full growth because it has been denied the amino acids necessary for

making new protein retains the power of growth, so that even though it

may be long past the ordinary age of maturity it can go on growing as

soon as the necessary materials are provided. The dependence of growth

on certain dietary accessories was spoken of in Chapter IV and need not

be repeated here.

The final use of food is as a source of energy for carrying on

metabolism. A good deal was said about that in an earlier chapter, but

there are a few additional points to be brought out here. The energy for

metabolism can come from any of the foodstuffs; these are present in the

blood stream in the form of sugar or fat or amino acids. A moment ago we

saw that in the case of the amino acids the nitrogen is removed before

they are ready for use as fuel. After this has happened the part that

remains is so similar to sugar that it can be thought of as the same

material and will be so considered by us. We have then to follow only

the two food substances, sugar and fat, through their use by the body

cells. Fat will be dealt with first since we have less to say about it.

It passes from the digestive tract into the blood stream in the form

known as an _emulsion_; all this means is that it is broken up into very

tiny particles which are kept from running together by some sort of a

film. In the case of the fats in the blood it is likely that this film

is composed of ordinary soap. In this state the fat circulates in the

blood stream until it is taken out by the tissues and burned. In the

process of this burning some very poisonous substances are likely to be

produced, but this happens only when large amounts of fat are being

burned by themselves. If sugar is present and is being burned at the

same time, there is no danger. Ordinarily in the body sugar is always

present, but in a certain disease, diabetes, about which more will be

said later, the body is not able to burn sugar as well as it ordinarily

does, and under these circumstances poisoning from the products of the

fat burning is apt to happen. This makes up, in fact, the serious danger

in diabetes. As we all know, fat is chiefly important as the form in

which food is stored in the body against a time of future need. We shall

return to the way in which the body does this after we have spoken of

the use of sugar as fuel.

Sugar is the great fuel substance of the body. About two-thirds or our

food ordinarily consists of starchy materials which are digested into

sugar. When we add to this the sugarlike remains of all the protein food

which is not actually used for the repair of tissues we see that this

substance makes up the great bulk of the material which is carried by

the blood to the tissues. This material is handled in the body in an

interesting way which depends on the curious fact that although sugar is

the most important fuel for living cells they cannot endure its presence

in them or in the fluids surrounding them except in very small amounts.

Sugar, as we all know, is very soluble in water, and it would be

perfectly possible for the blood to dissolve all the sugar that enters

it from the digestive tract and simply carry it in solution until the

tissues withdrew it for their needs; but this would mean that

immediately after every meal the percentage of sugar in the blood would

mount up to a high point from which it would gradually sink as the sugar

was taken out, to mount again after the next meal when absorption began

again. This does not actually happen because it is prevented by the

liver, which has as the most important of its many functions that of

storing the sugar that is taken up by the blood from the digestive tract

and dealing it out little by little as the cells of the body need it.

Back in the chapter on digestion we saw that all the blood that passes

through the intestinal tract is gathered up by the portal vein and

passes through the capillaries of the liver. It is during this passage

through the liver that the sugar is taken out of the blood and stored in

the form of a less easily dissolved material known as animal starch or

_glycogen_. The liver cells have the ability to convert sugar into

glycogen and they do this whenever the amount of sugar in the blood

passing through them is greater than the very small amount which is

suitable for the body cells. The blood that leaves the liver carries in

it only this small percentage. The liver cells have the ability to

change glycogen back into sugar, and this they do whenever the blood

that enters them is deficient in it, so that the blood leaving the liver

tends always to have the same amount of sugar in it. Whenever it enters

with more, there is a conversion of sugar into glycogen; whenever it

enters with less, there is a conversion of glycogen back into sugar.

So important is the protection of the body cells against having too much

sugar in the fluids surrounding them that the kidney acts to prevent

undue accumulation; this it does by withdrawing from the blood and

passing out into the urine any sugar that may be in the blood in excess

of the small amount which the tissues are able to endure. Thus we see

that if the liver did not have its function of converting the sugar into

glycogen we would have to change our eating habits completely, taking

only a little food at a time instead of eating it in three meals, since

otherwise most of our food would be wasted by being passed out from the

kidneys as fast as it was poured into the blood from the digestive

organs. There is a limit to the ability of the liver to change sugar

into glycogen. If the amount in the portal vein at any one time goes

above a certain figure, not all will be saved: a part will escape into

the blood stream, and since this will raise the percentage, the kidney

comes into action and passes it out. In order for this to happen, there

must be a very large amount of digested sugar in the small intestine

leading to rapid absorption. Since some starchy foods are easier to

digest than others some diets are more likely to result in the

appearance of sugar in the urine than others. Some of our common foods,

notably honey and corn sirup, consist largely of the kind of sugar the

body uses. These require no digestion at all, but are ready for

absorption as soon as they enter the small intestine. Naturally, if they

are eaten in any quantity, they are likely to flood the liver with sugar

beyond the amount which it can change to glycogen. Common table sugar

and the sugar of milk have to be digested before they are absorbed and

so are less likely to flood the liver. It is true, however, that either

of them if taken in very large amounts may do this. The digestion of

starch goes on much more slowly and so is absorbed more gradually and it

is doubtful whether the liver is ever flooded on a starch diet. Since

the presence of sugar in the urine is a common indication of diabetes,

it is necessary to know that other conditions may bring it about.

Obviously, as in the case of an examination for life insurance, a

perfectly healthy person might be rejected on account of the presence

of sugar in his urine, if it were not that the examining physician knows

of this other possibility and is on his guard against it.

The liver ordinarily turns glycogen back into sugar at just the rate

necessary to keep the amount in the blood constant. This means that when

functional metabolism is going on glycogen is being turned into sugar

more rapidly than when the body is quiet. One of the very interesting

discoveries of recent years is that in times of strong excitement

leading to the outpouring of adrenalin into the blood the rate of change

of glycogen into sugar is much increased, so that instead of the usual

small amount there is present in the blood a large concentration of

sugar. This is evidently advantageous in insuring ample fuel supplies to

the muscles at the time of an emergency. It is wasteful in that a large

part of the surplus sugar is passed out by the kidneys. The fact is

illustrated that in marshaling the bodily functions for meeting an

emergency economy is lost sight of.

In addition to these healthy conditions in which sugar may appear in the

urine there is the disease diabetes, in which the presence of sugar in

the urine is a conspicuous symptom. In diabetes there is a serious

disturbance of the whole fuel-supplying mechanism; the liver does not

carry on its function of changing sugar to glycogen and glycogen back to

sugar as perfectly as it should, and what is of much more importance the

muscles which are the chief users of sugar as fuel cannot use sugar in

anything like their usual manner. In fact in severe cases they appear to

be almost wholly unable to use sugar as fuel. Since the protein from

which the nitrogen has been removed classes itself with sugar in this

regard, the muscles are thrown back upon fat as their only source of

fuel, and this confronts the body with the danger already mentioned that

in the burning of fat when little or no sugar is being burned along with

it very poisonous products may be formed. Medical investigators have

devoted a vast amount of labor to the attempts to find a diet that can

be successfully eaten by diabetics. Starches and, almost equally,

proteins are not serviceable because they simply flood the tissues with

sugar, making an environment which is not good for the cells, and

keeping the kidneys busy getting rid of the surplus. Fats are dangerous

for the reason just stated. Quite recently real progress has been made

by means of the discovery that when the body is living on its own tissue

there will be no accumulation of sugar in the body fluids nor outpouring

of it from the kidneys. One who is being starved is living on his own

tissues and so by simply starving a diabetic his symptoms can be

relieved. This is not in itself a very promising expedient, since

evidently without food one cannot go on living very long. The point of

the treatment is that after starvation has proceeded until the body is

actually living on its own tissue it is possible to begin feeding

proteins cautiously until little by little a protein diet can be

established in which there is little or no indication of surplus sugar.

In other words it seems that when the body is compelled to live upon its

own tissue it uses proteins efficiently and will then go on using them

efficiently when they are supplied to it in the diet.

In an earlier section of the chapter we talked about the amount of

protein that the body requires; now we have to take up the matter of the

amounts of energy-yielding foods. It is evident that the amounts of

these depend upon the amount of metabolism; the total metabolism is made

up of the basic metabolism which is steady, shifting little day in and

day out, plus the functional metabolism which depends upon how actively

the body works. The main functional metabolism is that of the muscles

and it is that which varies from day to day. In the case of children

there is the additional metabolism of growth for which energy is

required and for which food must be eaten. In order to talk

intelligently about the use of foods for metabolism we must have a word

by which to express a definite amount of it. The word that we use for

this is “calory.” Primarily this word stands for a certain amount of

energy in the form of heat; since one kind of energy can be transformed

into another kind without changing the actual energy value we can use

this word as a measure of all kinds of energy and this has become the

custom. The calory as a unit of energy means really the amount of energy

in the form of heat required to raise the temperature of 1,000 grams of

water by 1 degree centigrade. We can translate it into more familiar

terms by stating that it equals almost exactly the amount of energy

required to raise a weight of 300 pounds to a height of 10 feet, or 30

pounds to a height of 100 feet, or any other combination of weight in

pounds multiplied by distance in feet which figures up to 3,000.

The total metabolism, as we said a moment ago, varies greatly day by day

because the extent to which we use our muscles is so different. The

basic metabolism is very steady and the functional metabolism of the

vital processes like breathing, the heartbeat, etc., is also pretty

steady, so that the metabolism for one who makes no use of his muscles

is fairly uniform. Of course it differs in large people as compared with

small, although curiously enough the difference is not proportional to

the weight but to the body surface. Just why this is so we do not know.

The average figure for the total metabolism of a resting man is about

1,900 calories a day; this includes the motions that are necessary for

eating and swallowing food, since life cannot go on indefinitely without

making those motions, but supposes that all other activities are done

away with. This figure, as we said a moment ago, holds pretty steady day

in and day out. To find the total metabolism on any day we have simply

to add to it the figure for the energy expenditure from the use of the

muscles, which will vary with the amount of work that is done. In

figuring this we have to reckon with the fact that the muscles are like

other machines in working at what is called in engineering a low

efficiency; by that we mean that the amount of energy that can be gotten

out in the form of useful work is less than the energy that is actually

consumed; there is a waste of energy which takes the form of heat. Our

muscles are under ordinary circumstances about twenty per cent

efficient, which means that for every calory-worth of actual muscular

work we do we use up five calories-worth of fuel; the energy of four

wasted calories takes the form of heat and we all know from experience

that the amount of heat thus produced inside our bodies warms us up very

quickly, when we are using our muscles actively.

