradual process.

CHAPTER XIII

THE BODY FLUIDS

We have learned to think of the cells which make up the body as

dependent on the fluid which surrounds them for the various materials

they require, and as a place into which they discharge the products of

their metabolism. We have seen furthermore that the fluids which bathe

the cells directly must be constantly renewed. The renewal is

accomplished by interchanges between this fluid and the blood, which

constantly flows through the tiny blood vessels that are everywhere

present in the body. In its course, in turn, it passes through the blood

vessels of the organs in which it is to be itself renewed; the digestive

organs for food supplies, the lungs for oxygen, the kidneys for the

discharge of waste material. We must now look further into the nature

and action of the various body fluids. Of course the foundation of all

of them is water. In this water must be dissolved everything that is

used by any of the cells for food or anything that any of these cells

produces. Under this latter head we have the waste products of ordinary

metabolism, or in the case of some cells special products of functional

metabolism. The presence of all these various materials would be bound

to make the body fluid an extremely complex mixture. In addition to

these various materials there are certain substances present besides

water to make up what we may call the structure of the body fluids as

distinguished from the materials which they are carrying from one place

to another. These structural materials include a number of salts of

which ordinary table salt (sodium chloride) is the most abundant as well

as the most familiar. In addition there are salts of lime and potash and

magnesium; all these latter in very small proportions. Just why the body

fluids should contain these salts is not very clear. We know that if

they were not present the cells of our bodies could not live, yet it is

true that there are a great many kinds of living cells that get along

perfectly well in fresh water, which may have no salts dissolved in it

at all. On the other hand there are more kinds living in the ocean and

exposed to the rather strong concentration of salts which make up ocean

water, and there are even some kinds of animals that live in strong

brine, so that evidently living protoplasm can adjust itself to

surroundings in which the strength of the salt in solution is widely

different. It is true that very few kinds of animals can endure being

changed suddenly from ocean water to fresh water or the reverse. One of

the best ways to clean the bottom of a ship that has become foul through

long sailing about the sea is to transfer it into a fresh water lake or

stream, where the accumulation of living animals and plants will be

killed and will drop off. Most kinds of ocean animals die rather

promptly if changed to fresh water, or fresh water animals if put into

the ocean. There are a few kinds of fish, like the salmon and shad,

which live in the ocean but lay their eggs in rivers, and these are able

to endure the change from the one kind of water to the other without

being destroyed. They, to be sure, make the transition gradually,

swimming up from the ocean into water that is less and less salty, until

they finally reach the fresh water stream itself. There are other kinds

of living things which can endure a much more abrupt change from salt to

fresh water and back again. In various parts of the world are large

rivers emptying into the ocean and so situated relative to towns and

cities that steamers make regular round trips from the ocean up to the

fresh water of the river and back again, and it will be found that on

the bottoms of these steamers are various plants and water animals which

endure the frequent shift from salt water to fresh and back again

without harm.

The percentage of salt in the body fluids of all the higher animals

including ourselves is only about one-fifth that in the ocean.

Furthermore the fluids of most kinds of land animals have about the same

percentage of salts dissolved in them. Naturally there has been much

speculation as to why there should be this percentage of salt in

preference to any other. One ingenious theory supposes that back in the

beginning of things, when the earth cooled down below the boiling point

of water, so that it was possible for water to collect on the earth, the

water in all the oceans was fresh. This would have to be true, since

water must have fallen in the form of rain; but in course of time some

of the salts in the earth’s crust would be dissolved, making the water

salty, and as time went on the ocean would become saltier and saltier.

This is still happening, for every river that discharges into the ocean

carries with it materials that it has dissolved from the underlying soil

during its passage from its source, and such material, when it once

enters the ocean, must stay there until the ocean water becomes

saturated with that particular substance. According to this theory there

was a time, then, when the ocean water was just about as salty as our

body fluids at the present time, and it supposes also that that was the

time when the ancestors of present land animals crawled out of the ocean

and took up their abode on land. Of course there is no way to prove that

this is so, but it does account for the particular percentage of salts

in our bodies as well as any other explanation we know anything about.

In addition to these various salts our body fluids contain in solution

moderate amounts of very complex chemical substances belonging to the

class of proteins. A fact about proteins which has not yet been

emphasized is that they make liquids in which they are dissolved sticky

or gelatinous. An excellent example of this is ordinary raw white of

egg, which is a solution of protein in water, and which shows the

gelatinous character very strikingly. Because of the protein that is in

solution in the body fluids, they have also this gelatinous character,

although to a much less extent than in the white of egg, because the

amount of protein in solution is so much less. As we shall show later,

this sticky quality of the fluid is of a good deal of importance in its

actual use in the body. It may be that the protein in the body fluids

serves other purposes as well. One interesting fact that needs to be

emphasized is that it is not used for fuel or building material for the

cells. The protein that comes to them as part of their food supply is

entirely distinct from that about which we are now talking, which is

part of the permanent structure of the body fluids. We shall speak of

the protein that serves to nourish the cells in a moment, when we are

talking about the relations of the fluids to the transportation of food

material.

In addition to the salts and proteins we have also dissolved a great

many very complex materials which may be looked upon as permanent or

relatively permanent constituents of the fluid, but about which we know

practically nothing chemically. We are sure that they are present,

because of certain effects which they produce, but the substances

themselves have never been made out by chemical analysis. These are

materials which are concerned with the resistance of the body to

infectious disease, and it will be necessary to say just a word about

infection to make clear the part played by them.

What we call an infection is the invasion of the body from the outside

by minute living organisms, either plant or animal, and the establishing

of them within the body, so that they grow and multiply. They carry on

their metabolism just as do all other living cells and produce various

chemical products as a result. There are many organisms living within

our bodies whose metabolic products apparently do us no harm, and so we

serve as hosts for these unbidden guests year in and year out without

even knowing of their existence. The products from other kinds of

organisms are poisonous to us, and when some of these organisms multiply

within us we discover it, because we are poisoned and become ill. Only

organisms whose products of metabolism are poisonous are counted

ordinarily as causing infection. In the strict sense we might be said to

be infected by the harmless organisms of which so many thrive within us,

but in the usual use of the word we speak of one as having an infection

only when his body has been invaded by organisms that produce poison. In

the case of most kinds of injurious organisms their multiplication

within the body unchecked would lead finally to its destruction. It is

necessary, therefore, that the body have some means either of checking

the development of the organisms or of neutralizing the poisons which

they produce. The body has this power, and the machinery for it consists

very largely of substances or structures in the body fluids. The

detailed story of these is too complex to be told here. We shall content

ourselves by saying that when an infection becomes established, as of

scarlet fever, for example, the poisons that are produced are poured out

by the organisms into the body fluids of the part where they happen to

be located, and are taken up from there by the blood and distributed all

over the body. The fever, headache, and other disagreeable symptoms are

due to these poisons. The interesting thing about it is that the poisons

themselves act toward the body as chemical regulators or hormones,

exciting some or perhaps all of the cells of the body to a special kind

of functional metabolism, which results in the manufacture of materials

which neutralize the poisons. Thus, one of the very important properties

of living protoplasm is to respond to the poisons from the metabolism of

other cells by producing neutralizing material. Whether one dies from an

infection or recovers from it depends on whether the cells are able to

produce enough neutralizing material to prevent themselves from being

killed or whether the poison is so abundant or so malignant that the

cells are destroyed in spite of their activity in pouring out the

neutralizing material.

Still another interesting thing about this whole matter is that every

kind of infecting organism has its own kind of poison, which differs

from that of the other kinds, and so the chemical effect of the poison

upon the cells is not the same for one infection as for another. The

functional metabolism of the cells in turn is adjusted to the kind of

poison, so that the material they pour out is suitable to neutralize the

particular poison which aroused them to activity in the first place, and

in most cases no other. If one gets well from any infection, there is a

surplus of the neutralizing material left in his body fluids, and, as

long as it remains, he is secure from another infection of the same

kind. This condition is defined as immunity. Since the neutralizing

materials are different for different infections, immunity against one

is in most cases of no avail against another. One may be immune against

scarlet fever, but be just as likely to catch pneumonia as a person who

has never suffered from any infection at all. It follows that an

individual who has had and recovered from a great many infections has a

correspondingly large assortment of neutralizing materials in his body

fluids. Some of these appear to persist throughout life, others

disappear fairly soon.

