The pulmonary artery, bringing venous blood, by its alternate expansion and recoil, draws the blood along until it reaches the pulmonary capillaries. These tiny tubes surround the air cells of the lungs, and here an exchange takes place. The impure, venous blood here gives up its _debris_ in the shape of carbon dioxid and water, and in return takes up a large amount of oxygen. Thus the blood brought to the lungs by the pulmonary arteries leaves the lungs entirely different in character and appearance. This part of the circulation is often called the lesser or pulmonic circulation.

The four pulmonary veins bring back bright, scarlet blood, and pour it into the left auricle of the heart, whence it pa.s.ses through the mitral valve into the left ventricle. As soon as the left ventricle is full, it contracts. The mitral valve instantly closes and blocks the pa.s.sage backward into the auricle; the blood, having no other way open, is forced through the semilunar valves into the aorta. Now red in color from its fresh oxygen, and laden with nutritive materials, it is distributed by the arteries to the various tissues of the body. Here it gives up its oxygen, and certain nutritive materials to build up the tissues, and receives certain products of waste, and, changed to a purple color, pa.s.ses from the capillaries into the veins.

[Ill.u.s.tration: Fig. 75.--Diagram ill.u.s.trating the Circulation.

1, right auricle; 2, left auricle; 3, right ventricle; 4, left ventricle; 5, vena cava superior; 6, vena cava inferior; 7, pulmonary arteries; 8, lungs; 9, pulmonary veins; 10, aorta; 11, alimentary ca.n.a.l; 12, liver; 13, hepatic artery; 14, portal vein; 15, hepatic vein.

All the veins of the body, except those from the lungs and the heart itself, unite into two large veins, as already described, which pour their contents into the right auricle of the heart, and thus the grand round of circulation is continually maintained. This is called the systemic circulation. The whole circuit of the blood is thus divided into two portions, very distinct from each other.

191. The Portal Circulation. A certain part of the systemic or greater circulation is often called the portal circulation, which consists of the flow of the blood from the abdominal viscera through the portal vein and liver to the hepatic vein. The blood brought to the capillaries of the stomach, intestines, spleen, and pancreas is gathered into veins which unite into a single trunk called the portal vein.

The blood, thus laden with certain products of digestion, is carried to the liver by the portal vein, mingling with that supplied to the capillaries of the same organ by the hepatic artery. From these capillaries the blood is carried by small veins which unite into a large trunk, the hepatic vein, which opens into the inferior vena cava. The portal circulation is thus not an independent system, but forms a kind of loop on the systemic circulation.

The lymph-current is in a sense a slow and stagnant side stream of the blood circulation; for substances are constantly pa.s.sing from the blood-vessels into the lymph s.p.a.ces, and returning, although after a comparatively long interval, into the blood by the great lymphatic trunks.

Experiment 90. _To ill.u.s.trate the action of the heart, and how it pumps the blood in only one direction_. Take a Davidson or Household rubber syringe. Sink the suction end into water, and press the bulb. As you let the bulb expand, it fills with water; as you press it again, a valve prevents the water from flowing back, and it is driven out in a jet along the other pipe. The suction pipe represents the veins; the bulb, the heart; and the tube end, out of which the water flows, the arteries.

[NOTE. The heart is not nourished by the blood which pa.s.ses through it. The muscular substance of the heart itself is supplied with nourishment by two little arteries called the _coronary arteries_, which start from the aorta just above two of the semilunar valves. The blood is returned to the right auricle (not to either of the venae cavae) by the _coronary vein_.]

The longest route a portion of blood may take from the moment it leaves the left ventricle to the moment it returns to it, is through the portal circulation. The shortest possible route is through the substance of the heart itself. The mean time which the blood requires to make a complete circuit is about 23 seconds.

192. The Rhythmic Action of the Heart. To maintain a steady flow of blood throughout the body the action of the heart must be regular and methodical. The heart does not contract as a whole. The two auricles contract at the same time, and this is followed at once by the contraction of the two ventricles. While the ventricles are contracting, the auricles begin to relax, and after the ventricles contract they also relax. Now comes a pause, or rest, after which the auricles and ventricles contract again in the same order as before, and their contractions are followed by the same pause as before. These contractions and relaxations of the various parts of the heart follow one another so regularly that the result is called the rhythmic action of the heart.