The actual muscular work done in a day by individuals varies from almost

nothing in the case of invalids confined to bed through about forty

calories for a person of decidedly sedentary habits and about 120

calories for the average clerical or professional man up to from 300 to

400 in the case of manual laborers. These figures are for the actual

muscular work done; to obtain the energy expenditure we have to multiply

each of them by five on account of the inefficiency of the muscles. If

we do this and then add to each product the constant figure of 1,900 for

the metabolism of rest we obtain for the total metabolism of a decidedly

sedentary person an average of about 2,100 calories; for an average

clerical or professional man 2,500 calories, for manual laborers from

3,200 to 4,000 calories. Of course, individuals may exceed even these

latter figures. It is believed that athletes in extreme competitions

such as for example a six-day bicycle race may liberate energy at the

rate of 10,000 calories a day, although they probably cannot keep this

up long enough actually to do that amount in a single day.

In order to satisfy the requirements of metabolism the food that is

eaten must yield corresponding amounts of energy. If it does not do so

enough of the tissues will be consumed to make up the deficiency. Of

these the first to be drawn upon will be the stored glycogen in the

liver and secondly the body fat. Only when the deficiency is great

enough so that all these are used up, does the protoplasm itself begin

to be drawn upon as a source of fuel. This happens in cases of prolonged

starvation and it is interesting to note that in this case the tissues

drawn upon are the muscles. When one wastes away as the result of

starvation, the only tissues that suffer serious loss at first are the

muscles; the rest of the body is fed at their expense. In this process

no muscle cells are actually destroyed--apparently each can sacrifice a

little of its protoplasm without being itself injured; the material thus

obtained is converted into amino acids and from most of these the

nitrogen is removed, leaving a fuel material which can be burned in the

cells all over the body to keep them going. It is only after extreme

starvation, when the muscles can no longer yield of their substance

without being themselves destroyed, that the other tissues begin to show

serious wastage. This explains why the brain of a starving man remains

clear almost up to the end.

If a surplus of food is eaten over the energy requirements, the liver

will store it in the form of glycogen so far as it is able; but if this

will not suffice, the excess will be changed into fat and stored in the

body in what are called adipose tissues. These are located in various

regions, one of the most important being directly under the skin. It is

the loading of this with fat that causes the bodily enlargement of fat

people. We do not know exactly how the fat is made; we do know that it

is not ordinarily food fat that has been simply transported to these

tissues and deposited there. There is abundant proof that the body can

manufacture fat even if there is none in the diet. Grazing cows that get

no fat of any kind produce milk with its regular percentage of fat in

the form of cream and do this day in and day out, showing that the fat

that the body makes does not have to come from fat in the food. Since

body fat represents ordinarily a storage of fuel against a future need,

we ordinarily think of it as made whenever the temporary storage in the

form of glycogen becomes inadequate to take care of the surplus of food

over the amount consumed in carrying on the metabolism. Since the vast

majority of people are neither gaining nor losing weight, the amount of

food that they take in each day must balance the average metabolism.

This is interesting because the amount of food that is eaten is

regulated chiefly by the sense of hunger, or by the hunger and appetite

together, and it is remarkable that these should cause us to eat so

accurately just the amount of food our metabolism requires. In order

that we may get some idea of how much energy is furnished in our common

foods a table is given below. The figures are for the numbers of

calories in a pound of the food material as purchased in the market. In

most vegetables and meats there is a loss of about ten per cent in

preparing them for the table, or, in the case of meat, in the bones. The

figures were prepared by officials of the United States Government in

arranging dietaries for the Army.

CALORIES PER POUND AS PURCHASED

Apples, fresh 219

Bacon 2,979

Bananas 298

Beans, dried 1,603

Beef, fresh 1,009

Bread 1,300

Butter 3,478

Cabbage 124

Celery 70

Cheese, American 1,948

Chocolate 2,858

Eggs 614

Fish, salmon, canned 679

Fish, fresh 368

Flour 1,651

Lard and substitutes 4,218

Milk 302

Pork, salt 2,948

Potatoes, white 311

Rice 1,631

Sugar 1,860

Tomatoes 106

Very few of our foodstuffs are exclusively of one kind of material; that

is they are not exclusively protein or exclusively fat or exclusively of

sugar or starch compounds. Most vegetables are mixtures of starch with

protein, fruits are mixtures of starch and sugar with a little protein,

both fruits and vegetables contain so much water that their actual fuel

value per pound amounts to little. Meats are mixtures of proteins with

fats. Milk is a mixture of proteins, sugar, and fat. Table sugar and

butter are as near pure unmixed foodstuffs as any of the things we

commonly eat. In an earlier part of the chapter we saw that about

two-thirds of our ordinary diet is of starch or starchlike materials.

This figure is given, not in weight of material, but in the energy

value. What it means is that about two-thirds of the energy for our

metabolism comes from the starch and sugar that we eat; the other

one-third is divided between fat and protein in the proportion of about

two to one. The energy we get from fat being about double that which we

get from protein--since a given weight of fat has about twice the energy

value of the same weight of protein--we actually eat about the same

amount of protein as of fat. The combined weight of the starch and sugar

is between four and five times that of the protein. These, of course,

are average figures representing not what we eat at any one meal, but

the way in which the foodstuffs are found to be divided in our diet

taken as a whole. Where meals have to be planned on a large scale, as in

armies or in institutions where the persons to eat the food have not

much choice in selecting them, it is necessary that the diets be

arranged both to give the proper amounts of material and also to furnish

them in about the correct proportions. The dietary experts who have

charge of these matters do this with tables similar to the one given

above. In domestic feeding arrangements, although usually the choice of

foods is determined by the state of the markets, the preferences of the

various members of the family, and the abilities of the cook, it is

astonishing how closely the result will correspond in the long run to

the figures here given. Most of us, without any effort on our part to do

so, eat a diet which is made up week in and week out in just about the

proportions here given. The experience of ourselves and of all our

ancestors indicates that these are the correct proportions for the human

race.

In talking about metabolism thus far we have spoken as though the

metabolism of the body at rest were about the same day in and day out

regardless of conditions. This is true in the main for healthy persons;

there is one condition which may bring about a change in the resting

metabolism of health which must be mentioned, and one or two coming

under the category of disease about which also something must be said.

The change in resting metabolism that comes about in health is one that

is seen when the diet contains an especially large percentage of

protein. For some reason which we do not understand the digestion,

absorption, and utilization of protein stimulate the resting metabolism

so that during the time that this protein is being used the metabolism

is higher than at other times. The curious thing about it is that the

increase of metabolism is not just enough to take care of the protein

itself, but goes so far to cause sugars or even fats to be burned at a

more rapid rate than usual. This fact is taken advantage of in treatment

for reducing flesh; where one lays on flesh it is evident that more food

has been taken than was required for metabolism so that the surplus has

been stored in the form of fat. The only way to reduce the weight is to

compel the body to burn up that stored fat. In theory the simplest way

to do this is simply to starve. If starvation is combined with very

vigorous exercise, the reduction of weight is bound to be rapid, since

metabolism cannot be carried on without fuel, and if not enough fuel is

supplied in the form of food, the body will have to furnish it and the

stored fat is the place from which it will be taken. Unfortunately this

is much simpler in theory than it is in practice. A good deal of

discomfort and sometimes even disturbance of health results from too

drastic efforts to reduce the weight by starvation. It is perfectly

feasible to do it by adopting and sticking to a practice of never eating

quite enough. This is a perfectly successful method, but requires great

strength of will to carry it out. Probably the easiest way to reduce

weight is to combine self-denial with a diet which consists chiefly of

protein. Thus the stimulating effect upon metabolism is obtained and the

result will be a gradual burning away of the body fat. The selection of

a diet to fit any particular individual can best be made under competent

medical advice, since personal peculiarities have to be taken into

account in selecting among the various foods those best adapted for the

purpose.

The variations in resting metabolism that fall under the head of disease

are, first, the increase of metabolism in fever, about which we shall

speak in detail in the next chapter, and, secondly, variations in

metabolism that result from variations in the activity of the thyroid

gland. The thyroid gland is an organ at the front of the neck; when it

is enlarged, as it is in some people, we have the condition known as

goiter. This gland is now known to manufacture and pour out into the

blood a hormone which is a regulator of metabolism. When it is produced

in normal amounts, the metabolism goes on at the rate that we find in

healthy people. If the gland is inactive and does not secrete enough of

the hormone, there is a reduction in the metabolism. Since this implies

a lowering of the vigor of the life processes, we might expect it to

have important effects. The most marked of these are in the nervous

system. Persons whose thyroid glands are relatively inactive are

mentally sluggish; the less active the gland is, the more marked is this

sluggishness, and in case of practically complete absence of the hormone

the condition amounts to idiocy. Occasionally a child is born without an

active thyroid gland; this child is doomed for life, unless artificial

aid can be procured, not only to complete idiocy but to all the other

results of lowered metabolism; these show themselves in dwarfishness and

a misshapen body. One of the conspicuous and beneficent discoveries of

medicine in comparatively recent years has been that extracts of the

thyroid glands of meat animals, when eaten by persons whose own thyroids

are not sufficiently active, supply the lack, and so they may be

restored to the normal condition. There are many people alive to-day who

are in all respects normal, but who, if they were to discontinue taking

thyroid extract, would relapse rapidly into a condition of idiocy.

The thyroid gland may sometimes become overactive as well as

underactive; when the former happens we have an increase in the resting

metabolism and a group of symptoms that indicate, so far as the nervous

system is concerned, a condition of overexcitability. Unfortunately

this does not mean exceptional mental power but rather mental

instability. The victims of this condition are exceptionally quick

nervously, but they are quick to take offense and quick to be disturbed

by all sorts of conditions. If the overactivity of the gland becomes too

pronounced, mental instability or even marked insanity results. Another

fact of the heightened metabolism is that large amounts of food must be

eaten to carry it on. Sufferers from overactivity of the thyroid gland

eat voraciously, but, in spite of doing so, are thin or even emaciated.