The next group of permanent constituents to be described consists of

some materials which seem to have nothing at all to do as long as

everything is going well: the body fluids bathing the cells, or, in the

case of the blood, circulating about from part to part through the blood

vessels. These come into play only when, as the result of injury, the

fluids begin to escape; namely, in the case of bleeding, or, as it is

technically called, hemorrhage. It is clear that if any injury is

started sufficient to allow the fluid to escape, there would be no more

reason why it should stop running out, unless prevented, than there is

why water should stop running out of an open faucet of itself. Since, as

we know, we do not bleed to death every time we get a slight cut, but

after a long or shorter time the bleeding stops of itself, there must be

some automatic arrangement by which the opening is plugged. All of us

are familiar with the way in which this is done. We know that in the

course of a few minutes after the bleeding begins, the blood tends to

set into a firm jelly, which is called the _clot_. This clotting is the

result of a chemical transformation which goes on in the blood as the

result of its escape from the blood vessels and its exposure to the

outside. The details of the chemical processes are too complicated to be

described here; all we need to remember is that the blood within the

body contains certain special materials which are soluble and therefore

float in solution along with the other soluble materials. When any blood

vessel is injured, so that the blood begins to escape, a series of

chemical changes is started automatically by which this soluble material

is changed and becomes insoluble, so that instead of remaining dissolved

in the blood it is precipitated out. In this form it is called _fibrin_.

Fibrin is a very sticky, stringy mass which forms a spongy network,

spreads itself over the injury, and clings firmly to the edges, thus

plugging the opening, unless the rush of blood is so strong that it

keeps washing the fibrin away as fast as it is formed. Bleeding from

small wounds will presently stop of itself, but if the hemorrhage is too

great for this, it is necessary that the blood flow be slowed down or

stopped artificially. This is done by locating and pressing upon the

large blood vessel through which the blood is escaping. In order to do

this successfully, one has to know something about the course of the

blood vessels, and this will be described in a later chapter.

There are a few people whose blood lacks some of the necessary chemical

substances to enable it to clot; such persons are known technically as

“bleeders.” Even a slight injury in one of them will cause serious, or

even fatal, hemorrhage, unless the escape of blood is stopped

artificially, since it will not stop of itself. An interesting fact

about this condition is that it runs in families; in other words it is

hereditary.

In addition to all these constituents of the body fluids which are

_dissolved_ in them, there are in that part of the fluids confined to

the blood vessels, which we call the blood, three kinds of structures

floating; these we have next to describe briefly. The first of these are

the _red corpuscles_. They give the blood its red color, although if

looked at singly they appear yellowish rather than red. Red corpuscles

are almost inconceivably tiny. They are red flexible disks, a little bit

thinner in the middle than at the edges, about one three-thousandth of

an inch in diameter. Some idea of the enormous numbers in the body can

be gathered, when we say that a drop of blood the size of the head of a

pin would contain four or five million of them. The red corpuscles are

made up of a sort of framework of protein within which is inclosed a red

coloring matter or pigment, known as _hemoglobin_. It is this pigment

that gives the blood its color, and in some respects it is one of the

most important of the nonliving substances in

[Illustration: STRUCTURE OF A DROP OF BLOOD AS SEEN UNDER THE MICROSCOPE

Two white (colorless) corpuscles _a_ appear. The remainder are red

corpuscles sticking together, forming _rouleaux_. (From Martin’s “Human

Body.”)]

the body. This is because it is the means by which the cells obtain

sufficient supplies of oxygen. As we have already seen, every cell is

constantly drawing from the body fluids about it the oxygen which is

required for carrying on its metabolism. The fluids in turn get oxygen

from the blood. It is necessary, therefore, for the blood to convey

abundant supplies. Oxygen will dissolve in water, as is proven by the

fact that fish and other aquatic animals are able to get enough oxygen

from the water in which they live to serve their needs; but it is not

sufficiently soluble to supply the needs of an active body like that of

man. It is necessary, therefore, to have a special additional means of

conveying oxygen besides its simply dissolving in the blood. This

additional means is furnished by the hemoglobin, which is an

“oxygen-carrying” pigment. What this means is that the hemoglobin has

the property of taking up oxygen chemically, whenever it is exposed to a

region where there is oxygen in abundance, and of giving it up again

whenever it passes through a region where oxygen is scarce. It is thus

that oxygen is conveyed from the lungs to the active tissues of the

body. We shall have more to say about this in the chapter devoted

especially to the matter of the oxygen supply.

We said a moment ago that the red corpuscles consist of a protein

framework inclosing hemoglobin. They are not living. They must,

therefore, have been made by living cells and poured out into the blood

stream. We might suppose that this was done once for all and that the

same red corpuscles are floating in our blood now that started floating

there when the blood was first formed; but, as a matter of fact, this is

not the case. There is a continuous breaking down of red corpuscles

which must be made good by a continuous manufacture of new ones. Most of

the larger bones in our bodies have a sort of spongy framework by which

the ends, where the joints are, are made stronger. Within the space of

these frameworks is a kind of marrow, known as _red marrow_, because it

has such a very abundant blood supply. It is in this red marrow that the

manufacture of red corpuscles goes on. There are throughout the red

marrow living cells which are constantly dividing and subdividing,

forming more and more so-called _daughter cells_. Within these daughter

cells hemoglobin is presently deposited; a little later they lose the

nucleus and probably the remainder of the living protoplasm as well,

leaving just the framework of nonliving protein with its contained store

of hemoglobin. This is the finished red corpuscle, and it breaks loose

from the red marrow and floats out into the blood stream. The rate of

manufacture of red corpuscles is very rapid; undoubtedly millions of

them are formed daily in the various red marrow regions of the body.

The total number of red corpuscles does not increase correspondingly,

because they are broken down at the same rate as they are formed. It is

now believed that in the _spleen_, which is a large organ of the abdomen

whose function has always been obscure, those red corpuscles which are

destined for destruction are picked out of the blood and broken down. We

commonly suppose that this fate overtakes corpuscles that are worn out

and are no longer efficient oxygen carriers, but we do not know, as a

matter of fact, that the corpuscles do lose their effectiveness in

course of time, nor have we any idea how the spleen could select out of

the millions in the blood those particular ones which are no longer

useful.

What the spleen does to the corpuscle is to break it up so that the

protein and the hemoglobin in it are set free in the blood stream. We do

not know what becomes of the protein; probably it is taken up and

utilized. We do know that the hemoglobin is decomposed in the liver. One

constituent of hemoglobin, in fact the constituent which gives it its

ability to carry oxygen, is the element iron. Iron is not particularly

abundant in living things, and we find that the body is thrifty with

regard to it. When the liver decomposes the hemoglobin, the iron is

saved in some way which enables the blood to carry it back to the red

marrow, where it can be used over again. There are also some portions of

the hemoglobin which are valuable as food material; the remainder, which

is of no further use, is discharged from the body as a part of the bile.

Besides the red corpuscles the blood contains what are known as the

white or, better, the colorless corpuscles. These, instead of being dead

structures, as are the red corpuscles, are actual living cells, which

float along in the blood stream or have the power of clinging to the

walls of the blood vessels and crawling along them; in places where the

blood vessels are very thin, they work their way right through, and so

get out into the spaces among the tissues. There are not nearly so many

colorless as red corpuscles, in fact, the latter are eight or nine

hundred times as numerous as the former. A great deal of study has been

devoted to colorless corpuscles to find out just what they do. We are by

no means certain that we understand fully all their activities, but we

do know that one very important thing that they do is to absorb into

their own bodies tiny foreign particles that may be present in the blood

stream or in the spaces among the tissues. There are not many kinds of

foreign particles that can get into these places. In fact, about the

only sort that can are the tiny plant or animal cells which are

responsible for disease. When these foreign organisms invade the body,

the colorless corpuscles may engulf them into their own bodies and

destroy them. In this way a great deal of infection is prevented. Of

course it may happen that the invading organisms are so numerous that

the colorless corpuscles can not get rid of all of them, or the

corpuscles themselves may show a diminished activity. It is an

interesting fact that both the number of colorless corpuscles in the

body and the vigor with which each colorless corpuscle attacks foreign

organisms vary from time to time, so that we are much more secure

against infection at some times than at others. In general we may say

that when the body is in a condition of vigorous health the colorless

corpuscles will be efficient. When we are run down, on the other hand,

these cells share with all other body cells this state of low vitality.

