CHAPTER V
THE CIRCULATION
Experiment 1. Anatomy of Mammalian Heart.—Get a sheep’s or beef’s heart from the butcher. Get the whole heart, not simply the ventricles (as usually sold). Note the blood vessels, four chambers, thickness of different walls, valves, cords, openings.
Experiment 2. Does Gravity affect the Blood Flow?—Hold the right hand above the head for a few minutes. At the same time let the left hand hang straight down. Then bring the hands together and see which is of a darker red because of containing more blood. Now reverse the position of the hands for a few minutes, and find whether the effect is reversed. (Entire class.)
Experiment 3. Study of Human Blood.—Examine a drop of blood under the microscope, first diluting it with a little saliva. See Fig. 60.
Experiment 4. The Circulation in a Frog.—Wrap a small frog in a moist cloth, lay on a slip of glass, place under the microscope, and study the circulation in the web of its foot.
Experiment 5. (Entire class.) Effect of Exercise upon the Pulse.—Tap a bell as the second hand of a watch begins a minute and let the pupils count the pulse at the radial artery on the wrist above base of thumb. Repeat standing, or after gymnastics or recess. Result?
Experiment 6. The Action of the Valves in the Veins.—Place the tip of the middle finger on one of the large veins of the wrist; with the forefinger then stroke the vein toward the elbow so as to push the blood from a portion of it, keeping both fingers in place. The vein remains empty between the fingers. Lift the finger nearer the heart and no blood enters the vein; there is a valve above which holds it back. Lift the other finger and the vein fills instantly. Stroke a vein toward the hand, and notice that the veins swell up into little knots where the valves are. Stroke in the reverse direction. Result?
Experiment 7. Finding the Capillary Pressure. This is found by pressing a glass plate or tumbler upon a red part of the skin. When the skin becomes pale the capillary pressure is counterbalanced.
Experiment 8. Emergency Drill.—Let one pupil come forward, mark with blue chalk or pencil the position on his arm of a supposedly cut vein. Let another pupil use means to stop the imagined blood flow. Experiment 9. Let another pupil stop the flow from an imaginary cut artery marked red. See text. Experiment 10. In a case of nose bleed do not let pupil lean over a bowl. (Why?) Cause him to stand rather than lie. (Why? See Exp. 2.) Apply cold water to contract arteries to nose, also have pupil hold a small roll of paper or a coin under upper lip (to make muscular pressure on arteries to nose). Experiment 11. Let one pupil treat another for a bruise (see p. 62). Experiment 12. Emergency drill, restoration from fainting (see p. 57).
The Cells have a Liquid Home.—The cells in the body of man, like the ameba, live in a watery liquid. This liquid is called lymph. The cells cannot move about as the ameba does to obtain food, so the blood brings the food near them and it soaks through the blood tubes into the lymph spaces next to the cells (see colored Fig. 3). The ameba gives off waste material into the water; the cells of the body give it off into the lymph to be carried off by the circulation. The blood, then, has two functions: (1) to take nourishment to the tissues; (2) to take away waste material from them.
The Organs of Circulation.—These are the heart, which propels the blood; the arteries, which take blood away from the heart; the veins, which take the blood back to the heart; and the capillaries (Fig. 53), which take the blood from the arteries to the veins.
Fig. 53.—Capillaries, connecting artery (b) with vein (a).
The heart is a cone-shaped organ about the size of its owner’s fist. It lies in a diagonal position behind the breastbone, with the small end of the cone extending toward the left. The smaller end (Exp. 1) taps or beats against the chest wall at a point between the fifth and sixth ribs on the left side. The breastbone and ribs protect it from blows. An inclosing membrane called the pericardium secretes a serous fluid and lessens the friction from its beating.
Why the Heart is Double.—There must be a pump to move the impure blood from the body to the lungs to get oxygen from the air, and there must be another pump to send the pure blood from the lungs back to the body. Hence there are two pumps bound together into one heart, beating at the same time like two men keeping step, or like two carpenters keeping time with their hammers. There are valves in the heart, as in other pumps. These valves are so arranged that when any part of the heart contracts and forces the blood onward, the blood cannot return after that part of the heart relaxes. Are the pumps placed one behind the other? Or is one above the other? Neither; they are side by side, with a fleshy partition between them (Fig. 54). The pump on the right moves the impure blood from the body to the lungs, and the one on the left moves the pure blood from the lungs to the body. There is no direct connection between the right and left sides of the heart.