They have rapid heartbeat, high blood pressure, and other symptoms

indicative of too great activity of the gland. The successful treatment

of this condition depends on the removal by surgery of enough of the

thyroid gland to reduce the outpouring of the hormone to the normal

amount. This feat is now accomplished successfully by our most skillful

surgeons, and the result has been the restoring to health and happiness

of large numbers of people whose lives were rendered miserable through

no fault of their own, but because their thyroid glands had become

unduly active. We do not know how the thyroid gland itself is

controlled; there is evidently something which causes it in the vast

majority of us to produce its hormone at the rate which keeps the

metabolism steady at what we look upon as the level of health. Deficient

thyroid is, at least in some cases, hereditary. Excessive thyroid

activity seems to be rather the secondary result of some preceding

disturbance of the nervous system, but as to that we cannot say with

confidence.

CHAPTER XIX

KEEPING WARM IN WINTER AND COOL IN SUMMER

A good deal of what has been said thus far in the book applies to nearly

all kinds of animals about as well as it does to man. We have now to

take up a feature found only in two great groups of the animal

kingdom--birds, and the four-legged animals, to which are given the name

of mammals. This peculiarity is commonly called warm-bloodedness. What

we really mean when we say that an animal is warm-blooded is that the

temperature of its body runs about the same summer and winter, day in

and day out. An animal that we call cold-blooded, on the other hand,

cools down when in a cold place, but is warm when in a warm place. It

therefore has a very variable temperature as compared with the almost

constant temperature of warm-blooded animals. The maintaining of the

warm-blooded condition, which means really the maintaining of a constant

temperature, involves only one thing, namely that the amount of heat

lost from the body shall exactly balance the amount of heat produced in

it. If more heat is produced than is lost, the temperature must rise; if

more heat is lost than is produced, the temperature must fall. These are

simple facts of physics which apply as well to the body of an animal as

to any other source of heat.

All animals produce heat, because in all of them metabolism is going on,

which means that in all of them energy is being liberated, and it is one

of the fundamental laws of physics that whenever any energy is liberated

all of it which does not take some other definite form is certain to

appear as heat. The other possible forms that the energy liberation of

the body may take are chemical energy in the manufacture of various

materials and the energy of motion. Whenever either of these processes

takes place a large amount of heat is produced in connection; this

means, from the standpoint of mechanics, as we have already seen, that

the body is an inefficient machine; if it were perfectly efficient, the

energy that it liberates might all go into the form either of motion or

of the manufacture of materials, with none left over to appear as heat;

but since it is only 20 per cent efficient, four-fifths of all the

energy that is displayed actually takes the form of heat. While this is

a marked inefficiency from the mechanical standpoint, from the

standpoint of bodily well-being it is by no means a bad thing, since it

is this constant production of heat which makes possible in birds and

mammals the maintenance of a constant and rather high temperature. That

it is a great advantage to the body to be warm all of the time is clear

when we compare the possibilities of our lives with those of

cold-blooded animals, like insects or frogs. Whenever the weather cools

down they necessarily become inactive since, as we saw in the third

chapter, when protoplasm is cooled, its metabolism necessarily slows

down, and when the cooling reaches a certain point the protoplasm

becomes completely inactive. The temperature of 98½ degrees F., which

our bodies maintain, is a temperature which is very well suited for an

active metabolism; by keeping this temperature all the year around they

are able to show this metabolism all the time instead of only in summer,

as in the case of the cold-blooded animals.

As we said a moment ago, the maintaining of a constant temperature is

altogether a matter of making the loss of heat balance the production of

heat; the production of heat is altogether a matter of the metabolism;

practically the whole of the resting metabolism takes the form of heat,

since apparently the expenditure of energy in keeping the protoplasm

alive is extremely wasteful; nearly all the energy that is actually

liberated takes the form of heat, almost none of it being used in actual

manufacture of material. This is, as has already been said, a very

constant metabolism; the functional metabolism on the other hand is

extremely variable depending on how hard we use our muscles; the total

amount of heat produced from hour to hour is scarcely ever the same

except when we are asleep; this means that the loss of heat, which has

to balance the production of heat, must vary from hour to hour exactly

parallel with the latter. In birds and in all mammals, except man, the

adjustment of heat loss to heat production is almost wholly automatic;

the animal or bird does very little to control it. We do see, however, a

few examples of effort on the part of animals either to prevent heat

from being lost too rapidly or to favor its more rapid loss. Thus, on a

cold day, a cat or dog is apt to lie curled up or with its legs bunched

under it; on a very warm day, on the other hand, it will lie stretched

out as much as possible, and in the case of the dog, with the tongue

hanging out full length. Birds sometimes are seen with feathers ruffled

up apparently in an effort to keep warm. Both birds and mammals seek

sheltered places in which to sleep, where they will be as little exposed

as possible to cold winds or rains. In man the automatic adjustment of

heat loss to heat production is very imperfect, in fact, it would not

enable men to live naked except in the tropics. It is true that outside

the arctic regions men might go naked and still make the heat production

equal the heat loss during the waking hours by very vigorous exercise,

but during the hours of sleep the loss of heat would be certain to be

much more rapid than its production and so the body temperature would

fall in severe weather enough to cause death. Man maintains himself

outside the tropics, then, by using artificial aids for maintaining the

balance between heat production and heat loss; these are of three sorts;

first the use of clothing, second the use of shelter, and third the use

of artificial heat. It is really a very curious fact when one stops to

think of it that, although many animals enjoy artificial heat and gladly

bask in it when opportunity offers, no animal has ever discovered the

simple fact that throwing sticks on a fire will keep it going, and so no

animals, except man, have ever made any real use of artificial heat.

When we think that the progress of human civilization has been

accomplished wholly in parts of the earth where clothing, shelter, and

fire are necessary to human existence, we realize that these, instead of

being mere incidents to our life, lie really at the very basis of

advancement. Hairless man is evidently a tropical animal; if he had not

devised means of maintaining himself outside the tropics, there is no

reason to suppose that he would have behaved any differently than have

the savages that inhabit those regions at the present time.

Clothing, shelter, and fire all operate to prevent us from losing heat

too rapidly and so have their great value when the weather is cool; we

also can and do bring about an increased heat production in cold weather

partly by exercising more actively and partly by increasing the amount

of protein in the diet, and so bringing into play the stimulating effect

of that substance on metabolism. This latter is seen perhaps most

strikingly in the experience of the Eskimos; when one of them comes in

from a hunting trip in the depth of winter very much chilled, he

scarcely stops to warm himself at all, but with the greatest haste gulps

huge quantities of meat that are barely thawed out, not really cooked in

our sense of the word; in fact, frequently the meat that is eaten is so

cold that it makes the Eskimo chillier than before; as soon, however, as

digestion and absorption have commenced, so that the stimulation of

metabolism can begin, the rapid production of heat in the body warms it

up to the point where complete comfort is obtained.

We have no other means of increasing heat production in our bodies

except by muscular exercise or eating protein. It is interesting to

note, however, that there is a form of involuntary muscular exercise

which comes into play when we need to produce more heat in order to

balance too rapid loss; this involuntary muscular action is the familiar

shivering which one does when chilly. It is reflex in the strict sense,

that is to say, the stimulation of cold on the skin arouses nervous

disturbances which pass to the muscles in various parts of the body and

set them into the violent movements which we call shivering. The

functional metabolism of shivering is much greater than we might at

first suppose, and the production of heat is correspondingly rapid. In

fact, if one who feels inclined to shiver encourages it instead of

attempting to keep from doing it, he will very quickly produce enough

heat within his body so that he will no longer have a disposition to

shiver.

We have seen above the various ways in which heat is produced in the

body, and the artificial means we employ to prevent heat from being lost

too rapidly. It is clear that all of these make up what we may call

coarse adjustments. They tend on the whole to cause heat to be produced

more rapidly when more is needed, or to prevent too rapid loss when

conditions are of a sort to bring it about. Nothing that has yet been

said accounts for the fine adjustment, that is, for the actual

maintaining from moment to moment of a balance of the heat loss with the

heat production. This fine adjustment is purely automatic and is carried

on in us by two distinct means. The first of these is by an automatic

regulation of the amount of blood flowing through the skin and so,

within certain limits, of the temperature of the skin. It is evident

that if the skin is warm heat will be given off from it more rapidly

than if it is cool; a condition in which the balance is being upset by

the failure of the body to lose heat rapidly enough can be corrected, at

least in part, by causing the skin to become warmer and so more heat to

be given off. The machinery for regulating the amount of blood flowing

through the skin is the vasomotor system, about which a good deal has

already been said. This system operates reflexly, stimulation of cold on

the skin tends to diminish the flow of blood through it by contracting

the blood vessels, while warmth on the skin has just the opposite

effect, causing the blood vessels to become flushed and the blood flow

to be more rapid. It is true that there is a reddening of exposed parts

of the skin in extreme cold, as we all see frequently on our nose or

ears, but this is a purely local effect and does not mean that the blood

is flowing through the region rapidly enough to keep up its temperature

and so favor the loss of heat. These changes in the amount of blood in

the skin are very effective in the fine regulation of heat loss. The

body can get rid of heat very much more effectively when the skin is

flushed than when it is pale. There are, however, limits to the

usefulness of this arrangement. Even when the skin is as pale as it can

become it still is warm, and on a cold day it continues to lose heat

more rapidly than is desirable. It is to prevent this that clothing is

worn. Clothing operates for people as fur does for animals. It

establishes a nonconducting layer between the skin and the outside. This

nonconducting layer warms up to the temperature of the body and so

hinders the escape of heat. The actual nonconducting material is the air

which is caught in the meshes of the fabric, or in the case of

fur-bearing animals among the different strands of the fur. The

effectiveness of clothing for conserving heat depends altogether on the

degree to which it imprisons air in its meshes. In this respect wool is

the most effective of all fabrics, cotton next, silk and linen having

very little effectiveness. This explains why wool is preferred for

winter clothing, linen and silk for summer.

The temperature of the body as a whole is, as we have seen, about 98½

degrees F.; this figure represents the maximum temperature that the skin

can reach, even when it is flushed to its utmost. In the warm weather of

the summertime the temperature of the air frequently mounts to or even

above that point and it is evident that under such circumstances the

body cannot give heat directly from itself to the surrounding air, and

no amount of flushing of the skin will enable it to do so. It is under

these circumstances that the second method of getting rid of heat comes

into play, namely, the evaporation of sweat. We have spoken of the sweat

glands in a previous chapter, but have left their action and the

description of the skin in which they lie for this point.