The average number of beats of the heart, under normal conditions, is from 65 to 75 per minute. Now the time occupied from the instant the auricles begin to contract until after the contraction of the ventricles and the pause, is less than a second. Of this time one-fifth is occupied by the contraction of the auricles, two-fifths by the contraction of the ventricles, and the time during which the whole heart is at rest is two-fifths of the period.

193. Impulse and Sounds of the Heart. The rhythmic action of the heart is attended with various occurrences worthy of note. If the hand be laid flat over the chest wall on the left, between the fifth and sixth ribs, the heart will be felt beating. This movement is known as the beat or impulse of the heart, and can be both seen and felt on the left side. The heart-beat is unusually strong during active bodily exertion, and under mental excitement.

The impulse of the heart is due to the striking of the lower, tense part of the ventricles--the apex of the heart--against the chest wall at the moment of their vigorous contraction. It is important for the physician to know the exact place where the heart-beat should be felt, for the heart may be displaced by disease, and its impulse would indicate its new position.

Sounds also accompany the heart's action. If the ear be applied over the region of the heart, two distinct sounds will be heard following one another with perfect regularity. Their character may be tolerably imitated by p.r.o.nouncing the syllables _lubb_, _dup_. One sound is heard immediately after the other, then there is a pause, then come the two sounds again. The first is a dull, m.u.f.fled sound, known as the "first sound," followed at once by a short and sharper sound, known as the "second sound" of the heart.

The precise cause of the first sound is still doubtful, but it is made at the moment the ventricles contract. The second sound is, without doubt, caused by the sudden closure of the semilunar valves of the pulmonary artery and the aorta, at the moment when the contraction of the ventricles is completed.

[Ill.u.s.tration: Fig. 76.--Muscular Fibers of the Ventricles.

A, superficial fibers common to both ventricles; B, fibers of the left ventricle; C, deep fibers pa.s.sing upwards toward the base of the heart; D, fibers penetrating the left ventricle ]

The sounds of the heart are modified or masked by blowing "murmurs" when the cardiac orifices or valves are roughened, dilated, or otherwise affected as the result of disease. Hence these new sounds may often afford indications of the greatest importance to physicians in the diagnosis of heart-disease.

194. The Nervous Control of the Heart. The regular, rhythmic movement of the heart is maintained by the action of certain nerves. In various places in the substance of the heart are ma.s.ses of nerve matter, called ganglia. From these ganglia there proceed, at regular intervals, discharges of nerve energy, some of which excite movement, while others seem to restrain it. The heart would quickly become exhausted if the exciting ganglia had it all their own way, while it would stand still if the restraining ganglia had full sway. The influence of one, however, modifies the other, and the result is a moderate and regular activity of the heart.

The heart is also subject to other nerve influences, but from outside of itself. Two nerves are connected with the heart, the pneumogastric and the sympathetic (secs. 271 and 265). The former appears to be connected with the restraining ganglia; the latter with the exciting ganglia. Thus, if a person were the subject of some emotion which caused fainting, the explanation would be that the impression had been conveyed to the brain, and from the brain to the heart by the pneumogastric nerves.

The result would be that the heart for an instant ceases to beat. Death would be the result if the nerve influence were so great as to restrain the movements of the heart for any appreciable time.

Again, if the person were the subject of some emotion by which the heart were beating faster than usual, it would mean that there was sent from the brain to the heart by the sympathetic nerves the impression which stimulated it to increased activity.

195. The Nervous Control of the Blood-vessels. The tone and caliber of the blood-vessels are controlled by certain vaso-motor nerves, which are distributed among the muscular fibers of the walls. These nerves are governed from a center in the medulla oblongata, a part of the brain (sec. 270). If the nerves are stimulated more than usual, the muscular walls contract, and the quant.i.ty of the blood flowing through them and the supply to the part are diminished. Again, if the stimulus is less than usual, the vessels dilate, and the supply to the part is increased.