Fig. 54.—Diagram of Heart.
Notice the two dark spots in the right auricle, and four dark spots in left auricle, where the veins enter. Does the aorta pass in front of, or behind, the pulmonary artery?
To trace one complete circuit of the blood (Fig. 54), let us begin with the blood in the capillaries of the outer tissues, such as the skin or muscles. The blood goes through small veins which unite into two large veins, through which it enters the receiving chamber, or right auricle, goes through the tricuspid valve into the expelling chamber, or right ventricle, then through a semilunar valve into the pulmonary artery leading to the lungs. Becoming purified while passing through the capillaries of the lungs, the blood goes through the pulmonary veins to the left auricle (Fig. 54), then through the bicuspid or mitral valve, to the left ventricle, whence it is forced through a semilunar valve into the largest artery of the body, called the great aorta (Fig. 54). Thence it goes to the smaller arteries, and then to the capillaries of the tissues in general, thus completing the circuit.
Fig. 55.—The Left Side of Heart (plan), showing the left ventricle at the moment when relaxing and receiving the blood from the auricle; and the same at the beginning of contraction to send blood into aorta. Notice action of the valve.
Structure of Veins and Arteries.—Seen under the microscope the arteries and veins show that they are made of three kinds of tissues arranged in three coats (Fig. 56): a tissue resembling epithelial tissue (Chap. 1), as a lining to lessen friction; an outer connective tissue (Chap. 1), to give elasticity; and a middle coat of muscular tissue to enable the vessels to change in size. Let us see why blood vessels must have these three properties.
Why the Blood Vessels must be Elastic.—The aorta and its branches are always full of blood. When the left ventricle with its strong, muscular walls contracts, the blood in the aorta and small blood tubes cannot move forward fast enough to make room for the new supply so suddenly sent out of the ventricle. Where can this blood go? If a cup is full, it cannot become more full; not so with an artery. The elastic connective tissue allows it to expand as a rubber hose does under pressure. The first part of the aorta having expanded to receive the incoming blood, the stretched walls contract because of the elasticity of the outer connective tissue coat and force blood into the portion of the aorta just ahead, forcing it to expand in turn. Thus a wave of expansion travels along the arteries. This wave is called the pulse.
The Pulse may be most easily felt in the wrists and neck. As the artery stretches and springs back, one beat of the pulse is felt. In men there are about seventy heart beats or pulse beats a minute. In women the rate is about eighty a minute. It is slowest when one is lying down, faster while sitting, still faster when standing, and fastest of all during running or violent exercise. (Exp. 5.) It should not be thought that the muscular or middle layer of the artery actively contracts and helps to send along the pulse wave; for this wave is simply the passive stretching and contracting of the outer connective coat, and travels like a wave crossing a pond when a stone is dropped into the water. The force of the pulse is furnished, not by the muscle fibers in the artery, but by the beat of the heart; the outer, or connective tissue, coat enables the pulse to travel. Why must there be a middle, or muscular, coat for variation in size?
Fig. 56.—Section of Artery, A, and Vein, V, showing inner coat, e (endothelial); middle coat, m (muscular); and third coat, a (connective tissue).
Use of the Middle Coat; Quantity of Blood and its Distribution.—The body of an adult contains about five quarts of blood. The blood furnishes the nourishment needed for the activity of each organ. The more vigorous the work of any organ, the greater is the amount of blood needed. The whole amount of blood in the body cannot be suddenly increased, but the muscular coat of the arteries going to the working organ relaxes, and allows the arteries to become enlarged by the pressure from the heart. Consequently, more blood goes to the active organ, and the other organs get along with less blood for the time. When we are studying, our brains get more blood; when running, the leg muscles get more; after a hearty dinner, the stomach and intestines get more than any other part of the body. Why is it difficult to do the best studying and digest a meal at the same time? We see that the muscular coat of the arteries is a very useful coat, for it enables the supply of blood to be increased in any organ which is in temporary need of it.