The skin serves a number of purposes; it is the great protective layer

for all the parts beneath; in order that it may serve for this it must

itself be reasonably free from injury. This is secured by having the

outer layers of the skin composed of a horny dead material. Just beneath

this dead layer is a layer of living cells tightly packed together and

having exposure to the body fluids only at their under side. Cell

multiplication goes on in these cells continuously and rapidly; as the

cells divide, they grow, the result being that those that are underneath

are constantly crowding those above them farther and farther out toward

the surface; this cuts them off from their food supplies and so they

die. This layer of dead cells becomes very closely packed and makes up

what is commonly called the horny layer of the skin or the “cuticle.”

The outer part of this cuticle is constantly rubbing off and, of

course, as this happens any dirt that may be clinging is rubbed off as

well. A large factor of cleanliness is the constant rubbing off from the

surface of the body of its outermost layer. The extent to which this

happens can be appreciated by anyone who has ever had a broken bone and

has had to have an arm or leg put up in a splint or cast for several

weeks. At the end of that time there is a great accumulation of dead

skin waiting to be rubbed off.

Bathing is a very efficient aid to cleanliness; when baths are for this

purpose, warm or hot water, with an abundance of soap, should be used;

the choice of soap is an individual matter determined largely by the

sensitiveness of the skin. Chemically there is no very great difference

between one kind of soap and another. Some of them contain materials

which are more irritating than those found in others and are to be

avoided by persons whose skins are easily irritated. The common belief

that a bath should not be taken immediately after eating rests on the

feeling that the flushing of the skin that results from the contact with

the warm water may divert blood unduly from the alimentary tract and so

interfere with the process of digestion. This may very well be true if a

very hot bath is taken; where the bath is only a degree or two above

body temperature little trouble is to be feared from this cause. A cold

bath shortly after eating sometimes gives rise to cramps either in the

stomach or in the muscles. Not much is known about these cramps or about

the causes which bring them about. In connection with its use as the

protective surface of the body the skin has scattered over it numerous

glands known as the _sebaceous glands_, which secrete an oily substance

that is spread more or less completely over the skin. This oil probably

helps to keep the skin soft and also to some degree to make the skin

waterproof. One of the reasons why soap has to be used in bathing is to

dissolve off this thin film of oil which is ordinarily over the surface

of the body.

Another use of the skin is as a great sense organ; as we have already

seen, the structures for touch, for temperature, and for pain are

present in the deeper layers of the skin. These consist of tiny cell

masses into which the tips of the sensory nerves pass. In the case of

the nerves of pain there are believed to be no special end organs, but

the nerves end nakedly among the cells of the skin.

The third use of the skin, and that with which we are particularly

concerned here, is as the fine regulator of body temperature. We have

already seen in part how the skin works in this connection, through

being warmer when there is more blood flowing through it and cooler when

there is less. Its other heat-regulating mechanism is made up of the

_sweat glands_. These, as the name implies, are tiny glands in the skin;

they consist of tubes opening on the surface and at their inner end

coiled up into a sort of knot. In this knot are a great many fine blood

vessels, so that each sweat gland has an abundant blood supply. So far

as we can tell, the sweat glands act as filters, corresponding in that

respect to the filtering tips of the kidney tubes. One interesting thing

about them is that the filtering action is controlled by nerves. These

nerves belong to the system which controls the smooth muscles and

glands; when the nerves become active the sweat glands are also active,

and there is rapid filtration of water and inorganic salts from the

blood into the sweat glands and out to the outside of the body. Small

amounts of organic materials come out also in the sweat, and these are

responsible for its characteristic odor. They evidently differ in

different people, since all dogs and some persons are able to recognize

individuals by their odor, and body odor is largely the odor of sweat.

The sweat glands are carrying on their filtering activity most if not

all of the time; we are not usually aware of it because the sweat comes

out in exceedingly fine drops which evaporate as fast as they are

formed; it is only when the sweat is poured out faster than it

evaporates that we become aware of its presence. If the air is very full

of moisture, sweat will stand on our bodies even though it is not being

formed more abundantly than usual. This is because of its inability to

evaporate into air already laden with moisture. Usually, however, when

sweat appears on the surface of the body it is because the sweat glands

have become more active. A couple of paragraphs ago we saw that the

sweat glands are under the control of nerves; these arise like the other

nerves that control smooth muscles and glands from a center in the brain

stem; this particular one is called the sweat center. It may become

active through various influences. The one most commonly arousing it is

a rise in the temperature of the blood. Whenever we begin to exercise,

heat is produced very rapidly, warming up the tissues in which the

activity is going on and, since the blood is flowing through them

rapidly, warming the blood up as well. This warm blood circulates all

through the body, tending to raise all parts of it to the temperature

of the active regions, or, as it passes through the skin, to cool by

loss of heat from the surface. When this warm blood enters the brain

stem it arouses the sweat center and an increased secretion of sweat

results. The importance of this is that the evaporation of water, no

matter at what temperature, requires a large amount of heat. This heat

is abstracted from the nearest place, which in this case would be the

body itself, so that the evaporation of sweat acts powerfully to cool

off the body. In hot weather this is really our only effective means of

getting rid of heat, for if the body is no warmer than its surroundings

it cannot lose heat directly, but sweat can evaporate, taking heat with

it, no matter how warm the surroundings may be. We said a moment ago

that a high percentage of water in the air would hamper the evaporation

of sweat. The practical workings of this fact are seen in connection

with heat prostrations in the summertime. Careful scrutiny of the

weather reports will show that these are much more numerous as well as

more severe on days of high relative humidity than on days that are

simply hot, but without much moisture in the air. As the sweat

evaporates from the body, the resulting water vapor has a tendency to

linger in the neighborhood and so to interfere with further evaporation.

Under these circumstances it is desirable that this moisture-laden air

be moved away and fresh air brought into its place. This is the benefit

we obtain from fanning. The air that is brought upon us by a fan feels

cooler than it really is; this we realize when we notice how comfortable

the draft of a fan is even on the very hottest days. When one fans

oneself in the old-fashioned way, the heat produced in working the fan

is often almost enough to balance the loss through the improved

evaporation of sweat. An electric fan, on the other hand, is a very

powerful aid to comfort in hot weather. In fact it is hard to think of

any mechanical device that has contributed more in recent years. An

amusing illustration of the importance of moisture in the air in

relation to bodily comfort is in the case of the man who thought he

would obtain a cool-weather office in hot weather by installing a block

of ice and causing his electric fan to play upon it. Very much to his

surprise and quite contrary to his expectations the room quickly became

unbearably sultry. The rapid evaporation of the ice filled the air so

full of water vapor that evaporation of sweat could not take place and

the persons in the room were much worse off than if they had contented

themselves with merely keeping the warm and comparatively dry air of the

room in rapid motion. Although the evaporation of sweat is a very

effective means of getting rid of heat from the body it does not work

perfectly in times of great muscular exertion in very hot weather.

Careful studies of men in steel industries and other places where hard

work is done in intense heat have shown that the production of heat

during the course of the working day slightly out-strips its loss, so

that by the end of the day it is the regular thing for the body

temperature to be up two or three degrees, to 100 or 101 degrees

Fahrenheit. Upon finishing the work the temperature quickly falls to

normal, so that there is no reason at present for supposing this daily

rise to be seriously harmful.

In fever the production of heat becomes greater than its loss for a time

and so the temperature goes up. It must not be forgotten that presently

the loss again balances the production; if it did not, the temperature

would keep on rising until death resulted; usually the greater loss of

heat from the body at the fever temperature is sufficient to restore the

equilibrium, but it will be clear that the temperature cannot fall again

until the heat loss exceeds the heat production. There has been a good

deal of discussion as to whether fevers are caused by an increase of

heat production or by an impairment of the machinery for getting rid of

heat. There is no doubt that in fevers the resting metabolism is much

greater than in health; it is not by any means always the case, however,

that the total metabolism of an individual lying quietly in bed with a

fever is greater than that of a healthy individual doing heavy manual

labor. We must therefore look to an interference with the process of

losing heat to account for the rise in temperature. The distribution of

blood through the skin in fever does not differ from that at other times

when the body is warm; it is the usual thing in fevers for the skin to

be much flushed. There is, on the other hand, a definite impairment of

the sweat-secreting mechanism. It is one of the most familiar facts

about fever that the skin is not only hot but very dry. It is also well

known that by inducing sweating the temperature can often be brought

down. It is evident that the poisons which are responsible for the fever

have at least two effects; to increase general resting metabolism to a

high level and at the same time to interfere with the ordinary

regulation of the sweat glands, so that, even though the temperature of

the blood is several degrees above normal, the sweat glands are not

stimulated to active production of sweat.

A fever is very uncomfortable to the person suffering from it, but is

not in itself particularly dangerous, unless it mounts to a very high

degree. The body wastes away rapidly, of course, because of the

increased metabolism which is usually not balanced by any taking of

food. Theoretically fever patients should eat as abundantly as laborers;

practically this is impossible of accomplishment in most cases because

of the depressed condition of mind and body of the patient. In some

particularly wasting diseases, like typhoid, feeding has been carried on

with some degree of success. The old practice with regard to fevers was

to restore the patient to comfort as quickly as possible by getting rid

of the fever, that is, by lowering his temperature to normal; at the

present time the tendency in medicine is not so much to strive with the

result of the poisoning as to seek to rid the body as quickly as

possible of the poison and of its source. For that reason modern

physicians are much less concerned with the degree of fever shown by the

patient than with the extent to which the poisons in his body are being

controlled or are becoming masters.

In all our discussion of the regulation of body temperature it is

important to remember that our feelings of cold or of warmth depend

altogether on the temperature of the outermost one-quarter inch of our

body surface. No matter how cold or how warm we may feel, as soon as we

get to that depth below the surface, we find the body temperature the

same, namely 98½ degrees. We feel cold when the skin is cool; the skin

is cool because not much blood is flowing through it; the stimulation of

the surrounding cold on the skin has aroused reflex vasoconstriction and

so diverted the blood out of the skin into other parts of the body.

This means that the mechanism for preventing loss of heat is working as

effectively as it can and, therefore, that the heat which the body

produces is being conserved. Experience shows that in reality the

feeling of cold is very largely a matter of the condition of the

extremities. If the hands and feet are warm, one rarely finds the cold

uncomfortable; on the other hand, cold extremities produce a very

general feeling of discomfort, even though the rest of the body may be

warm. The lesson to be drawn from this is to pay particular attention to

keeping the extremities warm. Where workmen are standing on a floor it

is much more important that the floor be warm than that the air of the

room as a whole be so. Of course, it must not be cold enough to allow

the fingers to be chilled and stiff where manual work is being done, but

it has been found that a considerably lower factory temperature is

endured when the floors are warm than when the floors are cold.