Now the vaso-motor center may be excited to increased activity by influences reaching it from various parts of the body, or even from the brain itself. As a result, the nerves are stimulated, and the vessels contract. Again, the normal influence of the vaso-motor center may be suspended for a time by what is known as the inhibitory or restraining effect. The result is that the tone of the blood-vessels becomes diminished, and their channels widen.

The effect of this power of the nervous system is to give it a certain control over the circulation in particular parts. Thus, though the force of the heart and the general average blood-pressure remain the same, the state of the circulation may be very different in different parts of the body. The importance of this local control over the circulation is of the utmost significance. Thus an organ at work needs to be more richly supplied with blood than when at rest. For example, when the salivary glands need to secrete saliva, and the stomach to pour out gastric juice, the arteries that supply these organs are dilated, and so the parts are flushed with an extra supply of blood, and thus are aroused to greater activity.

Again, the ordinary supply of blood to a part may be lessened, so that the organ is reduced to a state of inactivity, as occurs in the case of the brain during sleep. We have in the act of blushing a visible example of sudden enlargement of the smaller arteries of the face and neck, called forth by some mental emotion which acts on the vaso-motor center and diminishes its activity. The reverse condition occurs in the act of turning pale. Then the result of the mental emotion is to cause the vaso-motor nerves to exercise a more powerful control over the capillaries, thereby closing them, and thus shutting off the flow of blood.

Experiment 91. Hold up the ear of a white rabbit against the light while the animal is kept quiet and not alarmed. The red central artery can be seen coursing along the translucent organ, giving off branches which by subdivision become too small to be separately visible, and the whole ear has a pink color and is warm from the abundant blood flowing through it. Attentive observation will show also that the caliber of the main artery is not constant; at somewhat irregular periods of a minute or more it dilates and contracts a little.

[Ill.u.s.tration: Fig. 77.--Some of the Princ.i.p.al Organs of the Chest and Abdomen. (Blood vessels on the left, muscles on the right.)]

In brief, all over the body, the nervous system, by its vaso-motor centers, is always supervising and regulating the distribution of blood in the body, sending now more and now less to this or that part.

[Ill.u.s.tration: Fig. 78.--Capillary Blood-Vessels in the Web of a Frog's Foot, as seen with the Microscope.]

196. The Pulse. When the finger is placed on any part of the body where an artery is located near the surface, as, for example, on the radial artery near the wrist, there is felt an intermittent pressure, throbbing with every beat of the heart. This movement, frequently visible to the eye, is the result of the alternate expansion of the artery by the wave of blood, and the recoil of the arterial walls by their elasticity.

In other words, it is the wave produced by throwing a ma.s.s of blood into the arteries already full. The blood-wave strikes upon the elastic walls of the arteries, causing an increased distention, followed at once by contraction. This regular dilatation and rigidity of the elastic artery answering to the beats of the heart, is known as the pulse.

The pulse may be easily found at the wrist, the temple, and the inner side of the ankle. The throb of the two carotid arteries may be plainly felt by pressing the thumb and finger backwards on each side of the larynx. The progress of the pulse-wave must not be confused with the actual current of the blood itself. For instance, the pulse-wave travels at the rate of about 30 feet a second, and takes about 1/10 of a second to reach the wrist, while the blood itself is from 3 to 5 seconds in reaching the same place.

The pulse-wave may be compared to the wave produced by a stiff breeze on the surface of a slowly moving stream, or the jerking throb sent along a rope when shaken. The rate of the pulse is modified by age, fatigue, posture, exercise, stimulants, disease, and many other circ.u.mstances. At birth the rate is about 140 times a minute, in early infancy, 120 or upwards, in the healthy adult between 65 and 75, the most common number being 72. In the same individual, the pulse is quicker when standing than when lying down, is quickened by excitement, is faster in the morning, and is slowest at midnight. In old age the pulse is faster than in middle life; in children it is quicker than in adults.