Why the Blood Vessels must be Smooth.—The inner coat of the heart and other blood vessels is made of tissue like the epithelial tissue which forms the epidermis and the smooth lining of the mouth and other organs. The purpose of this lining is to lessen friction, and thus save the work of the heart. The friction is greatest in the capillaries because of their small size. The inner coat of smooth cells is the only coat that is prolonged to form the capillaries (see Fig. 57).
Fig. 57.—Capillaries Magnified, showing Cells forming their walls. Notice that each cell has a nucleus and three branches.
The capillaries are small, thin, short, and very numerous. They are very small so that they may go in between the cells of the tissues. The capillaries are very thin so that the nourishment from the blood may pass readily into the tissues, and the waste material pass readily into the blood. They are very short so that the friction may be less; and they are very numerous so that all parts of the tissues may be supplied with blood, and that the blood may flow very slowly through them. Because of the number of the capillaries, their total volume is several hundred times larger than the volume of the arteries that empty into them, or of the veins that flow from them. Hence the blood flows slowly through the capillaries, as water flows slowly through a lake along the course of a river. All the changes between the blood and the lungs, and between the blood and the tissues, take place in the capillaries, and the object of the other parts of the circulation is merely to move the blood continually through the capillaries.
The effect of gravity is to retard the flow in certain parts of the body and aid the flow in other parts, according to the position of the body (Exp. 2).
Fainting is usually due to lack of blood in the brain, which in turn results from a weakening of the heart beat. Since the brain cannot work without fresh blood, fainting is accompanied by unconsciousness. Recovery from fainting is aided by loosening the clothing at the neck and by placing the head of the patient a little lower than the body so that the weight of the blood may aid the flow to the brain. Dashing a little cold water in the face shocks the nerves and arouses the heart to stronger beats.
The veins have valves placed frequently along their course (Fig. 58). These valves are pockets made by a fold in the inner coat of the wall of the vein. When a boy places his hand in his pocket, the pocket swells out; but if he rubs his hand on the outside of the pocket from the bottom toward the top, it flattens down. So with the action of the blood upon the valves in the veins. (Repeat Exp. 6 in class.)
Fig. 58.—Valves in Veins. (Jegi.)
How Muscular Exercise aids the Heart.—When a muscle contracts, it hardens and presses upon a vein which goes through the muscle, and the blood is pressed out of the vein (see Fig. 58). The blood cannot go toward the capillaries, for the valves fill and close when it starts that way; so it must all go out toward the heart. When the muscle relaxes, the blood that has been pressed forward cannot go back because of the valves, but the valves nearer the capillaries open, and the veins are filled from the capillaries (Fig. 53). When the muscle contracts again, the same effect on the blood movement is repeated. We see, therefore, that every contracting muscle converts into a pump the vein running through it, and when a person works or exercises, many little pumps are working all over the body, aiding the heart in its function. This aid makes the blood flow faster and relieves the heart of part of its work, so that it beats faster, just as a horse might trot faster if another horse helped to draw the load (Exp. 3). The pressure of a contracting muscle upon an artery does not aid the blood flow in the artery because the latter is destitute of valves.
How Breathing aids the Heart.—Breathing is a blood-pumping process as well as an air-renewing process. When the chest expands, blood is drawn into it. When the chest contracts, the flow of blood away from it is aided. As the chest expands, the downward pressure of a great, broad muscle, the diaphragm (Fig. 74) compresses the liver, stomach, and other abdominal organs, and forces the venous blood upward into the expanding chest, thus helping it on its way to the heart. But if the abdominal wall is weakened by tight lacing or by the pressure of belts and bands which support the clothing, the weak abdominal wall yields to the downward pressure of the diaphragm, and no compression of the liver or aid to the circulation will result.
Fig. 59.—The Ventricles of a Dog’s Heart relaxed (above), and contracted (below).
How the Blood Vessels are Controlled.—Evidently the blood vessels are not regulated by the will. We cannot voluntarily increase the beating of the heart, or cause it to slacken its action. Even an actor cannot cause his face to turn pale or to blush at will. This is because the tiny muscles in the walls of the blood vessels are involuntary muscles. They are controlled by nerves of the sympathetic system called vaso-motors. They are not subject to the will (see Fig. 25). The nerve center which controls the blood vessels is located in the top of the spinal cord at the base of the brain. When cold air strikes the skin the nerves near the arteries are stimulated, the arteries in the skin contract, and the skin turns white. When the heat from a hot fire strikes the skin, the nerves are soothed, the arteries relax, and the face becomes red. When the stomach is filled with food, the heart beats faster and sends more blood to aid in digestion. When we run fast, the heart beats fast to supply more blood to the muscles, but it slows down as sleep comes on, that the body and brain may rest.