This explains why people who allow themselves to become run down are

more likely to fall victims to infection than those who are in vigorous

health, and emphasizes, of course, the importance of a habit of life

that shall keep the body vigorously healthy at all times. It should be

understood that the colorless corpuscles do not show the same

effectiveness against all kinds of foreign organisms; some kinds of

disease germs are able to bring about infection in the body quite

regardless of the activity of the colorless corpuscles. From other

kinds, on the other hand, they give the body very complete protection.

The most familiar example of the action of the colorless corpuscles is

in the formation of what we all know as pus. There are a few kinds of

organisms that, instead of getting into the body and becoming scattered

through its fluids, establish themselves at certain points and by growth

and multiplication accumulate at those places in large numbers. Examples

are pimples and boils. In these cases the pus-forming organisms have

located just under the skin and are multiplying there at a great rate.

They produce poison which is absorbed from the place where they are and

distributed through the body. This poison appears to have some sort of

chemical attraction for the colorless corpuscles; at any rate the

corpuscles gather from all around to the place where these organisms are

located and engulf as many of them as they are able, but in so doing

they themselves are destroyed, and pus, as we know it, is simply a mass

made up of the dead bodies of the colorless corpuscles along with the

organisms which they have destroyed and which in turn have destroyed

them. In the ordinary pimple or boil the colorless corpuscles ultimately

win the combat and the infection is completely overcome. It happens

occasionally, however, that a pus-forming infection becomes established

in some place where it manages to maintain itself in spite of the

attacks of the colorless corpuscles. This happens very frequently in the

tonsils, so that persons who have infected tonsils may have pus

formation going on month in and month out. This may also take place at

the roots of the teeth. In fact it is now commonly believed that

whenever the nerve to the tooth dies, infection is certain sooner or

later to become established at the tip of the root. These places where

pus formation is going on continually are known as “pus pockets.” For a

long time very little attention was paid to them. Persons occasionally

suffered acute distress from gumboils or had attacks of sore throat

owing to the infected condition of the tonsils, but beyond these no

particular attention was paid to the pus-forming organisms, unless, as

occasionally happened, an especially virulent type became established

which brought on the condition commonly known as blood poisoning. Within

the last few years the discovery has been made that there is a steady

discharge of poison from every pus pocket into the body fluids and so to

the blood stream. The amount is ordinarily so small that the body as a

whole is not seriously affected, but now and then, either because of a

larger production of poison or because of a lower resistance on the part

of the body, serious ill effects are produced. Among them may be

mentioned acute (inflammatory) rheumatism. This is not only an extremely

painful condition, but it is very likely to leave serious after-effects,

as, for example, injured heart valves, to give trouble for the rest of

life. The discovery of these facts has given us great respect for pus

pockets, so we no longer treat them carelessly. Infected tonsils are

looked after, not simply because they bring about sore throat now and

then, but even more because of the poison which they are likely to send

through the body. It is probably not too much to say that the practice

of dentistry has been revolutionized since the significance of pus

pockets has been discovered. Formerly the killing of nerves to relieve

aching teeth or to insure them against future aching was a common

practice. The modern dentist, on the other hand, kills nerves to teeth

with the very greatest reluctance and only as a last resort, because he

knows full well that in so doing he is opening the way for the

establishment of pus pockets with the train of ills that is likely to

follow. At the present time methods of curing pus pockets at the base of

the teeth are not very adequate, so that the extraction of the tooth has

to be resorted to, but there is every reason to believe that the dental

profession will shortly find methods by which pus pockets can be

controlled without having to sacrifice the teeth.

The third kind of structures which are present in the blood stream are

much smaller than the red corpuscles, but are nowhere near so numerous.

They are called the _platelets_ and are disk-shaped bodies composed

chiefly of protein material, and probably, although not certainly,

living cells. Their presence in the blood remained unsuspected up to

about the end of the last century, not so much because of their very

small size, as because they go to pieces very quickly after the blood is

shed. By the time a drop of blood could be gotten under the microscope

they would be all gone. They were discovered only as the result of the

adoption of a method of treating the blood which preserved them long

enough so that they would still be present when the blood was looked at

under the microscope. They seem to have something to do with the changes

that take place in the blood by which it is caused to clot. Whether this

is their only function, we do not know.

We have now described the substances which are present in the blood and

in the other tissue fluids as a fairly permanent part of their make-up.

In addition to these there are present all the materials that are in

transit to the cells or from them. These include all kinds of foodstuffs

on their way from the digestive organs to the cells. As we shall see in

detail later, the digestive organs work over the food that we eat before

passing it on to the blood, so that the actual food materials that are

being transported by the blood are the digested products of the food

rather than the food itself. For example, there are to be found in the

blood, in addition to the regular blood proteins which were described a

moment ago, the digestion products of the food proteins on their way to

serve the needs of the various cells of the body. There will be found

also the digestion products of the other classes of foodstuffs. The body

fluids contain also the waste products of cell metabolism on the way to

be discharged from the body and the special products, such as the

hormones, which are manufactured by certain cells and carried through

the blood stream to act upon other cells. Of course we realize that not

all of the materials that are manufactured by cells are poured out into

the blood stream. Such materials as saliva, gastric juice and the like

are passed directly from the cells in which they are manufactured into

tubes by which they are conveyed to the digestive canal, where they

carry on their special work of digesting the food.

Among the things which are present in the body fluids should be

mentioned the two gases oxygen and carbon dioxide. We should expect

oxygen to be present in the body fluids, because it is necessary for the

metabolism of cells and can get to them only by being carried in the

blood stream. We have seen in the red corpuscles the special method by

which an abundance of oxygen is transported. Carbon dioxide is the

gaseous product of the oxidation of carbon and is found in large amounts

wherever there is burning, since carbon is the chief constituent of all

fuel and whenever carbon is burned, carbon dioxide is formed. Since the

fuel materials that are burned in the living cells consist largely of

carbon, carbon dioxide is produced in them as well. They have to get rid

of it, and the only way they can do so is by passing it out into the

fluids that surround them, which in turn pass it on to the blood. The

way in which the body handles these two gases makes up a special

subdivision of the subject of physiology and will be so treated in the

chapter on respiration.

CHAPTER XIV

THE CONVEYER SYSTEM OF THE BODY

In the last chapter we talked about the body fluids and saw that they

can be subdivided into the tissue fluids, which surround the cells, and

the blood, which is inclosed in a system of pipes and which carries the

materials to and from the tissue fluids. We now have to take up this

matter of transporting the material in more detail. The first step will

be to see how materials that are in the blood get from it to the tissue

fluids, and how materials that are in the tissue fluids get from them

into the blood. Unless these interchanges can take place freely, there

is no way in which the blood can serve as a conveyer system. In order to

see how the interchanges are carried on, we shall have to look first at

some features of the construction of the system of pipes through which

the blood flows. As we all know, the large blood vessels are either

arteries or veins, the arteries being blood vessels which are carrying

blood away from the heart, and the veins vessels that are carrying blood

toward the heart. If we start with an artery and trace it through the

body, we find that it is continually giving off branches which in turn

give off smaller branches, until finally the subdivisions are so small

that we cannot trace them any further with the naked eye. In the days

before the microscope was discovered, there was a great deal of question

as to how these finest branches ended. At first no one suspected that

there was connection between the fine subdivisions of the arteries and

the fine subdivisions of the veins through which the blood could pass.