[Ill.u.s.tration: Fig. 79.--Circulation in the Capillaries, as seen with the Microscope.]

As the pulse varies much in its rate and character in disease, it is to the skilled touch of the physician an invaluable help in the diagnosis of the physical condition of his patient.

Experiment 92. _To find the pulse_. Grasp the wrist of a friend, pressing with three fingers over the radius. Press three fingers over the radius in your own wrist, to feel the pulse.

Count by a watch the rate of your pulse per minute, and do the same with a friend's pulse. Compare its characters with your own pulse.

Observe how the character and frequency of the pulse are altered by posture, muscular exercise, a prolonged, sustained, deep inspiration, prolonged expiration, and other conditions.

197. Effect of Alcoholic Liquors upon the Organs of Circulation.

Alcoholic drinks exercise a destructive influence upon the heart, the circulation, and the blood itself. These vicious liquids can reach the heart only indirectly, either from the stomach by the portal vein to the liver, and thence to the heart, or else by way of the lacteals, and so to the blood through the thoracic duct. But by either course the route is direct enough, and speedy enough to accomplish a vast amount of ruinous work.

The influence of alcohol upon the heart and circulation is produced mainly through the nervous system. The inhibitory nerves, as we have seen, hold the heart in check, exercise a restraining control over it, very much as the reins control an active horse. In health this inhibitory influence is protective and sustaining. But now comes the narcotic invasion of alcoholic drinks, which paralyze the inhibitory nerves, with the others, and at once the uncontrolled heart, like the unchecked steed, plunges on to violent and often destructive results.

[Ill.u.s.tration: Fig. 80.--Two Princ.i.p.al Arteries of the Front of the Leg (Anterior Tibial and Dorsalis Pedis).]

This action, because it is quicker, has been considered also a stronger action, and the alcohol has therefore been supposed to produce a stimulating effect. But later researches lead to the conclusion that the effect of alcoholic liquors is not properly that of a stimulant, but of a narcotic paralyzant, and that while it indeed quickens, it also really weakens the heart's action. This view would seem sustained by the fact that the more the intoxicants are pushed, the deeper are the narcotic and paralyzing effects. After having obstructed the nutritive and reparative functions of the vital fluid for many years, their effects at last may become fatal.

This relaxing effect involves not only the heart, but also the capillary system, as is shown in the complexion of the face and the color of the hands. In moderate drinkers the face is only flushed, but in drunkards it is purplish. The flush attending the early stages of drinking is, of course, not the flush of health, but an indication of disease.[34]

198. Effect upon the Heart. This forced overworking of the heart which drives it at a reckless rate, cuts short its periods of rest and inevitably produces serious heart-exhaustion. If repeated and continued, it involves grave changes of the structure of the heart. The heart muscle, endeavoring to compensate for the over-exertion, may become much thickened, making the ventricles smaller, and so fail to do its duty in properly pumping forward the blood which rushes in from the auricle. Or the heart wall may by exhaustion become thinner, making the ventricles much too large, and unable to send on the current. In still other cases, the heart degenerates with minute particles of fat deposited in its structures, and thus loses its power to propel the nutritive fluid. All three of these conditions involve organic disease of the valves, and all three often produce fatal results.

199. Effect of Alcohol on the Blood-vessels. Alcoholic liquors injure not only the heart, but often destroy the blood-vessels, chiefly the larger arteries, as the arch of the aorta or the basilar artery of the brain. In the walls of these vessels may be gradually deposited a morbid product, the result of disordered nutrition, sometimes chalky, sometimes bony, with usually a dangerous dilatation of the tube.

In other cases the vessels are weakened by an unnatural fatty deposit.

Though these disordered conditions differ somewhat, the morbid results in all are the same. The weakened and stiffened arterial walls lose the elastic spring of the pulsing current. The blood fails to sweep on with its accustomed vigor. At last, owing perhaps to the pressure, against the obstruction of a clot of blood, or perhaps to some unusual strain of work or pa.s.sion, the enfeebled vessel bursts, and death speedily ensues from a form of apoplexy.