We feel cold in cold weather because the body is conserving heat; in

warm weather, on the other hand, we feel warm because the skin is

flushed and the body is losing heat as rapidly as it can. The practical

bearing of this is that it might be disastrous to feel particularly warm

in cold weather. Not, of course, if the feeling of warmth be due to a

very rapid production of heat, as in vigorous exercise, but if it is due

simply to a flushing of the skin when not much heat is being produced,

the body is being deprived of what heat it has and a condition which

cannot long be endured in cold weather will result. It happens that

exactly this situation is met with in connection with the use of

alcohol. One of the important effects of alcohol is to produce a

flushing of the skin; this is a direct drug effect and has nothing

whatever to do with the amount of heat that is being produced in the

body. The result is that one who, by a few drinks, fortifies himself,

when starting out for a long cold ride, feels very warm and comfortable

as long as the alcoholic effect persists; but as soon as this passes off

he is very much colder than he otherwise would be, since during all this

time he has been losing heat rapidly instead of conserving it. It used

formerly to be not at all uncommon in the cold parts of the country for

persons to perish from exposure on long drives because they had

attempted to keep themselves warm by taking alcohol, when if they had

conserved their bodily heat instead of wasting it for the sake of

comfort, they would probably have reached their destinations in safety.

While we are on the topic of temperature regulation and the part played

by the skin generally in bodily activities a word should be said about

bathing for other purposes than cleanliness. There is a good deal of

discussion about whether the morning cold bath is really a health

measure or is simply a fad; there can be very little doubt that persons

who react well to the cold bath find that it contributes definitely to

the enjoyment of life. It is particularly true that after a night of

disturbed rest the morning cold bath makes one feel much more ready for

the duties of the day, whether he actually is so or not. The value seems

really to be in what is known as the _reaction_ in which the first chill

is followed by a warm glow. The bath should under no circumstances be

prolonged beyond the time when a good reaction is obtained. Brisk

rubbing with a harsh towel helps the oncoming of the reaction very much.

For those who would like to take a cold bath but have found it too much

of a hardship a hint may be helpful, namely, that if one enters the bath

with the feet warm there is practically no discomfort connected with

taking it. To step into a cold bath when the feet are already cold

causes them to begin aching almost instantly and usually nullifies all

the benefits the bath affords. One should either enter the bath

immediately after rising, before there has been any time for the body to

become chilly, or if one prefers to shave or carry on other activities

first, it is a good plan to give the feet a momentary preliminary bath

in hot water. When this condition is fulfilled, the cold bath will be

both enjoyable and beneficial to the majority of people. Very hot baths

have a distinct relaxing effect; they favor the onset of sleep and can

be taken to best advantage just before retiring. There is nothing in the

common idea that a hot bath opens the pores of the skin; the only pores

the skin has are the openings of the sweat glands, and these do not vary

in size. What a hot bath does do, is to cause a marked flushing of the

skin as the result of which the body loses heat rapidly. It is

necessary, therefore, after a hot bath to wrap oneself warmly so that

excessive loss of heat shall not occur.

In finishing this topic, the subject of catching cold must be brought

up, together with its relation to the regulation of body temperature.

There is no doubt at the present time that common colds result from

infection; in that sense they are acute diseases. It seems to be the

fact, however, that the organisms which cause them are pretty constantly

present in the air passages of our throats and lungs ready to invade the

body whenever opportunity offers. Very recently it has been shown that

rapid chilling of the skin is accompanied by contraction of the blood

vessels in the linings of the throat and air passages. The effect of

thus cutting down the blood supply to these linings seems to be to make

them more ready of access by the organisms of colds. At any rate the

fact that when the skin is suddenly chilled a cold frequently develops

is best explained on this basis. Since colds are infectious, the

organisms causing them may sometimes succeed in penetrating the linings

of the air passages even when no change in the latter has occurred. It

may therefore happen, and frequently does, that one develops a cold

without being able to refer it to any time when there was a sudden

chilling of the skin. Also it can easily happen that the chilling may

not lead to a cold, since if the organisms do not happen to be abundant

or if the general bodily resistance is high, the infection will not get

a foothold, even though the linings of the air passages would permit the

organisms to enter. One’s best precaution against colds would naturally

be to avoid having the organisms which produce them present in the

throat and lungs. Since this usually cannot be done, the remaining

practical measures are to keep the general bodily health as good as

possible and to avoid conditions which lead to a sudden rapid chilling,

such as sitting in a cold draft when heated.

CHAPTER XX

THE PERPETUATION OF THE RACE

We have tried to take up one by one the chief things that happen in the

body, but thus far have emphasized their importance entirely in

connection with bodily well-being; that is, we have seen how the body

maintains itself against the competition of other living things and in a

world full of hazards. Before leaving the subject entirely, a short

account must be given of the way in which the _race_ is maintained upon

the earth as distinct from the maintenance of individuals. Early in the

book we showed that every one of us starts life as a single cell which,

by dividing and subdividing, along with continuous growth, finally

develops into our large and complicated body. This cell is really the

union of two cells; one furnished by each parent; in the body of the

parent it formed part of what is known as the _germinal tissue_, this

name being applied because the cell from which we start may be spoken of

as the germ of life. As the cell from which the body is to come begins

to develop, the cells formed from it quickly become different one from

the other, so that very early it can be distinguished that certain cells

are to form the future nervous system, others the future muscles, and so

on. One of the earliest of these groups of cells to become

distinguishable is that which is to become the germinal tissue in the

newly formed individual. A great deal of importance has been attached

of recent years to this fact that the germinal tissue is set aside

almost from the beginning of development, and when we remember that the

next generation will be derived from this germinal tissue and from no

other tissue, we see that there is a very close relationship of the

germinal tissue from generation to generation. The importance of this is

in connection with heredity, namely, with the question of the

resemblance of the child to the parent. We all know that sometimes

children resemble their parents very closely, at other times there is

almost no resemblance at all. The main fact of heredity is, as we have

just seen, that the child comes from germinal tissue of the parent and

not from the parent as a whole. We can think of an individual as a

complex body carrying within itself and nourishing an independent group

of cells which are to serve as the starting point for the next

generation, but with which the individual himself has nothing to do

except to provide nourishment. A little later we shall try to show just

why we think of the germinal tissue as so little dependent on the rest

of the body. What we want to do first is to point out how this notion

must affect our views of heredity. If the germinal tissue is simply

harbored and nourished by the body, but otherwise quite independent of

it, it follows as a matter of course that things the parent does,

provided that they do not disturb the nourishment of the germinal

tissue, can have very little effect upon it. For example, anything that

the parent may achieve during his lifetime cannot be passed on by

heredity to his offspring. This is entirely contrary to the idea of

heredity which is held by nearly everybody. Just a little over one

hundred years ago a Frenchman by the name of Lamarck published writings

in which he argued very strongly in favor of the inheritance of

“acquired characters,” meaning by this that the achievements of parents

could be and were transmitted to the offspring. It is evident that this

would be a very great advantage; the progress of the race would be

secure if the children could inherit directly the achievements of their

parents. Of course, it might work the other way around and children

might inherit unfavorable acquisitions of their parents as well as

favorable. Since the time of Lamarck there has been not only hot

discussion of his ideas but also a vast amount of experimentation to

find out whether acquired characters actually can be inherited. One of

the most famous experiments is that of a man who cut off the tails of

mice at birth and did this generation after generation in the hope of

being able to produce a race of tailless mice; after a great many

generations he abandoned the attempt with the conclusion that the

artificial removal of tails from parents would not cause the offspring

to be born in the tailless condition. More recently similar experiments

have been carried out on kinds of animals that have much shorter lives

and in which the generations are correspondingly more frequent. In some

cases thousands of generations have been studied without any indication

that changes brought about during the lifetime of the parents can be

transmitted to the offspring.

One of the first questions that is sure to be asked as soon as this

inheritance of acquired characters is denied, is how changes can then be

brought about; the answer to that question is found in the experience of

animal and plant breeders. One fact of nature which should be

emphasized, is that individuals are never exactly alike except perhaps

in the case of so-called identical twins which have a special heredity

spoken of in Chapter V. Even brothers and sisters always have pretty

marked differences. These differences among individuals are some of them

probably more or less accidental results of the way in which the germ

develops into the complete body; others are the result of differences in

the germ cells themselves. The distinction between these is that where

the germ cells are different, the difference will be transmitted by

heredity to future generations; if it is a modification that comes on in

connection with development and is not due to a difference in the germ

cell, it will not show itself in future generations. What the animal and

plant breeders have to do is to watch for changes in their stock, and

when the kinds that they are looking for appear, they breed them in the

hope that the desired features will prove to be hereditary, so that a

race can be established showing them. A very striking example of this

kind of breeding is in the development of hornless cattle. Of late years

it has been the practice to dehorn cattle regularly and this dehorning

has never caused the offspring to be born without horns, but on the

other hand it has happened occasionally that calves have been born which

were naturally hornless and since this absence of horns was due to a

difference in the germ tissue, the character was found to be hereditary

and it has been possible to establish breeds of hornless cattle. Thus by

selecting cases where a change occurred naturally in the germ tissue, a

result has been obtained which could not possibly be gotten by any

amount of work directly upon the parents.

What we have tried to make clear thus far is that heredity is absolutely

a matter of the condition of the germinal tissue. What we have now to do

is to see what the germinal tissue is like and how the process of

heredity actually goes on. In Chapter V something was said about the

nucleus; it will be remembered that the nucleus contains a substance

known as _chromatin_, which, as seen under the microscope, looks like a

tangled skein or network of fine threads, but is in reality a number of

tiny structures known as _chromosomes_. We now know that these

chromosomes are the actual controllers of heredity. Of course we do not

know at all how they exert their control; what we do know is that when

certain factors are present in the chromosomes of the germinal tissue of

the parents, the offspring derived from that germinal tissue will have

traits that would not be present if the chromosomes had been different.