About the beginning of the seventeenth century William Harvey became

convinced that there must be fine vessels leading across from the

smallest arteries to the smallest veins and that the blood must pass

through these. He came to this conclusion without ever having seen these

small vessels, since at that time there was no microscope by which such

tiny structures could be seen. Before Harvey’s time it was supposed that

the blood ebbed and flowed in the arteries and in the veins. He showed

that the blood flows in one direction constantly, leaving the heart by

way of the arteries and coming back into it by the veins. Harvey was,

therefore, the discoverer of the circulation, one of the most important

discoveries in physiology. After the microscope was perfected the tiny

tubes connecting the finest arteries with the finest veins were made

out. They were found to be very small in diameter and to have very thin

and delicate walls. They were also found to be extremely numerous. The

finest subdivisions of the arteries that can be seen with the naked eye

are scattered very thickly through all the tissues of the body which

have a blood supply, and they go on subdividing microscopically, so that

the finest vessels are scattered more and more thickly through the mass

of living substance. The very finest of all, which are the tubes

connecting the smallest arteries with the smallest veins, are called

_capillaries_, from a Latin word meaning a hair, to indicate their very

small size. They are so close together in most parts of the body that it

would be difficult to thrust a

[Illustration: Photo, Paul Thompson

THE TEST FOR BLOOD PRESSURE]

[Illustration: Photo, Paul Thompson

PART OF PROCESS OF URINALYSIS INDICAN TEST]

[Illustration: A NETWORK OF CAPILLARIES

The artery _a_ and vein _v_ (highly magnified). (From “The Human

Mechanism,” Hough and Sedgwick)]

pin in anywhere for any distance without striking against one or more of

them. The capillaries are spread so thickly that there are not many

places in the body where living cells are more than a very small

fraction of an inch from one of them. The cells do not, however, lie

right against the capillaries, but are separated from them by tiny

spaces filled with tissue fluid. In order for material to get from the

blood to any living cell, then, it must pass through the wall of the

capillary into the fluid which fills the tissue space and from that in

turn to the cell itself. The wall of the capillaries is so delicate that

if the blood flowing through any capillary contains more of any

substance than is present in the tissue fluid surrounding that

capillary, some of it will pass out through the wall and into the

tissue fluid, just about as freely as though there were no wall there at

all. The arrangement can be illustrated by a familiar example; if a drop

of ink is allowed to fall into a glass of water, it will color only a

small part of the water at first, but quickly spreads out until each

part of the water is as deeply colored as any other part. If the glass

of water were to be divided in half by a very delicate membrane, and the

ink dropped in on one side, it would spread out in the same way, passing

through the membrane in so doing, until again all the water in the glass

was equally colored. Of course, the quickness with which the ink could

pass through the membrane would depend on how delicate the membrane was.

We could imagine membranes which would not let any ink at all through,

and every degree from that up to membranes so delicate as to offer no

obstruction at all to the passage of the ink. The walls of the

capillaries rank as membranes of such delicacy as apparently to offer

almost no obstruction to the passage of materials through them. They

hold back the red corpuscles and the platelets fairly well, so that they

do not pass out of the blood and into the tissue spaces, unless the

capillaries are actually injured. The colorless corpuscles are able to

make their way through the capillary walls and so also do nearly all the

substances that are dissolved in the blood. It is an interesting fact

that the blood proteins do not pass freely through the capillary walls,

although the digestion products of food proteins, do. It will be

remembered that in the chapter on Body Fluids the importance of the

sticky quality of the blood proteins was spoken of. It is now believed

that it is because of this gelatinous nature that the blood proteins

are not able to pass out through the capillary walls, and this is

supposed to be important in the proper working of the circulation. In

fact there is a condition of greatly lowered vitality to which the name

“shock” is applied, in which the blood proteins escape through the

capillary walls to so great an extent as to interfere with the proper

working of the body. It has been found possible to prevent this in a

very large measure by the simple expedient of injecting some substance

like mucilage directly into the blood stream. We are to think of the

capillary walls, then, as allowing materials to pass freely through them

in either direction, from the blood into the tissue spaces or from the

tissue spaces into the blood, with the exception of the red corpuscles,

platelets, and the blood proteins, and as thus keeping the tissue fluids

supplied with whatever materials the blood contains or taking from the

tissue fluids the waste products of cell metabolism, which the cells are

pouring out. With this arrangement clearly in mind, all that remains for

the understanding of the conveyer system is to see how the blood is kept

in motion and distributed among the various organs of the body, and then

to consider where the blood in turn gets its supplies of materials which

it can pass on to the tissue fluids, or how it gets rid of the

substances which the tissue fluids have passed on to it from the cells.

At the beginning of the chapter we said something about the arteries and

veins and their branching into smaller and smaller subdivisions with the

final connecting link between the smallest arteries and the smallest

veins in the form of capillaries. We are now to consider in detail the

movement of the blood through these tubes, and to do that it will be

necessary to speak again of this arrangement. In describing the

circulation we usually begin with the heart. The heart itself will be

taken up presently. First let us trace the blood vessels from the heart

through the body and back to it again. The large main artery leading

from the heart is known as the _aorta_. This springs from the upper side

of the heart, bends over in an arch, and passes down through the chest

into the abdomen.

[Illustration: THE HEART AND BLOOD VESSELS DIAGRAMMATICALLY REPRESENTED

_L_, lung; _M_, intestine; _P_, liver; dotted lines lymphatics.

(Martin’s “Human Body”)]

Branches are given off from the aorta all along its length. The very

first of these come off before the aorta gets away from the heart, and

are the arteries by which the tissues of the heart itself are supplied

with blood. A little farther on are large arteries, one for the left arm

with a large branch running up the left side of the neck, another for

the right arm with a large branch running up the right side of the

neck. Each of these in turn gives off branches all along to provide for

the tissues of the arms, neck, and head. It is worth while noting that

the arteries running up the neck to the head are large in proportion to

the size of the head itself; this is because the brain, as the most

important organ in the body, requires and receives a disproportionately

large blood supply. Besides the brain the head contains numerous muscles

and also the salivary and tear glands, all of which carry on active

metabolism and therefore require abundant blood supply. The main

branches of the aorta to the head and arms are given off from the arch;

as the aorta passes down through the chest it gives off small branches

to the muscles of the chest wall, and then passes into the abdomen. Here

are located two of the three important arrangements for renewing the

blood; namely, the digestive organs and the organs of excretion

(kidneys). Large branches from the aorta pass to the digestive organs

and others to the kidneys; smaller branches lead to the muscles of the

abdominal wall, and also to the various secreting glands that are

located in the abdomen. At the lower end of the abdomen the aorta

divides, giving one large branch for each leg. As we have already seen,

if we follow any of these subdivisions through its finer and finer

branchings, we shall finally be able, with the aid of a microscope, to

trace it to capillaries, where the interchanges between blood and tissue

fluid occur, and beyond the capillaries to tiny veins which unite with

other tiny veins from other capillaries into larger veins. These again

continue to come together into main veins corresponding in every part of

the body with the main arteries. All these veins finally unite into two,

one for the lower part of the body, called the _inferior vena cava_,

and one for the upper part of the body, called the _superior vena cava_.

These two come together just at the entrance to the heart. One special

feature of the blood supply to the digestive organs may as well be

mentioned here; it is that the blood which flows through the capillaries

of the stomach and intestines is all reassembled into a vein known as

the _portal vein_, which instead of passing directly into the inferior

vena cava goes first to the liver, where the vein breaks up into another

set of capillaries, the liver capillaries, beyond which is another vein

which leads into the inferior vena cava. The result of this arrangement

is that all blood passing into the capillaries of the stomach or

intestines is obliged to pass again through the capillaries of the liver

before going on into the main stream of the circulation. This is an

important feature of the renewal of the food supplies of the blood.

We have now traced the blood from the heart through the body back to the

heart again, and have seen how in its course some of it will pass

through such active tissues as muscles or brain or glands, so that the

interchanges can go on by which the fluids in these active tissues can

take up needed materials and give off wastes. Also a part of it flows

through the digestive organs, where food materials can be taken up, and

another part flows into the kidneys, where wastes can be gotten rid of.