Parts of the Blood.—The blood which flows from a cut finger seems to be a bright red throughout. When a drop of it is looked at through a microscope, however, the liquid itself is seen to be almost as clear as water. This liquid is called the plasma. Floating in it are millions of biconcave disks containing a pigment (hemoglobin) which gives the red color to the blood. The disks are called red corpuscles (Fig. 60). A few irregularly shaped bodies, nucleated and almost transparent, and called white corpuscles, are also found in the blood. The red corpuscles go only where the plasma carries them (Exps. 3, 4). The white corpuscles sometimes leave the blood vessels entirely. At times one may be seen shaped like a dumb-bell, half of it through the wall of the blood vessel and half still in the blood vessel. After the corpuscle is out, no hole can be found to account for its mysterious passage. The white corpuscles consist of protoplasm. The red corpuscles contain no protoplasm. Hence the latter are not really alive.
Fig. 60.—Human Blood Cells (magnified 40,000 areas), showing many red cells and a single white blood cell on left, larger than red cells. (Peabody.)
Fig. 61.—Side and Front Views of Frog’s and Man’s Red Corpuscles, drawn to same scale. Compare outline, concavity, diameters.
The Use of Each Part of the Blood.—The plasma keeps the blood in a liquid state, so that it may flow readily; the plasma also transports the food that has been eaten and digested, and carries carbon dioxid to the lungs and other waste material to the kidneys. The red corpuscles transport the oxygen from the lungs to the tissues. The white corpuscles devour and destroy irritating particles, such as drugs, poisons, and germs. They are of great importance in purifying the blood and as a protection against disease. One is shown in Fig. 60.
The sounds of the heart beat may be heard by applying the ear to the chest. They are two, a long, dull sound and a short, clear one. The first comes from the vibration of the bicuspid valve together with an unexplained tone arising from large contracting muscles, in this case the walls of the ventricles. The second, or short, clear sound, is produced by the sudden closing and vibration of the semilunar valves.
Changes in the Composition of the Blood as it passes through the Various Organs.—When the blood is forced out by the heart, part of it goes to the stomach and intestines through arteries which divide into capillaries. These capillaries absorb all kinds of food from the alimentary canal except the fats (see p. 64), and unite to form the portal vein, which takes the absorbed food to the liver. In the liver some of the impurities of the blood are burned up and changed into bile. The blood, purified and laden with food, is carried from the liver to the heart, where it reënters the general blood stream. The blood flow from the food tube through portal vein and liver to the heart, as just described, is called the Portal circulation.
Renal circulation. Two branches from the aorta carry blood to the kidneys. There the urea and a large amount of water are taken out, and the purified blood is emptied into the large vein that leads up to the heart.
Pulmonary circulation (Fig. 67). This is the circulation through the lungs. During this circulation carbon dioxid gas is removed from the blood and oxygen is added to it.
Fig. 62.—Blood Clot separated from serum.
Some impurities and a large amount of water escape from the blood as it passes through the skin.
Coagulation.—So long as blood is in an uninjured blood vessel it remains a liquid. In a few minutes after it flows from a blood vessel, it forms into a stiff, jellylike mass called a clot (Fig. 62). The process of forming the clot is called coagulation, and it is brought about by the albuminous substance called fibrin, which is always in the plasma of healthy blood. On exposure to air the fibrin forms into a network of fine threads throughout the mass (Fig. 63) and the corpuscles become entangled in the meshes. The clot consists of the fibrin of the plasma and corpuscles; the watery portion of the plasma, called the serum, separates from the clot (Fig. 62). The property of coagulating is a great safeguard, as a clot often plugs up a cut blood vessel. What is the difference between serum and plasma?
Fig. 63.—Network of Fibrin in Human Blood (enlarged).