Space does not permit us to tell at length how the facts that are now to

be described were discovered. They date from the gardening experiments

of an Austrian monk by the name of Mendel, who for many years grew

ordinary garden peas and studied from season to season the varieties

that appeared. The facts of heredity that he found to be true of peas,

have since been shown to apply just as well to ourselves, and since this

book deals with human beings we shall try to make the description apply

directly to human heredity. The first thing to get clearly in mind is

that every single thing about any one of us which can or does differ

from the corresponding feature in another person, may be a _hereditary

trait_, and, if it is, will have a factor controlling it somewhere among

our chromosomes. Since the number of hereditary traits is legion,

including not only the size and shape of all parts of our bodies inside

and out, but our mental peculiarities and moral tendencies as well,

there must be a huge number of controlling factors, or _determiners_, as

they are often called. As a matter of fact, there are probably not quite

so many determiners as traits, because a single determiner may govern

more than one trait. But even so, the number of determiners is too large

for comfort in trying to describe their working. The best way to go

about it is by pretending that we are not so complicated as we really

are. We shall do this by setting the number of different determiners

human beings may possess at fifty-two, not because that is anywhere near

their real number, but because it is the number of combined large and

small letters in the alphabet, and we propose to use letters to stand

for determiners, as an easy way to keep them separate in the

description.

Every cell in our bodies has its nucleus with its equipment of

chromosomes. An interesting fact already spoken of in Chapter V is that

according to our best knowledge the chromosomes in any cell of any one

of us are exactly like those in all the other cells of the same person.

The chromosomes in the muscle cells are exactly like those in the nerve

cells, and both correspond exactly with the chromosomes of the ordinary

cells in the germinal tissue. In every one of these cells the

chromosomes are arranged in pairs. It happens that in human beings the

number of pairs is twenty-four, with one pair incomplete in some of us

for a reason that will be explained presently; this is an awkwardly

large number for our present purpose, since we have allotted only

fifty-two determiners altogether, so we shall do some more pretending

and set the number of pairs at nine. One more change will have to be

made from the real state of affairs before we can go on with the

description; this is to suppose that our various bodily features can

show only one difference, instead of the many of which they are really

capable. For example, we shall suppose that the hair can be either black

or light, but never red; the nose can be Roman or Greek, but never

Irish. By making this supposition, we can let the large letters stand

for one set of hereditary determiners and the small for the same

features but with exactly contrary traits. According to this

arrangement, if we had two persons side by side one of whom had only

large-letter determiners in his chromosomes and the other only small,

they would have the same general human make-up, but in every possible

detail one would be the exact opposite of the other.

It has been proven by complicated studies which we cannot take time to

describe that the determiners are grouped in the different chromosomes

in a definite plan. We saw a moment ago that the chromosomes are in

pairs. This pairing is an essential part of the arrangement, for every

hereditary bodily feature actually has two determiners governing it,

which lie in corresponding positions in the two chromosomes of the pair.

To illustrate how this works out, let us suppose that the chromosomes of

the first pair contain determiners A, B, and C. Each of these three

determiners will be present in both members of pair number one, and if

this is true of any cell it will be true of all the cells, and in any

other human being either they or the corresponding small-letter

determiners, a, b, and c, will occupy pair one of the chromosomes.

Furthermore, they will lie in a row within the chromosomes, always in

the same order; thus if A and C are at the ends with B in the middle in

one chromosome, every other chromosome that contains these three

determiners will have them in the same order.

[Illustration: DIAGRAM ILLUSTRATING VARIOUS ARRANGEMENTS OF DETERMINERS

IN A SINGLE CHROMOSOME PAIR

1 and 2, Pure; 3, Hybrid.]

Thus far we have planned our diagrams as though only large letter

determiners were concerned; but we saw a moment ago that there is a

complete set also of small-letter determiners, which control a set of

contrary hereditary traits, and we intimated that these will sometimes

be found in the chromosomes in positions corresponding with those

occupied by the equivalent large-letter determiners. It might happen,

for example, that pair one of the chromosomes would contain large-letter

determiners A, B, and C, while pair two would contain small-letter

determiners a, b, and c. Evidently the person in whom this combination

was present would differ from one all of whose chromosomes contained

large-letter determiners, since part of his traits would be established

by small-letter determiners.

We are now ready to go back to the germinal tissue and trace the process

of heredity as it actually works out in the developing offspring. In the

ordinary cells of the germinal tissue, as we have already seen, the

chromosomes are exactly like those in the other cells of the body. Cell

multiplication goes on actively in the germinal tissue of both parents;

in the mother this leads to the production of germ cells which are

called eggs, and in the father to the production of cells that are

called sperm. During the process certain changes occur, so that neither

eggs nor sperm are exactly like the original cells of the germinal

tissue. If we look back at the description of cell division in Chapter

V, we shall recall that the chromosomes split lengthwise and are pulled

apart. Now that we have learned about determiners, we will realize that

every determiner splits in half, because otherwise there would not be an

equal distribution of determiners between the two cells. Much of the

cell division in the germinal tissue is precisely like this, but at a

certain stage in the production of both eggs and sperm there is one cell

division in which the pairs of chromosomes are simply pulled apart,

without there having been any previous lengthwise splitting. The result

of this is to leave the resulting cells with only half as many

chromosomes as the other cells of the body have. Some further changes

take place in these cells before they become ripe eggs or ripe sperm,

but there is no further disturbance of the chromosomes, so that eggs and

sperm contain only one member of each pair, instead of both members, as

do all other body cells.

The first step in development is the coming together of the egg cell

with one sperm in the process that we call fertilization. The sperm

penetrates the egg and its chromosomes line themselves up with the

egg’s, restoring the pairing that is present regularly. Immediately

afterward the cell divisions begin that make up development, and in all

of these the usual lengthwise splitting of determiners takes place.

Every cell in our body contains its pairs of chromosomes, one member of

each pair tracing back directly to the egg cell while the other traces

directly to the sperm. Thus half of our determiners came from the

maternal germ tissue and half from the paternal.

[Illustration: CHROMOSOMES IN CELL DIVISION

Figure 1, chromosome splitting in ordinary cell division, in which each

determiner splits in half, contrasts with Figure 2, reduction division,

in which the chromosomes of the pair are simply pulled apart.]

We shall now begin to see how heredity works out. Suppose chromosome one

in the egg has large-letter determiners A, B, and C, while the

corresponding chromosome in the sperm has small-letter determiners a, b,

and c. When these line up after fertilization, restoring pair one, we

have A opposite a, B opposite b, and C opposite c. Since we have

supposed the large and small letters to stand for contrary hereditary

traits we introduce here a conflict and must ask at once how it is

settled. One of the things the monk Mendel worked out in his studies of

heredity in peas was this particular problem. He found that where there

is conflicting heredity one of the determiners usually dominates over

the other, and when this happens the trait in the offspring will be like

that of the parent which contributed the dominant determiner. To

illustrate: suppose A is dominant over a, then in the case in question

the offspring will be like the mother in feature A. In some kinds of

animals and plants conflicting characters blend in the offspring,

producing an intermediate appearance. A good example of this latter case

is in the common flower, the four-o’clock. In this plant white blossoms

and red blossoms are due to determiners that occupy the same positions

in the chromosomes; therefore, if white and red flowered plants are

crossed, these conflicting determiners come together when the sperm and

egg chromosomes pair. Since neither determiner dominates over the other,

the color of the flowers in the offspring is neither white nor red, but

pink.

Any individual whose chromosome pairs contain conflicting determiners is

called a “hybrid”; there may be every degree of hybridism, from the

simplest, in which all the chromosome pairs are alike except one, up to

the most complete, in which all the pairs are unlike. It is easy to tell

a hybrid by its appearance in the cases in which there is blending

inheritance, but not so easy when one trait dominates over the other,

for then the hybrid will look like the parent that furnished the

dominant determiner. The sure way to detect hybrids is by the study of

their offspring. Suppose we have two parents, both of whom are hybrids

in respect to chromosome pair one. This pair in both will contain

determiners A, B, and C lined up opposite a, b, and c. Since we have

supposed the large letters to be dominant over the small, both will have

the same appearance, dependent on the presence in their chromosomes of

A, B, and C. Since in the formation of eggs and sperm the chromosome

pairs are pulled apart, half the eggs produced by the mother will

contain determiners A, B, and C, and the other half a, b, and c, and

half the sperm of the father will similarly contain one set and half the

other. Since it is an absolute matter of chance which sperm encounters

which egg in fertilization we can safely conclude that in the long run

all the chances will be realized equally. Calling, for convenience, the

large-letter eggs E and the small-letter e, and, similarly, the

large-letter sperm S and the small-letter s, this means that E can be

fertilized either by S or s, and e also by either S or s. The possible

combinations are ES, Es, eS, and es. In terms of actual determiners

these combinations are ABCABC, ABCabc, abcABC, and abcabc.

The combinations just given represent the possible offspring from a pair

of hybrid parents. If we look them over, we see at once that only half

of them are hybrid, namely, combinations ABCabc and abcABC; the other

half are pure breed, the chromosome pairs being exactly alike; but these

pure breeds are of two kinds, one having only large-letter determiners,

the other only small-letter. If it is a case where the large letters

dominate over the small, the large letter pure breeds and the hybrids

will look alike, but the small-letter pure breeds will look different,

since in them the traits governed by the small-letter determiners have a

chance to show themselves. A very good illustration of this is seen in

eye color in human beings. Brown eyes are dominant over blue; in other

words, the determiner that causes eyes to be brown dominates over that

responsible for blueness in cases where both come together in hybrids. A

person who has brown eyes may be either a pure breed in that respect or

may be a hybrid; there is no way to tell the difference from the

appearance; but if the brown-eyed person has offspring, and any of them

turn up with blue eyes, it is proof positive that the parent is a hybrid

so far as eye color is concerned. Moreover, the blue-eyed child is not a

hybrid in this respect; his brown-eyed brethren may or may not be; in

the long run two-thirds of them will prove so; the other third will be

pure breed, having in their chromosome pairs only brown-eye determiners.

Where the hybrids differ from the pure breeds, as in the case of the

four-o’clocks, given earlier, it is easy, of course, to tell which are

pure and which are hybrid. When pink four-o’clocks are interbred, the

chromosomes will combine just as described above for hybrids, since the

plants that have pink flowers are hybrid. One-half the offspring will

have pink flowers, showing that they are hybrid; one-fourth will have

white flowers, proving that in them the white-flower determiners have

separated out, and the other fourth will have red flowers, because in

them the red-flower determiners are the only kind present. This and

similar experiments have been tried hundreds of times, and whenever the

numbers of offspring have been great enough to allow the chances to

equalize, the proportion of different kinds of offspring has always

agreed almost exactly with expectation. Of course in human beings the

families are not large enough for this always to work out accurately,

but even so the agreement is often striking.