This leaves us to consider only the passage of the blood through the

lungs, where the supply of oxygen is to be taken up and the gaseous

waste product, carbon dioxide, is to be disposed of.

The most urgent requirement of the body is the requirement for oxygen.

There is under ordinary circumstances at all times some surplus of food

materials stored in the cells, so that even though the renewal of their

surrounding fluids from the blood should stop, they could keep on going

for a time on the material that is stored within them; but there is no

such storage of oxygen. The cells in the body lead almost a

hand-to-mouth existence so far as their oxygen supply is concerned. They

are constantly withdrawing oxygen from the tissue fluids surrounding

them, and these fluids are just as constantly withdrawing it from the

blood; therefore any failure of the blood to be properly supplied with

oxygen results very promptly in a condition of oxygen hunger in the

cells. This means prompt cutting off of metabolism, since metabolism is

a matter of oxidizing fuel, and oxidation cannot go on unless the oxygen

is provided. This urgent need for oxygen is met in the body by having

the arrangement for supplying it to the blood much more perfect than the

other renewal arrangements. We saw a moment ago that only part of the

whole blood stream passes through the digestive organs at any given time

and only part of the stream passes through the kidneys. The whole

stream, on the other hand, passes through the capillaries of the lungs.

This is brought about by having an arrangement whereby the combined venæ

cavæ after entering the heart communicate with an outlet in the form of

an artery leading to the lungs. This artery, which is called the

_pulmonary artery_, breaks up into capillaries in the lungs, which

reunite into the _pulmonary vein_ which comes back to the heart again.

It is from the pulmonary vein that connection is made with the aorta,

starting the blood on its course through the body again. We see then

that the blood passes through the heart twice in each complete round,

once as it comes in from the body at large on its way to the lungs, and

again as it comes in from the lungs on its way to the body at large.

We have spoken of the heart thus far as a single organ; it is actually

two hearts side by side, and these would work just as well if they were

at a distance instead of being built into one organ. There has probably

been more misunderstanding of the heart by people in general than of any

of the other parts of the body. This is because from the earliest times

the heart has been looked upon as the seat of the affections, and so

powers and properties have been attributed to it to which it is not at

all entitled. As we tried to make perfectly clear in a former chapter

all intelligence and all feelings are located in the brain; the heart

cannot possibly take any more active part in these than can the stomach,

liver, kidneys or any of the other parts of the body which are concerned

with the maintaining of the tissues in good working order. Probably no

one really knows how it came about originally that the heart was endowed

with these peculiar gifts. It is true that in time of strong emotion

there are changes in the activity of the heart which we can perceive.

These occur because the heart is under the same kind of nervous control

as are the smooth muscles and glands, and shares with them in the

disturbances which accompany emotion; but the real seat of these

emotions is, of course, the brain. As a matter of fact the heart is

nothing but a muscular pump whose sole function is to keep the blood in

motion. From what has already been said it is clear that the heart

cannot relax its activity for more than an instant without disastrous

results; the pressing need of the tissues for oxygen requires that the

blood be kept moving. If there were any other way in which the needs of

the cells could be supplied except through the movement of the blood,

the heart could be dispensed with perfectly well. We have emphasized

this about the heart because it is much easier to understand its working

if one thinks of it simply as a pumping organ than if one is attributing

to it mystic functions connected with our higher emotions.

[Illustration: DIAGRAM SHOWING THE RELATION OF THE TWO HALVES OF THE

HEART

_ra_ and _rv_, right auricle and ventricle; _la_ and _lv_, left auricle

and ventricle; _ao_, aorta; _vc_, venæ cavæ; _pa_, pulmonary artery;

_pc_, pulmonary capillaries; _pv_, pulmonary vein. (Martin’s “Human

Body.”)]

We showed a moment ago that the heart is really a double pump. The

relation of the two halves is shown in the diagram. One of the two

pumps, that on the right side of the heart, receives the blood from the

body at large and pumps it out into the pulmonary artery and through

the capillaries of the lungs; the pump on the left side of the heart

receives the blood from the lungs through the pulmonary vein and pumps

it out into the aorta and so through all the other capillaries of the

body. Since the circuit of the body is much more extensive than the

circuit of the lungs, the work of pumping is correspondingly greater,

and we find the left part of the heart a much more powerful pump than

the right. The heart operates as a _reciprocating pump_, by which we

mean that it alternately fills and empties. In this respect it is like

ordinary pumps except those of the rotary variety. Any reciprocating

pump must have a chamber which will fill and which can then be emptied

forcibly. In order that it shall not empty itself back through the pipe

from which it filled there must be a valve in the intake pipe which

shall close as the pump is being emptied. If, as is the case in the

heart, it is emptying itself into a system which permits backflow, there

must be another valve in the outlet pipe to prevent the fluid that has

been expelled from running back in. Each of the heart pumps consists,

then, of a chamber, which alternately fills and empties itself, and two

valves, one on the intake and one on the outflow side. In ordinary pumps

the forcible emptying is performed by a piston which moves through the

pump chamber expelling the liquid ahead of it and then has to draw back,

making room for the chamber to fill again. In the heart the forcible

emptying is accomplished by muscular action. The wall of the heart

consists of a great many muscle fibers so arranged that when they

contract they pull the walls of the heart together, making the cavity

smaller, or even obliterating it completely. The contraction of these

fibers makes up what we are familiar with as the beat of the heart. The

frequency with which they contract varies a good deal in different

individuals. The average is about seventy-two a minute; but it may be as

slow as forty-eight or fifty, or may run up to one hundred and forty or

one hundred and fifty a minute. Whatever the rate, in every case there

is an alternation of contraction and relaxation; during the relaxation

the cavity is filling with blood through the intake valve, the outflow

valve being closed, so that no blood that has once been pumped out can

rush back in again. By the contraction of the muscles the heart is

emptied, the outflow valve being open, and the intake valve being closed

to prevent an escape of blood backward into the veins through which it

flowed in. The part of the heart that carries on this active pumping

work is known as the _ventricle_; that on the right side, which receives

the blood from the body and pumps it to the lungs, is the right

ventricle; and that on the left side, which receives the blood from the

lungs and pumps it to the body, is the left ventricle.

In addition to the ventricles, which are the active pumps, each side of

the heart has an additional chamber known as the _auricle_, whose

purpose is to serve as a reservoir into which blood can flow during the

time that the ventricles are emptying themselves. If it were not for the

auricles, the movement of blood into the heart would have to stop with

every beat, because while the ventricles are contracting the intake

valves are closed and there would be no place to which blood could flow,

but since each side has its auricle, the flow of blood goes on during

the beat of the ventricles, the auricles filling up. The intake valve,

in order to operate properly, should be located between the auricle and

ventricle, and this is where it is. The vein opens directly into the

auricle without any valve between; the auricle opens into the ventricle

with the intake valve at the point of junction. The intake valves are

given rather formidable names; they are sometimes spoken of as the

_auriculo-ventricular valves_; that on the right side of the heart

between the right auricle and the right ventricle is composed of three

flaps of membrane, and has therefore been named the _tricuspid valve_.

The intake valve on the left side of the heart, which is composed of but

two flaps, is known as the _mitral valve_. As soon as the beat of the

ventricles is over and the ventricular muscle relaxes, the blood which

has accumulated in the auricles presses the intake valve open and blood

begins to flow through it directly into the ventricle. Both the intake

and outflow valves are composed of stout but thin sheets of membrane, so

that very little pressure is required to operate them. The weight of the

blood that is accumulated in the auricles during the beat of the

ventricle is more than sufficient to force the valve open and allow the

blood to flow on through into the ventricle. In a heart that is beating

seventy-two times a minute, there cannot be much time occupied either in

filling or emptying. As a matter of fact both these intervals are

measured in tenths of seconds. If we take a heart that is beating at the

average rate of seventy-two times a minute the whole of a single beat

amounts to eight-tenths of a second. The beat of the ventricle takes

about three-tenths of a second or three-eighths of the whole time; the

period of relaxation of the ventricle, during which it is filling with

blood through the open intake valve, is about five-tenths of a second or

five-eighths of the whole time. The movement of blood is rapid enough

so that this five-tenths of a second allows the ventricle to fill; in

fact much less time than this is required, for in a heart that is

beating at twice the average rate, the ventricle still fills with blood

between beats.