Veins and Arteries compared.—The veins have thin, soft walls and the arteries have thick, tough, elastic walls. When a vein is cut, it may usually be closed by pinching the walls of the end together. If an artery is cut, the walls will not readily stick together, but often stand open until the end of the artery is tied. For this reason, and because an artery is subject to the direct pressure of the heart, a cut artery is more dangerous to life than a cut vein. Because of the toughness of the arteries, and because they are located close to the bones, they are less likely to be cut than the veins, which are softer and nearer the surface. The veins begin in capillaries and empty into the auricles; the arteries begin at the ventricles and empty into capillaries; and there is a semi-lunar valve at the origin of each artery.
Cuts and Bruises.—1. Wash a cut under running water. 2. Stop the bleeding. The washing in cold water may do this. Elevating an injured arm or leg will aid the blood greatly in forming a clot at the opening. 3. Bandage firmly with a strip of cloth and sew the end. Keep wet the part of the bandage where the cut is; this lowers the temperature of the wound. It may be necessary to hold a gaping wound closed with strips of surgeon’s plaster placed across the cut. A handkerchief folded first into a triangle and then into a narrow bandage is often useful. A cut artery may be known from a cut vein by the brighter color of the blood, and by the flow being stronger at each heart beat, while the flow from a vein is uniform. Pressure to stop the flow of blood from an artery should be applied between the cut and the heart; but when the blood comes from a vein, the pressure should be applied to the side of the cut farthest from the heart.
Apply hot water immediately for several minutes to a bruise. Either a bruise or a cut may be washed with a weak solution of some antiseptic such as carbolic acid. After washing a bruise it may be bound with a cloth soaked in witch hazel or arnica.
The Lymphatic System
This system contains and conveys a liquid called the lymph. It consists of lymph spaces, lymph tubes, (lymphatics), and lymphatic glands. Lymph corresponds nearly to the blood without the red corpuscles. It is the familiar liquid seen in a blister, or oozing out where the skin has been grazed without breaking a blood vessel.
Necessity for Lymph and Lymph Spaces.—The body cannot be nourished with the albumin, sugar, oxygen, and other digested food in the blood, until this food passes out of the blood vessels. The food leaves the blood through the thin walls of the capillaries. Many of the cells do not touch the capillaries, and the lymph penetrates the spaces between the cells to reach them (see colored Fig. 3). If there were no lymph spaces, these cells could not get any food. The lymph bathes the cells, and the cells absorb what they want from the nourishing fluid. The red corpuscles bearing the oxygen cannot pass through the capillary walls. Oxygen, being a gas, readily passes through the walls and reaches the cells through the lymph in the lymph spaces. The waste materials must go back into the blood; carbon dioxid passes back through the capillary walls and is taken to the lungs; how the other waste materials formed in the cells pass back will soon be explained.
Need of Lymphatics.—The plasma continually passes into the tissues, but it cannot return directly into the blood. The lymph contains waste material which must be removed, and also much unused food which nature, like an economical housekeeper, will offer to the tissues again. There are vessels called lymphatics that take the lymph back into the blood (see Fig. 64).
The Lymphatic Circulation (Fig. 64).—The blood flow does not begin nor end, but makes a never ending circle. The countless lymphatics begin, with open ends, in the lymph spaces between the cells (colored Fig. 3). The smaller lymphatics unite into larger ones until finally they all unite into two large ones that empty into the large veins under the collar bones, near the neck. The one that empties under the left collar bone (3, Fig. 66) is called the thoracic duct because it goes up through the thorax just in front of the spinal column (1, Fig. 66). The other at the right side of the neck is called the right lymphatic duct (see Figs. 64, 65).
Fig. 64.—Surface Lymphatics of Hand.
In persons with the dropsy, the lymph accumulates in the lymph spaces and is not drained away by the lymph flow. Dropsy usually shows itself first by swelling of the feet and the leg below the knee. (Why? See Exp. 2.)
There is a set of lymphatics called lacteals, situated in the abdomen, which have the function of absorbing digested fats from the intestine (Figs. 66, 100, and colored figure 2).
Fig. 65.—Diagram to show the Two Parts of the Body drained by the Two Lymph Ducts.