[Illustration: CARING FOR THE BABY’S EARS

Important for adults and babies alike]

[Illustration: THE BABY’S FOOTPRINT

A means of positive identification]

[Illustration: BABIES PHYSICALLY AND MENTALLY ACTIVE]

In practical animal breeding the blended inheritance just described is

not very useful, for even though a blended character might appear which

is just what the breeder has been looking for, it will not occur in more

than half the offspring and can not ever be depended on to show itself

in any particular individual. This explains largely why pure-bred stock

is always more desirable than hybrid, and why breeders strive so eagerly

to obtain desired traits in pure-bred animals. In plants, blended

characters are much more valuable, for two reasons; first, because the

offspring are so numerous that even though half of them come out pure,

and so lack the desired blend, there are enough left that have it to

make the crop worth while; and second, because propagation by cuttings

is possible in very many kinds of plants, which means that the same

plant is kept going in hundreds of places, and for tens or even hundreds

of years. A trait that is desirable can be perpetuated indefinitely by

this means, even though it may be a blending of several hereditary

traits, which would separate out in a few generations by ordinary means

of propagation.

There are several more things in heredity that must be taken up while we

are on the subject, so we shall have to return to the chromosomes for a

while. We have seen that there are several determiners to each

chromosome; for convenience, we assigned three apiece to our

chromosomes, except the ninth, which has to get along with two; but in

reality the number to each chromosome is often much greater. This

grouping of the determiners, several to a chromosome, carries an

interesting consequence with it, in that all the hereditary traits

controlled by one chromosome have to go together in reproduction. In

the example we have already used A, B, and C are together; therefore any

individual that shows character A must show B and C as well. The most

striking instances of this are certain traits that are bound up with

sex, but we cannot describe these further until we have looked into the

heredity of sex, which we shall do in a minute. First a word must be

said about occasional exceptions that turn up to the rule that we have

just stated. In the study of thousands of specimens now and then one has

been found in which there has evidently been a swapping about of

determiners. We can illustrate the situation by supposing chromosome one

is found to contain determiners A, b, and C, instead of A, B, and C; one

small-letter determiner has traded places with a large. Of course, the

effect of this is to permit different combinations of hereditary traits

than ordinarily occur, and at the present time students of heredity are

actively engaged in following this up to see how it happens, and what

advantage can be taken of it. This crossing over of determiners from one

chromosome to another takes place only among such as are actually in

contact at times within the nucleus as seen under the microscope, which

confines it to the members of corresponding pairs.

In man, and in many of the lower animals, sex is a hereditary character.

That means that there is a determiner for it which is grouped with other

determiners in one of the chromosomes. In man the determiner is for

femaleness; there is no special determiner for producing the male sex;

it is produced whenever the female determiner is missing from one

chromosome of the pair, and this is brought about by having the whole

chromosome that should make up this pair absent. At the beginning of

this chapter the fact was mentioned incidentally that a good many of us

have only 47 chromosomes, instead of the 48 that are characteristic of

human beings. The distribution is really almost exactly half and half,

for all males have 47 and all females 48. This means that the cells of

the germinal tissue of females have 24 complete pairs, while the

corresponding cells in males have only 23 complete pairs and one

chromosome over. This extra chromosome is the one that contains the

determiner for femaleness; each of the chromosomes of pair 24 in females

contains this determiner also. These are often spoken of as sex

chromosomes.

Now when in the course of the production of egg cells within the

mother’s germinal tissue the pairs of chromosomes are pulled apart, each

separate cell, and so each egg, will contain the full number of

chromosomes, 24, including the sex chromosome. But when the same thing

happens in the course of the production of sperm only every other one

will have the full number; the remaining half having only 23, and all of

this half lacking the chromosome that contains the determiner for

femaleness. There are, then, always equal numbers of two kinds of sperm,

one with 24 and the other with only 23 chromosomes. If the egg is

fertilized by a sperm containing 24, including the sex chromosome, the

pairing of chromosomes is complete in the egg, and the offspring will be

a female; if, on the other hand, the fertilizing sperm is one that

contains only 23 chromosomes the pairing in the egg will be incomplete;

the single sex chromosome of the egg will not be paired with a

corresponding one from the sperm and the egg will develop into a male.

Since it is a pure chance whether fertilization will be accomplished by

a sperm of 24 or one of 23 chromosomes, we should expect the sexes to

appear in exactly equal numbers, taking the world as a whole. As a

matter of fact, whenever extensive birth data have been accumulated they

have shown a very slight excess of male births over female. We are not

able to explain this at the present time. It is possible that the 23

chromosome sperms are a little more vigorous for some reason than those

that have 24, and so are able to fertilize slightly more than their

share of eggs.

We spoke a moment ago of hereditary traits whose determiners are bound

up in the sex chromosomes. All such behave interestingly in heredity for

the simple reason that they can never be transmitted from father to son,

but only from father to daughter. This is because, as we have just seen,

the sex chromosome in the sperm always causes the egg which that sperm

fertilizes to develop into a female. The single-sex chromosome which

males possess invariably comes from the mother. An interesting example

is the common type of color blindness known as Daltonism. Normal color

vision is hereditary and the determiners which establish it are in the

sex chromosomes. Occasionally a person is found in whom these

determiners are defective. If this person is a male, he will be color

blind, but if a female not, unless both sex chromosomes are defective in

this regard, since normal color vision is dominant over color blindness;

so if one sex chromosome is normal the vision will be also. The woman,

in this case, will be a hybrid with respect to color vision; one of her

sex chromosomes containing a normal determiner, the other a defective.

This works out in heredity as follows: If a color-blind man is married

to a woman who has no color blindness in her heredity, none of his

children will be color-blind because he cannot transmit the sex

chromosomes which carry the determiners for color blindness to his sons,

but only to his daughters; all the latter will be hybrid with respect to

the character, since all of them come from fertilized eggs which

received sex chromosomes from the sperm. If these daughters, in turn,

marry men who are free from color blindness, some of their sons may be

color-blind, but none of their daughters can be. The only way in which

women can be color blind through inheritance is by descent from

color-blind fathers and from mothers who are either themselves

color-blind or are hybrid with respect to the trait. The result of this

difference in the heredity of the two sexes is to make color blindness

many times as frequent among men as among women. In round numbers four

men out of every hundred have this type of color blindness, while only

six or seven women in ten thousand show it.

We have left for discussion only one topic dealing with heredity, but

this is the most baffling of all, since it deals with the problem of how

the various kinds of determiners came into existence. It is evident that

if given one parent with all large-letter determiners and the other with

all small-letter, we might, in the course of many generations, get a

great variety of combinations and so a great many different-appearing

individuals. But unless we have various kinds of determiners to start

with, there is no way in which this can be done. We do not pretend to

know very much about how the innumerable determiners that are in

existence came about, but we have one clue that is thought to point the

way. In some animals, and many plants, descendants put in their

appearance from time to time that are so different from their ancestors

as not to be accountable according to ordinary laws of heredity. These

have long been known, and the name of “sport” has been applied to them

by breeders. Since the facts about determiners have been learned, it has

been clear that these “sports” cannot have all their determiners like

those present in their parents, and it has come to be believed that

occasionally spontaneous changes take place within individual

determiners. Since the determiners are undoubtedly complex chemical

structures, we know of no reason why this might not happen. Probably it

is much more common an occurrence in some kinds of plants and animals

than in others. The name of “mutant” has been applied to the plant or

animal in which this has happened, and the process is called “mutation.”

It is important since it is the most likely way in which the innumerable

kinds of determiners that are now in existence came into being.

We suppose that since life first put in its appearance on the earth

there have been uncounted mutations, a vastly greater number than are

now represented by determiners. Many of the mutants could not compete

with their brothers and sisters of ordinary descent and so promptly

died, but occasionally it might happen that a mutant would be as well

fitted for life as its relatives, in which case it would establish

itself, and in course of time become ancestor to a whole line like

itself. If this happened often enough, and time were allowed for it to

work out, all the kinds of plants and animals that are now in existence

might have come by descent from a very few ancestors. The geological

history of the earth shows that there has been plenty of time, even

though valuable mutations did not occur oftener than once in a thousand

years.

Our description of racial perpetuation should be finished by an account

of the development of the fertilized egg. Snugly ensconced within the

body of the mother, in an organ devoted solely to the purpose, the egg

passes rapidly through the early stages, living at first on fats and

proteins stored within itself. After these are exhausted it draws

supplies from the body fluids of the mother. In the course of a few

weeks it has developed its own conveyer system, with its own beating

heart and its own stock of blood. There is never any actual mingling of

the blood of the developing child with that of the mother; capillaries

of the maternal circulation come into intimate contact with capillaries

of the circulation of the child. Here interchange of all sorts of

material goes on; food and oxygen pass from the mother’s blood to the

child’s and waste materials from the child’s blood to the mother’s.

During all this time the mother is eating, breathing, and excreting

wastes for two. She cannot bring any nervous influences to bear on the

developing child, since there is no connection between her nervous

system and the child’s; she can, however, influence it chemically

through the blood. Poisons that get into the blood of the mother can

pass from it into the blood of the child. These may be the poisons of

auto-intoxication, or drugs that the mother has taken. In either case

they may do the child harm. We do not know very much about this, but it

may be that a considerable percentage of children that are born with

abnormalities that are not hereditary come by them through chemical

influences received from the mother’s blood.

When the development of the child has gone far enough so that it can do

its own breathing, feeding, and discharging of waste materials, it is

expelled from the body of the mother in the process that we know as

birth. This does not imply by any means that parental care and

responsibility are at an end. Food, protection, and warmth must be

provided. Education must be attended to, for the nervous system of the

new-born infant is absolutely undeveloped. It has, through heredity,

certain possibilities of achievement; their realization hinges upon the

bringing to bear of worth-while influences. Upon the attainment of

maturity the child will be expected to assume his place in society, and

society has a right to the best that he is able to offer. In preparation

for this it is the duty, both of the parents and of society itself, to

provide throughout the formative years as nearly as possible the

environment best suited to the development of those traits which make

for usefulness. Environment cannot overcome the limitations of heredity,

but environment can bring out the best that is in us.