A word remains to be said about the beat of the auricle. During most of

the period when the ventricle is relaxed the auricle is also quiet and

blood is pouring directly through it from the veins into the ventricle;

but just an instant before the ventricular beat begins, one-tenth of a

second to be exact, the auricle contracts, emptying what blood it

contains into the nearly filled ventricle; thus, when the ventricle

beats, which it does immediately, closing the intake valve at the same

time, the auricle is empty and so is able to accommodate the inflow of

blood from the vein during the three-tenths of a second that the intake

valve is shut. Both sides of the heart work exactly together, the two

auricles beating simultaneously, and the two ventricles. Of course it is

necessary that this be so, for if they did not keep pace exactly, one

with the other, there would be a piling up of the blood either in the

lungs or in the veins leading from the body to the heart, and the

efficiency of the circulation would be seriously impaired.

We can get a good deal of information about the way our hearts are

behaving simply by holding the hand against the chest directly over the

heart or by pressing the ear against the chest of some one else and

listening to the heart’s action. The physician makes use of a

stethoscope, which is merely an apparatus for conducting clearly the

sounds which the heart makes, so that it is not necessary to apply the

ear to the chest. When one listens thus to the heart he finds that with

every beat there are two distinct sounds: the first is a rather dull

sound which comes just at the beginning of the beat of the ventricle,

the second is a sharp sound occurring just at the end of the ventricular

beat. As we saw in Chapter IX, sound is always the result of vibrations,

and a great deal of study has been devoted to an attempt to find out

where the vibrations come from that cause the heart sounds. It is now

generally believed that the first sound is partly the result of

vibrations set up in the contracting heart muscle and partly due to

vibrations from the sharp closing of the intake valves. The second sound

is known to be wholly due to the sudden closing of the outflow valves.

The sounds are chiefly of importance in that they enable the physician

to determine whether the valves are holding tight or whether there is a

leakage of blood through them. In case the intake valve leaks, there

will be a backward jet of blood from the ventricle into the auricle with

every beat of the heart. This will cause a sort of hissing or murmur

which can be heard with the stethoscope in connection with the first

sound. If the outflow valve is the one that leaks, blood will squirt

back into the ventricle from the aorta, while the ventricle is relaxing.

The murmur in this case will come just after the second sound. The

skillful physician by comparing the loudness of the murmur when the

stethoscope is pressed at different points on the chest and back can

determine whether the leaky valves are on the right side or the left

side. Thus an accurate diagnosis of imperfect valves can be obtained. Of

course the heart will not work well if its valves are not tight any more

than will an ordinary pump, so that persons suffering from this trouble

cannot have as good a circulation as those whose valves have nothing

the matter with them. It is true that in most cases of imperfect valve

action there is a compensation in the form of an increase in the size

and strength of the heart muscle, so that the circulation is maintained

in spite of poor valve action by harder work on the part of the heart.

It is evident that in a case of this kind exceptional strains on the

heart are more dangerous than if the heart is normal to begin with, so

that persons with faulty valve action must avoid physical strains, such

as sharp running after street cars or trains, which would be borne with

impunity by ordinary individuals. Since faulty valves are a frequent

result of acute rheumatism, which in turn comes from pus pockets, and

since no way is known to cure a defective valve, once the trouble has

developed, it is evident that prevention is of the utmost importance.

Physical efficiency is very seriously hampered by poor heart action.

One feature of the heart action with which we are all perfectly familiar

is that both the rate and the vigor of the heartbeat vary greatly from

time to time. When one is lying quietly, the heartbeat is at its lowest

point. It becomes more rapid as one sits up, still more rapid upon

standing, increasing still more with the taking of any form of muscular

exercise, and in vigorous muscular exercise attains its greatest

rapidity and force. The rate in this latter case may be fully double

that of the quiet standing position, and, as the vigorous thumping tells

us, the force is also very much increased. As we saw in Chapter VII the

heart muscle works automatically, contracting and relaxing without being

stimulated through the nervous system. The _variations_ in rate and

force, however, are the result of nervous action. The heart muscle, as

we have already seen, is under the same sort of nervous control as the

smooth muscles and glands. It has passing to it two sets of nerves, one

to slow it down, the other to speed it up. Both these sets of nerves

arise from centers in the brain stem, and both these centers appear to

be discharging continuously. So it works out that the heart muscle is

under the constant influence of two opposing sets of nerves, and its

actual rate and vigor depend upon the balance between them. This has the

effect of making the heart extremely responsive to nervous influences.

The slightest relaxation on the part of the nerves whose function is to

cause slowing will lead to a prompt increase of rate, since the nerves

that tend to cause increase are active all the time. Various things may

bring about changes in the nervous balance governing the heart; chief of

these are muscular activity and emotional disturbance. Practically all

the changes in the heart action that we observe from moment to moment

can be explained as being due to one or the other of these causes. There

are, however, two additional points to be noted briefly; the first is

that after muscular exercise the heart slows down very gradually, not

returning to its ordinary resting rate for a half hour to an hour after

the exercise is over, depending on how long the exercise was kept up.

The explanation of this long-continued rapid beat is found in the great

outpouring of waste products as the result of the exercise. We have

already learned that the functional metabolism of muscular work involves

the oxidation of a large amount of energy-yielding material and

therefore brings about the production of large amounts of oxidation

products. Their presence in the blood serves as a stimulus to

[Illustration: THE MICROSCOPE IS NECESSARY TO UNDERSTAND THE STRUCTURE

OF THE BODY

In this laboratory the workers are examining blood smears]

[Illustration: Photo, Cornell University Medical School

MICROPHOTOGRAPH OF BRAIN TISSUE]

the nerve center in the brain stem, which acts to quicken the heart, and

this keeps the rapid beat going until these products are gradually

gotten rid of from the body. Another somewhat similar case is the

prolonged rapid heartbeat following a violent emotion. The explanation

of this we saw a couple of chapters ago in the outpouring of adrenalin

that accompanies emotion. One property of adrenalin, as already noted,

is to quicken the heart; so, as long as any adrenalin remains in the

blood stream, the heartbeat will be faster than normal.

In the above paragraphs we have tried to make clear that the blood is

kept in motion through the body by the work of the heart, and that the

heart’s activity varies in accordance with the needs of the body; in

muscular exercise there is a great increase in metabolism, which means a

greatly increased demand both for food supplies and for oxygen. To meet

this increased demand it is necessary that the blood circulate more

abundantly, and in the automatic speeding up of the heart through the

nervous system we have the means by which this is done. In the case of

strong emotion, as already emphasized, the bodily reactions are such as

put the body into the best possible condition for meeting the emergency.

Evidently a quickened heartbeat, by insuring abundant supplies of oxygen

and of foodstuffs, contributes to this end. The slowing of the

heartbeat, when one lies down, is evidently helpful in enabling the

heart itself to recover from any strains that may have been put upon it.

The heart is a muscle, and like any other muscle carries on its

functional metabolism, which means that it is oxidizing fuel materials

and producing waste products. Since it is absolutely necessary that the

heart go on beating regularly year in and year out for perhaps eighty

or a hundred years, any relief from activity that it can get by slowing

down during sleep is evidently an advantage. It has been calculated that

the heart muscle really enjoys an “eight-hour day,” by which is meant

that on the average the functional metabolism of contraction is going on

in heart muscle only about one-third of the time, eight hours out of

each twenty-four. During the active waking time the metabolism takes a

larger percentage than that, but during sleep enough less to even up.