What makes the Lymph Flow?—The heart does not, for its pressure is not transmitted beyond the blood tubes. The successive pressures of a working muscle move the lymph forward in the lymphatics in the same way that the blood is moved forward in the veins, and the valves keep it from moving back. When riding a trotting horse, or in a jolting vehicle, the lymph is moved beyond the valves at every jolt (Fig. 64). Without exercise the lymph stagnates, and the body becomes poisoned by its own wastes. At every expansion of the lungs lymph is drawn into the chest; and it is forced out of the chest at every contraction. Deep breathing is as great a benefit to the body in moving stagnant lymph as it is in purifying the blood.
Fig. 66.—Chief Lymphatic Vessels and Glands of trunk.
1, 3, Thoracic duct (emptying at 3); 2, receptacle for chyle (lacteals below it).
The lymphatic glands are kernel-like enlargements along the lymphatics, and they contain a great many lymph cells which purify the lymph as it passes through them. The lymphatic glands are numerous in the armpits and the groins. The cells in the lymph glands multiply, and some of them are carried by the lymph into the blood to become those remarkable little bodies, the white corpuscles.
Hygiene of the Circulation
Effects of Work, Fresh Air, and Rest on Corpuscles and Plasma.—Work uses up the nutritious elements in the blood. A few hours after food is eaten the nutritious materials in the blood are found to be increased. By the breathing of fresh air the carbon dioxid in the plasma is diminished and the oxygen in the colored corpuscles is increased, changing the blood to a brighter red. Sleep gives time for the exhausted cells and depleted blood to be replenished. Loss of sleep means longer hours of activity and greater consumption of nutriment with shorter hours for replacing the nutriment. The pale skin of one who has lost sleep tells of the exhausted condition of the blood.
How the Muscles help the Circulation.—The imperative need of muscular exercise to keep the body sound exists because of the lack of other means to cause movement in the veins and lymphatics. Good food, pure air, and plenty of exercise are necessary for healthy blood. Many so-called “blood purifiers” are advertised to entrap the ignorant. It is impossible to imagine how “blood purifiers” can aid the blood. The blood is purified, not by putting anything into the blood, but by something going out of it as it passes through the skin, kidneys, liver, and lungs. These organs all send out impurities brought to them by the blood.
The one great hygienic effect of muscular exercise is an active circulation, and from an active circulation nine chief effects may be traced. The effects upon the body will be given in order, beginning with the surface—skin, fat, muscles, bones; and the effects upon the internal organs are given in order of position, beginning with the highest—brain, lungs, heart, digestive organs.
Effects of Exercise and Improved Circulation.—1. The skin is made fresh, pink, and smooth from the flushing of the capillaries; it is purified by the perspiration and the renewal of cells. 2. If the fat is too great in amount, it is burned up; if it is too small in amount, the better nourishment brought by the blood increases it. 3. The muscles are better fed (see Fig. 48) and grow firm, strong, and large. 4. The skeleton is held in proper position by the stronger muscles, and deformity is prevented. 5. The brain. The pure, fresh blood, loaded with oxygen from expanded lungs, flushes every capillary of the brain, clears the mind, and doubles or trebles its power to work. 6. The lungs are expanded by deep breathing if the exercise be rapid and vigorous. A slow stroll or saunter is not of value. 7. The circulation. Every contracting muscle aids the heart in its work. The deep breathing moves stagnant lymph. 8. The stomach. Exercise burns up the food and increases the appetite. 9. General effects. Exercise promotes good humor, decreases loafing, cigarette smoking, gossiping, and other vices.
The effect of tobacco on the heart, if cigarettes or cigars are used, is sometimes to cause attacks of irregular beating; the heart flutters faintly for a while, then palpitates strongly, then flutters again. This condition is called tobacco heart, or trotting heart.
Effect of Alcohol upon the Circulation.—After a person has taken an alcoholic drink his face and skin are likely to become flushed, and perhaps his heart beats faster. Most investigators have found that the alcohol itself does not directly increase or strengthen the action of the heart. Hence it is probably wrong to call alcohol a heart stimulant. The flushing of the skin is believed to be due to the relaxing effect of alcohol. It relaxes, it paralyzes, the vasomotor nerves which control the little muscle fibers in the walls of the blood vessels. The relaxing and enlarging of the blood vessels decreases the resistance to the blood flow, and the heart beats faster under its lighter load. The narcotic effect of alcohol is much more powerful than its irritating or stimulating effect. The effect of alcohol in causing fatty degeneration of the muscles often weakens the heart and other blood vessels.