CHAPTER XXI

CHILD AND MAN

Everyone is familiar with the beguilingly helpless picture the tiny baby

presents. The disproportionately large head, with aimlessly rolling eyes

and toothless mouth, the frail and delicate limbs, waving in the air or

clutching spasmodically at anything within reach, the expressionless

face, on which, for the first few days, only sensations of discomfort

are registered, all mark a creature whose survival hinges absolutely on

unremitting care; a far cry from the competent self-sufficiency of the

average person of mature years. These surface marks of helplessness are

by no means the most significant. Buried from view beneath the soft and

velvety skin are characteristics of even greater meaning to those on

whom falls the responsibility for the rearing of the new life.

At the time of birth the bony skeleton is very incomplete; there is a

spot just above the forehead where the skull bones have not yet grown

completely together, leaving a space where the brain is protected from

injury only by the overlying skin. This spot can be detected easily in

very young infants by its pulsations in time with the heartbeats. In

most of the other bones the deposit of lime salts to which they owe

their stiffness has gone only a little way, so that it would be

impossible for the child to stand or walk even though it knew how. In

this respect the contrast between the human infant and the new-born of

such animals as horses or cattle is very striking, for the latter walk

stumblingly from almost the moment of birth, and efficiently within a

few hours thereafter.

Not only is the infant devoid of teeth, but in various other regards his

digestive apparatus is undeveloped. Not only is he unable to chew solid

food, but he could not digest most of it if it were served already

chewed. During the early months of life the child is emphatically a

one-diet being. His alimentary tract deals successfully with the mixture

of proteins, fats, and sugars of which milk consists, provided the

proportions are substantially those of mother’s milk and the quantity at

a feeding is not too great. He can do this because the enzymes needed

for digesting these particular substances are manufactured by his

digestive glands from the very beginning, and because the muscles of his

stomach and intestines can churn and propel the soft curd into which the

milk is converted as soon as it enters the stomach. The fact that cow’s

milk sets into a tough curd accounts for much of the difficulty some

babies have in thriving upon cow’s milk.

There is no starch in milk, and neither the saliva nor the pancreatic

juice of the infant contains the enzyme by which starch is digested. It

is wholly useless to begin feeding starch-containing foods until this

enzyme begins to be manufactured, which usually takes place when the

child is about eight months old. Even then the introduction of starch

into the diet should be gradual and cautious.

Modern science has discovered no better food for infants than mother’s

milk, and no substitute more satisfactory in general than suitably

modified cow’s or goat’s milk. Under modern conditions of life,

particularly in cities where milk has to be transported over long

distances, and where much time necessarily elapses before the milk can

be placed in the hands of the consumer, there is serious danger of

contamination of the milk with disease germs. In all enlightened

communities this danger is fully recognized, and the entire milk supply,

so far as possible, but that part of it destined for the feeding of

children in particular, is safeguarded by all available means. A fairly

reliable index to the degree of enlightenment of any community is the

quality of the milk which its children receive. One recent discovery of

considerable importance, and incidentally an interesting illustration of

the way in which correct procedure may be hit upon in advance of the

scientific knowledge which justifies it, is the finding that orange

juice contains one of the dietary accessories, known as vitamines,

essential to health, and present in raw milk, but destroyed when milk is

heated. For years, physicians had recommended orange juice for babies.

It is of vital importance when the milk must be pasteurized.

Both the heartbeat and the breathing in the young child are much more

rapid than in the grown person. It is believed that this quickness of

heart action and of breathing rate are related to the smaller size of

the infant as compared with the adult and are of no very marked

significance. At any rate it is true in general that the smaller the

animal the more rapidly does its heart beat and the more quickly does it

breathe. A very noticeable fact about young children is the

susceptibility to all sorts of influences of the mechanism by which the

breathing is controlled. Every passing interest reflects itself in

heightened breathing. Violent emotion often leads to such extreme

overbreathing as to drain the child’s blood of considerable of its store

of carbon dioxide, whereupon the rapid breathing gives place to

prolonged holding of the breath. Many a young mother has been seriously

alarmed by the length of time her offspring in a tantrum is able to

refrain from drawing its breath. Contrary to the appearance of things,

which would indicate that the child is holding its breath out of spite,

and in the hope of getting even with its parent, the cessation of

breathing is largely or wholly automatic, indicating the way in which

nervous and chemical influences have interacted to suspend the

respiration.

The child is born with all its muscles in place, and all fully formed in

that every muscle fiber the child will ever have has been produced

previous to birth. In fact, as soon as the full equipment of muscle

fibers has been laid down the body loses the power to form more, so that

if, through injury, one is so unfortunate as to have some of his muscle

fibers destroyed he will have to get along for the rest of his life with

those that are left. The gaps in the muscle tissue that are produced by

injuries are filled up by a kind of connective tissue known as scar

tissue. The muscle fibers are all present, but smaller and weaker than

they will be later.

The connections between muscles and nerves are also pretty well

established at the time of birth, so that the body and limbs can be

moved freely, even if not at all efficiently.

Not only are the motor nerves formed and in connection with the muscles

at and even before birth, but the sensory nerves and most of the central

nervous system are ready to begin functioning as well. Some reflex

actions, including a few that require quite elaborate muscular and

nervous coordination, are performed very shortly after birth. One famous

example, that has been much cited as tending to reinforce the belief in

man’s descent from tree-dwelling ancestors, is the curling of the

fingers about a slender rod that is pressed against the palms. In most

very tiny babies the grasp thus secured is strong enough so that the

child can be raised and held in the air, supported wholly by its own

grip. Sneezing, which is really a very complex act, requiring accurate

cooperation on the part of many muscles, is done successfully by very

young infants. The reflex of sucking, which is of paramount importance,

in that without it the child would almost inevitably starve, is present

practically from birth. Another important early reflex is that of

crying. It is a curious thought that this reflex, by which bodily

discomfort is made known, and through which relief may be summoned, is

revealed at the very instant of birth, in connection with the drawing of

the first breath, while the contrary reflex of laughter, by which bodily

well-being is expressed, puts in an appearance only after some weeks or

even months.

It is difficult to determine just how far the sense organs have arrived

in their development at the time when the infant begins its independent

existence. That touch and those senses related to bodily discomfort, of

which pain is most important, are operative from the first is shown by

the occurrence of the reflexes described above, which are brought into

action by those particular senses. There is good reason to believe that

the sense of hunger comes into play within two or three days at the

latest. The sense of thirst does not have much chance to reveal itself

early in life, for with a diet exclusively liquid, and feedings

separated by only a few hours, there is no reason why the child should

become thirsty. As the feedings become less frequent, and particularly

as solid food begins to be added, there is real danger that the child

may be insufficiently supplied with water.

Muscle sense and the equilibrium sense, if present at all, must be in a

very imperfect condition at first. They seem to differ from the senses

described thus far in that they depend on practice for their

development. At any rate the bodily movements are largely aimless in the

beginning, and it will be observed that the baby has the appearance of

experimenting with its extremities, placing them repeatedly in

particular positions and seeming to gain precision thereby. The eye

movements, and especially those by which both eyes are focused on a

single object, depend for their accuracy on the working of muscle sense.

In the estimation of the parents a distinct mark of progress is

registered the first time the baby follows a movement with its eyes. As

soon as it does this accurately, and also brings both eyes to bear on

any object, its muscle sense is known to be in efficient operation, so

far, at least, as the eye muscles are concerned. Equilibrium sense first

shows itself when real balancing motions of the body are made.

The senses of taste and smell may be operative to some degree in very

young children, but it is doubtful whether they have either the breadth

or the acuteness that will be shown later in life. Recognition of

disagreeable tastes or smells seems to appear earlier than appreciation

of agreeable. This is in line with the general fact that the

self-protective reactions are developed very early.

Hearing and sight are probably in working order practically from birth.

It is customary to test the sight of the new-born by passing a light

directly in front of the eyes. If sight is present there will be some

appreciable eye movement, suggesting that the eyes are attempting to

follow the moving light. There is no reason to believe that there is any

such thing as definite looking at objects thus early. So long as the

eyes continue to roll aimlessly about, and before they begin to focus

accurately, they are more likely concerned with distinctions between

light and shadow, than with perceptions of form or size. In general we

may say of the senses that those concerned immediately with bodily

discomfort are about as fully developed at birth, or shortly thereafter,

as they ever will be, while those that have to do with the general

adjustments of the body to its environment reach full efficiency more

gradually.

The higher parts of the brain, especially those concerned on the one

hand with the mental life (the cerebrum), and on the other with the

complicated reflex acts involved in locomotion (the cerebellum), are not

ready to begin active functioning when the child is born. Indeed some

parts of the cerebellum do not take on final form for from eight months

to two years afterward. It is thought by some that the question of

whether a child will learn to walk early or late depends, in part at

least, on how soon his cerebellum reaches complete development.

Most parents are fully alive to the importance of abundant warm covering

when their children are to be taken out into the cold, but there is much

less appreciation of the harm that may be done by too much clothing in

extremely hot weather. Of special importance is the avoidance of

exposure to sharp drops in temperature. The adult adjusts himself more

or less automatically to these, whereas the infant does so to only a

limited extent.

Finally, the hold of the infant upon life, that quality that we know as

ruggedness or vitality, is much slighter than it will be after a few

years. Not only is the susceptibility to many kinds of infectious

diseases very much greater, and the power of resisting them very much

less, but the ill effects of poisons, whether taken in with the food or

breathed in with the air, are more pronounced. Thus the vitiated air of

slum dwellings, saturated with the effluvium from unwashed bodies and

unclean clothing, while trying enough for the average grown person, is

deadly for all but the toughest babies. Even in the ordinarily well-kept

home, especially in the winter time when ventilation is apt to be

neglected, the air within the house tends to become unsatisfactory from

the standpoint of the infant’s welfare.

The wise parent, and wise he must be at this time, relaxes his care in

just proportion as the child achieves ability to do things for himself.

Since bodily development is more rapid than mental the close supervision

of food, clothing, and physical occupation is necessary only during the

early years, but the task of building up, through the slow processes of

education, the sort of mind which will be able to do its proper share in

dealing with the difficulties which confront the coming generations is

one to which may well be devoted the best thought and effort not only of

the parents, but of organized society as a whole.

Typographical errors corrected by the etext transcriber:

that their less curvaturos=> that their less curvatures {pg 114}

as fast as it it formed.=> as fast as it is formed. {pg 180}

Russian invesigator showed=> Russian investigator showed {pg 240}

quantities of axygen=> quantities of oxygen {pg 254}

unless artificial air can be procured=> unless artificial aid can be

procured {pg 303}