The heart empties itself into the large arteries; the left heart into

the aorta, the right heart into the pulmonary artery. Both these

arteries, as well as their subdivisions, are highly elastic. The very

best quality of rubber tubing is not superior to our arteries as samples

of elastic tubes. The blood, as we have already seen, is quite sticky,

and the capillaries through which it must pass in its course around the

body are microscopically tiny. The heart pumps the blood out of itself

at the rate of four or five quarts a minute or more, according to

whether it is working moderately or at high speed. To force this amount

of the sticky blood through the tiny capillaries evidently requires very

considerable force. As a matter of fact, the force is sufficient so that

if it were applied to working a fountain it would force a jet to the

height of nearly eight feet. Evidently pumping blood into elastic

arteries with this force and against the resistance offered by the tiny

capillaries causes the arteries themselves to be not only filled but

overfilled, so that their walls are greatly stretched. This fact, that

our arteries are elastic and are kept on the stretch by the pressure of

the blood within them, is of the very greatest importance to the proper

flow of blood and this in turn is so important to our well-being that

some of our most serious chronic diseases are traceable to the loss of

elasticity on the part of the arteries.

We must remember that once every second, or oftener, the heart is

shooting a jet of blood into the large artery which is already stretched

with blood and which can empty itself only through the tiny capillaries

at the tips of its finest subdivisions. On account of the inertia of the

blood stream, room is made for this additional jet of blood by

stretching the arteries near the heart more than they were stretched

before. The result is that there is an inequality in the amount to which

the arteries are stretched, those near the heart being stretched more

than those farther along. As quickly as possible this inequality of

stretch will be equalized by a spreading of the additional tension out

over all the arteries in the form of a wave. This wave makes up what we

know as the pulse. It can be felt in any artery that is near enough to

the surface so that the finger tips can press upon it. The radial artery

at the wrist is the one commonly used by physicians for feeling it.

There is a large artery in the neck in which the pulse can also be felt

readily, and if one takes the pulse of another person in the neck with

one hand and in the wrist with the other he can easily satisfy himself

that the pulse in the neck always comes an instant earlier than that in

the wrist. This is simply because the pulse spreads from the heart as a

wave, and the distance to the neck is not so great as that to the wrist.

By the time the finest subdivisions have been reached, the tension is

equalized throughout the arterial system, and there is no more pulse.

The advantage of this is that the blood flows through the capillaries

in a steady stream and not in a series of jerks. This, in fact, is the

chief, but not the only, benefit we derive from having elastic arteries.

Since the heart operates as an intermittent pump, it is evident that

unless the arteries were elastic and so could take up the shock, the

blood would have to pass through the capillaries in a series of jerks,

exactly corresponding with the beats of the heart. There is abundant

proof, which we shall return to in a moment, that to have the blood move

through the capillaries in this jerky fashion would be disastrous.

Before taking that up, however, we wish to show that by having elastic

arteries the actual work of the heart is less than it would be if the

arteries were stiff. The reason is really very simple. As was stated a

few pages back, the heart is actually emptying itself only during

three-eighths of every beat. If the arteries were stiff tubes, and

therefore not able to take up any of the blood within themselves,

exactly as much would have to pass out through the capillaries during

this three-eighths of the beat as was pumped in by the heart. In other

words, if the heart were pumping five quarts a minute, five quarts would

have to pass through the capillaries in three-eighths of the minute

instead of having the whole minute in which to do it. Since the arteries

are actually elastic, they are able, by stretching a little more, to

make room for part of the blood and so spread the time of its passage

through the capillaries out over the whole time instead of confining it

just to the period when the ventricle is contracting. Evidently it would

take more work to pump five quarts of blood through the capillaries in

three-eighths of a minute than in a whole minute.

We measure the work of the heart by what we call blood pressure, about

which we are hearing so much these days, so that it will be well to

explain as clearly as possible just what is meant by it. The blood

pressure really means the pressure of the blood within the large

arteries. It could be measured with any ordinary pressure gauge, if it

were not for the fact that we cannot very well cut into our bodies to

apply gauges to the arteries. For this reason it has been necessary to

invent means of finding out what the blood pressure is from the outside.

The way it is done is to put a band around the arm, press this band down

upon the arm until it squeezes the large arm artery shut, and then, by

means of a suitable gauge, find out how much pressure was required. Of

course, it is necessary to be able to tell when the artery has actually

been squeezed shut, so that the determination of blood pressure in human

beings is the work of an expert. Furthermore, blood pressure, as should

be clear from what has already been said, varies with every heartbeat.

It is at its maximum the instant the heart finishes emptying itself into

the artery, and falls off steadily, reaching a minimum just before the

next beat comes. The more elastic the arteries, the less difference

there will be between the maximum and the minimum blood pressure. The

reason for this will be clear when we think that if the arteries were

entirely rigid there would have to be a very high pressure during the

time the heart was actually beating to force the blood out through the

capillaries, but that between beats the pressure would fall off to zero.

The more elastic the arteries are, the more nearly do they exert a

steady pressure on the blood within them, and so the less will be the

difference between the maximum and the minimum pressure. When the

physician determines blood pressure, he really determines both the

maximum and the minimum pressure, in order that he may be able to judge

whether or not the arteries are as elastic as they should be. High blood

pressure just by itself might not mean much more than that the heart was

beating more rapidly than it should, but a high maximum pressure and a

low minimum pressure means nonelastic arteries. This in turn means that

the blood is forced through the capillaries in jets rather than in a

steady stream, and we may judge of the importance of having a steady

flow through the capillaries when we recall the well-known medical

proverb that “a man is as old as his arteries.” It is an actual fact

that the chronological age of an individual need not have much to do

with his physical age. If his arteries continue elastic over a long

period of years, he will be physiologically youthful, while if his

arteries become rigid he will be physiologically aged, no matter how few

his actual years upon earth may have been. Unfortunately we do not know

as much as we would like to about the causes of loss of elasticity in

the arteries. It does appear, however, that self-indulgence of various

kinds is apt to lead to loss of elasticity. For example, even the

moderate use of alcohol is now generally recognized by the medical

profession as a cause of impairment of elasticity in the arteries. It is

probable that intemperance in the use of various foods and drugs leads

also to this same condition.

We have just seen that the heart is obliged to maintain high blood

pressure in order to force the blood through the tiny capillaries. It

will be clear that the actual amount of pressure will depend in part

upon how much blood is forced through in a minute and in part upon the

extent to which the capillaries offer resistance. It is a familiar law

of friction that the smaller the tube the greater will be the resistance

it will offer to the passage of liquid through it, so that if the

capillaries change in size their resistance to the flow of blood through

them is bound to vary. The walls of the capillaries are very sparsely

provided with muscle fibers, but the very finest subdivisions of the

arteries, which are really no larger in diameter than the capillaries,

have much more smooth muscle in their walls. These muscles, as we have

already seen, can by their contraction or relaxation make the tiny

vessels smaller or larger. We have examples of this in the flushing and

pallor of the skin. What we wish to do now is to show how the flow of

blood through different parts of the body is controlled by changes in

the caliber of these tiny tubes. The muscles in the vessel walls have

the double nervous control commonly found in smooth muscles, and both

sets of nerves trace back to centers in the brain stem. One of these

centers causes the muscles to contract and the vessels to become

smaller; the effect of this is, of course, to increase the resistance to

the passage of blood through them. The nervous center which brings this

about is called _vasomotor_, or, more properly, the _vasoconstrictor_

center. Vasoconstriction means literally causing contraction of the

vessels, which is exactly what this center does. Not all the blood

vessels in the body are acted upon through the vasoconstrictor center.

Those of the skin and of the abdominal organs are under its control, but

those of the skeletal muscles are not. The result of activity of this

center, then, is to make it more difficult for blood to flow through the

skin and through the abdominal organs, but the ease of flow of blood

through the muscles is not affected. Of course, it will follow

automatically that the blood stream will be diverted in a large part

from the former regions into the latter. The skin and abdomen together

make up so large a part of the whole body that marked constriction of

the blood vessels in these two regions is bound to cause a considerable

increase in blood pressure.