Climate and Brain Work.—In going to sleep the vessels in the skin dilate and blood is drawn from the brain to the skin. It is difficult to go to sleep when cold, for cold sends the blood to the brain and keeps the mind active. On the same principle, mental work is difficult in very warm weather because of the enlarged capillaries in the skin and the withdrawal of blood from the brain to the skin. This increases the perspiration and keeps the temperature of the body down to normal, but it deprives the brain of blood needed for good mental work. Mental workers in warm weather and in warm climates should seek every condition favoring coolness. Benjamin Franklin was accustomed to strip himself almost entirely of clothing when he was writing and wanted his brain to work at its best. The wearing of barefoot sandals and the thinnest cotton clothing, light in color, helps to prevent mental inertia in hot weather. In the Gulf states in summer and in our tropical islands the best mental work can be done by rising at dawn and working before the hot part of the day begins. Some of the greatest thinkers in the world have lived in warm climates (Greece and India), but they wore very few clothes and ate moderately of the simplest food (see p. 44).
Congestion is a swelling of the blood vessels of some part, with the accumulation of blood therein. Congestion is active when a rapid flow of blood distends the capillaries. Example, flushing of face when running. Congestion is passive when there is a narrowing of the outlet of the capillaries, the blood moves slowly and partly stagnates in the swollen vessels. Example, when the nose feels stopped up during a cold. If a syringe is worked so fast that the rubber tube swells, this is like active congestion; if the end of the tube is pinched together so that moderate pumping causes it to swell, this is like passive congestion.
Inflammation is congestion where the vessels of any part are strained and injured. White corpuscles collect there to repair the vessels and devour the blood that escapes and stagnates there. They also destroy germs that have usually found lodgment and begun to multiply. The serum of the blood also destroys the germs by the antitoxins in it. Inflammatory troubles are: colds, rheumatism, diarrhœa, and all diseases with name ending “itis.” An inflamed part is red, swollen, hot, and painful.
Prevention and Care of Colds.—A cold is an inflammation of a mucous membrane. Colds are prevented by so living as to encourage a free, vigorous circulation, and especially by not coddling the body so tenderly that the circulation becomes deranged by the least exposure. The circulation may be deranged by overheating as well as by chilling the body; usually it would be more appropriate to say that the person caught “a hot” than “a cold.” At the first sign of a cold vigorous exercise, a cold bath, or going outdoors into cold air may aid in sending fresh blood to remove the stagnation and stop the inflammation. A warm foot bath and hot drinks may relieve by drawing blood from the congested mucous membrane. After the cold has become fixed such measures will not help, but the cure is aided by helping the skin to keep its full share of blood. The cold must run its course. The cells will be given every chance to repair the injury and destroy the germs (if any) by avoiding hard work, eating moderately of digestible food, avoiding drugs, especially infallible drugs advertised in newspapers, even if recommended by otherwise intelligent people. Repeated colds tend to become a disgusting disease called chronic catarrh. Constricting the blood vessels of the skin causes congestion of the (internal) mucous membranes. A skin tenderly protected constricts more readily than one accustomed to cold. Cold is the best preventive of cold. Cold baths, pure air, light clothing, free breathing, moderate eating, ward off colds. Fussing with sprays, gargles, and drugs will not; for the main factor in bringing on a cold is not germs, nor temperature, but the state of the system itself. Persons who have suffered much with colds have found that after substituting cotton underwear for woolen, colds became very rare. Linen will have a similar effect, but it is not as durable, soft, or heat-retaining as cotton (see p. 16).
Practical Questions.—1. Through what kind of skin do the blue veins in the wrist show most plainly? 2. Which is more compressible, a vein or an artery? 3. Why are those who take little exercise likely to have cold feet? (p. 57.) 4. Where does the so-called venous blood flow through an artery? 5. What vein begins and ends in capillaries? (The portal vein, colored Fig. 5.) 6. To what purifying organ, after leaving the lungs, does the heart send part of the blood for further purification? (Colored Fig. 5.) 7. What keeps the blood moving between the beats of the heart?