A fairly high blood pressure is necessary for bodily well-being, because

only thereby is the brain assured of sufficient nourishment. To see why

this is so, we have only to remember that the brain is unfavorably

situated for receiving ample supplies of blood. It is at the top of the

body, so that the influence of gravity has to be overcome in forcing

blood up to it. Also it is completely inclosed in the bony skull, which

it in turn fills so completely that there is almost no room for the

accommodation of extra blood in it. In all other parts of the body a

rise in blood pressure stretches the arteries and so leads to there

being actually more blood within them, but there is no room for the

arteries in the brain to stretch, so that the total quantity of blood in

the brain cannot vary greatly from time to time. The only way in which

an increased blood supply can be obtained is by causing the blood to

flow more rapidly. This is precisely what happens every time the blood

pressure in the body rises. Whenever the vasoconstrictor center sends

nervous discharges into the blood vessels of the skin and abdominal

organs, causing them to contract, there is a diversion of blood from

them directly into the skeletal muscles and also a rise in blood

pressure due to the increase in resistance to the circulation, which

causes blood to flow more rapidly through the brain. The net result,

then, is improvement in the blood supply to the skeletal muscles and to

the brain. So important is the blood supply to the brain that the

vasoconstrictor center discharges actively throughout the waking part of

the day. If for any cause the activity of this center diminishes, there

will be an increased circulation in the skin and in the abdominal

organs, the blood pressure will fall, the circulation through the brain

will decrease, and along with it there will be a decrease in brain

function. After this passes a certain point unconsciousness will result.

This is what happens when one faints. For some reason or other the

vasoconstrictor center becomes inactive, and the series of events just

described is set in motion. Fainting ordinarily cures itself

automatically, because when consciousness is lost, the individual falls

over; this brings his head down on the level with the rest of his body,

makes it easier for the blood to flow through it, and so in a moment or

two consciousness will be regained. It is a mistake to try to scramble

to one’s feet immediately, because until the vasoconstrictor center

recovers its ordinary activity, raising of the head above the level of

the rest of the body is bound to result in its failure to receive

sufficient blood, and so faintness will come on again. It has long been

a practice to dash cold water in the face of a fainting person. The

physiological value of this is in the sudden stimulation of the sensory

nerves in the face by which is set up a stream of nervous discharges

which will play upon the vasomotor center, and arouse it again to

activity. Almost any vigorous sensory stimulation may have the same

effect.

During sleep the vasoconstrictor center is usually not very active; the

cause of sleep is not completely understood, but one of the most

satisfactory theories regarding it is that during the period of waking

the vasoconstrictor center becomes gradually more and more fatigued, and

so requires more and more stimulation to keep it active. This

stimulation may come in part through the ordinary channels of the sense

organs and in part from the higher brain centers, as when one keeps

awake by an effort of the will. Upon going to bed sensory stimulation is

cut off to a very large extent, also the will to remain awake is no

longer present. Under these circumstances the fatigued vasoconstrictor

center is under a minimum of stimulation and tends, therefore, to lessen

its activity. The result is that the blood pressure falls and presently

the circulation through the brain drops below the level of consciousness

and the individual is asleep. During the period of sleep the fatigued

center recuperates, so that it becomes more susceptible to sensory

stimulations and, in course of time, is aroused by such stimuli as

accompany the returning day to sufficient activity to restore the brain

to consciousness. Of course it will be perceived that there are many

things about sleep which are not satisfactorily explained on this

theory; in fact no one at the present time pretends that we can account

for it completely on this basis of changes of circulation through the

brain. It is believed, however, that they have a good deal to do with it

and it is certainly true that in healthy individuals the course of sleep

follows very closely the activity of the vasoconstrictor center. Undue

wakefulness in persons not suffering from disease can nearly always be

explained on the basis of excessive activity on the part of this

center. The activity may be the result of chemical stimulation, as when

persons are kept awake by drinking coffee or strong tea in the evening,

or by the persistence of adrenalin in the blood following a period of

great excitement. Mental activity, itself, tends to keep the

vasoconstrictor center whipped up, so that one who allows his mind to

work actively during the time when he should be asleep is very apt to

find sleep refusing to come when it is desired. The center is often

stimulated from the digestive tract; gastric irritation, even though not

acute enough to be recognized as indigestion, may cause wakefulness by

arousing nervous disturbances which play upon the vasoconstrictor

center. Flatulence, namely the presence of large volumes of gas in the

digestive tract, frequently acts as a mechanical source of irritation by

which wakefulness is induced. Evidently the factors favoring healthy

sleep are the inducing of fatigue, preferably by physical exercise, the

avoidance of chemical irritation or of excitement in the hours just

before going to bed, and finally the adoption of dietary habits which

shall insure good digestion. If an individual in whom all these

precautions are combined still continues chronically wakeful, the

trouble is sufficiently deep-seated to call for competent medical

attention.

Besides the vasoconstrictor center about which we have just been talking

there is in the brain stem a center which relaxes the tension of the

muscles in the walls of the blood vessels. This is known as the

_vasodilator_ center, and it acts in opposition to the vasoconstrictor.

In most parts of the body it does not appear to have any very great

importance for the simple reason that the blood is under such high

pressure that any relaxation of effort on the part of the

vasoconstrictor center leads at once to a forcing open of the blood

vessels. There are a few regions, however, in which the action of the

vasodilator center is of real importance; one of these is in the

skeletal muscles. These, as should be perfectly clear by this time, are

the seats of our most active functional metabolism. When the muscles are

active, great amounts of oxygen and food are being withdrawn from the

blood and large amounts of waste material, including carbon dioxide,

poured out into it. Only a very rapid circulation will take care of this

situation. At the same time the skeletal muscles are the most compact of

our living tissues. The cells are crowded together in making up the very

strong muscular machine by which our movements are performed. As the

muscles contract, they squeeze hard on the blood vessels passing through

them. In view of this situation it is very important that the blood

vessels be opened as widely as possible during muscular activity, and so

we find that the vasodilator center acts to improve the circulation

through the muscles, while they are active. In time of special

emergency, as we saw a moment ago, the vasoconstrictor center is at the

same time engaged in cutting down the blood supply to the skin and to

the abdominal organs, thus insuring to the muscles the maximum possible

nourishment through the blood stream.

Besides the skeletal muscles there is active functional metabolism in

the various secreting glands; a feature of their activity is that it

must be very great at certain times, but falls to little or nothing at

others; thus during the actual taking and digesting of food the various

glands which secrete digestive juices are exceedingly active, but in

the intervals they may be doing little or no work. They require a very

copious blood supply when they are functioning, but need very little

between times. The blood vessels flowing through all these glands are

under the influence of the vasodilator center, so that they are opened

as widely as possible while the glands are active. They, therefore,

receive much more blood in proportion to their size than they would if

it were not for the action of this center. Between times their blood

supply falls off to that which suffices for inactive tissues generally.

In time of emergency the action of the vasoconstrictor center upon the

vessels through these glands is such as to cut off the blood supply to

them almost completely. This is well illustrated in the dry mouth of the

frightened man, showing that the salivary glands have suspended

activity, a suspension due largely, if not wholly, to the cutting down

of the blood supply through them.

In the section just completed we have tried to give some idea of the way

in which the circulating blood provides the various tissues with the

materials they require and is adjusted automatically to meet variations

in demand from different tissues. Just one more point needs to be noted

in completing the account of the conveyer system. The tissue fluids

which serve as the connecting links between the circulating blood and

the individual cells are necessarily full of waste products, because

these come directly from the cells to the tissue fluids and afterward

pass on from them to the blood. The result is that the cells are

constantly bathed in a solution of their own waste products. There is

only one way in which relief from this condition can be obtained and

that is by moving the used tissue fluid away bodily and letting it be

replaced by fresh fluid. As a matter of fact, this happens; there is an

oozing of fluid through the walls of the capillaries from the blood into

the tissue spaces; this of course pushes the fluid already in those

spaces on ahead of it. If there were no place to which the fluid could

go, the result would be a swelling, as the tissue became more and more

filled with fluid. This is avoided ordinarily by a drainage system

whereby the tissue fluids are carried off as fast as more fluid comes in

from the blood, but when liquid is poured out faster than it will drain

off, as from a bruise or wrench, we do get a swelling.