2. _The Greater Circulation_.--From the left auricle, the blood is forced past the bicuspid valve to the left ventricle; thence it is driven through the semilunar valves into the great aorta, the main trunk of the arterial system. Passing through the arteries, capillaries, and veins, it returns through the venæ cavæ, ascending and descending, gathers again in the right auricle, and so completes the "grand round" of the body. Both these circulations are going on constantly, as the two auricles contract, and the two ventricles expand simultaneously, and _vice versa_.

THE VELOCITY OF THE BLOOD varies so much in different parts of the body, and is influenced by so many circumstances, that it can not be calculated with any degree of accuracy. It has been estimated that a portion of the blood will make the tour of the body in about twenty-three seconds (FLINT), and that the entire mass passes through the heart in from one to two minutes. [Footnote: The total amount of blood in an adult of average weight is about eighteen pounds. Dividing this by five ounces, the quantity discharged by the left ventricle at each systole, gives fifty- eight pulsations as the number necessary to transmit all the blood in the body. This, however, is an extremely unreliable basis of calculation, as the rapidity of the blood is itself so variable. Chauvreau has shown by experiments with his instrument that, corresponding to the first dilation of the vessels, the blood moves with immense rapidity; following this, the current suddenly becomes nearly arrested; this is succeeded by a second acceleration in the current, not quite so rapid as the first; and after this there is a gradual decline in the rapidity to the time of the next pulsation.] (See p. 314.)

DISTRIBUTION AND REGULATION OF THE HEAT OF THE BODY.--1. _Distribution_.--The natural temperature is not far from 98°. [Footnote: The average temperature is, however, easily departed from. Through some trivial cause the cooling agencies may be interfered with, and then, the heating processes getting the superiority, a high temperature or fever comes on. Or the reverse may ensue. In Asiatic cholera, the constitution of the blood is so changed that its disks can no longer carry oxygen into the system, the heat-making processes are put a stop to, and, the temperature declining, the body becomes of a marble coldness, characteristic of that terrible disease.--DRAPER.] This is maintained, as we have already seen, by the action of the oxygen within us. Each capillary tube is a tiny stove, where oxygen is combining with the tissues of the body (see note, p. 107). Every contraction of a muscle develops heat, the latent heat being set free by the breaking up of the tissue. The warmth so produced is distributed by the circulation of the blood. Thus the arteries, veins, and capillaries form a series of hot- water pipes, through which the heated liquid is forced by a pump--the heart--while the heat is kept up, not by a central furnace and boiler, but by a multitude of little fires placed here and there along its course.

2. _Regulation_.--The temperature of the body is regulated by means of the pores of the skin and the mucous membrane in the air passages. When the system becomes too warm, the blood vessels on the surface expand, the blood fills them, the fluid exudes into the perspiratory glands, pours out upon the exterior, and by evaporation cools the body. [Footnote: Just as water sprinkled on the floor cools a room.--_Popular Physics_, p. 255.] When the temperature of the body is too low, the vessels contract, less blood goes to the surface, the perspiration decreases, and the loss of heat by evaporation diminishes. [Footnote: Thus one is enabled to go into an oven where bread is baking, or into the arctic regions where the mountains are snow and the rivers ice. Even by these extremes the temperature of the blood will be but slightly affected. In the one case, the flood gates of perspiration will be opened and the superfluous heat expended in turning the water to vapor; and, in the other, they will be tightly closed and all the heat retained.]

LIFE BY DEATH.--The body is being incessantly corroded, and portions borne away by the tireless oxygen. The scales of the epidermis are constantly falling off and being replaced by secretion from the cutis. The disks of the blood die, and new ones spring into being. On the continuance of this interchange depend our health and vigor. Every act is a destructive one. Not a bend of the finger, not a wink of the eye, not a thought of the brain but is at some expense of the machine itself. Every process of life is thus a process of death. The more rapidly this change goes on, and fresh, vigorous tissue takes the place of the old, the more elasticity and strength we possess.

CHANGE OF OUR BODIES.--There is a belief that our bodies change once in seven years. From the nature of the case, the rate must vary with the labor we perform; the organs most used altering oftenest. Probably the parts of the body in incessant employment are entirely reorganized many times within a single year. [Footnote: To use a homely simile, our bodies are like the Irishman's knife, which, after having had several new blades, and at least one new handle, was yet the same old knife.]

THE THREE VITAL ORGANS.--Death is produced by the stoppage of the action of any one of the three organs--the heart, the lungs, or the brain. They have, therefore, been termed the "Tripod of Life." Really, however, as Huxley has remarked, "Life has but two legs to stand upon." If respiration and circulation be kept up artificially, the removal of the brain will not produce death. [Footnote: When death really does take place, _i. e._, when the vital organs are stopped, it is noticeable that the tissues do not die for some time thereafter. If suitable stimulants be applied, as the galvanic battery, transfusion of blood, etc., the muscles may be made to contract, and many of the phenomena of life be exhibited. Dr. Brown- Sequard thus produced muscular action in the hand of a criminal, fourteen hours after his execution.]

WONDERS OF THE HEART.--The ancients thought the heart to be the seat of love. There were located the purity and goodness as well as the evil passions of the soul. [Footnote: Our common words, hearty, large-hearted, courage (_cor_, the heart), are remains of this fanciful theory.] Modern science has found the seat of the mental powers to be in the brain. But while it has thus robbed the heart of its romance, it has revealed wonders which eclipse all the mysteries of the past. This marvelous little engine throbs on continually at the rate of one hundred thousand beats per day, forty millions per year, often three billions without a single stop. It is the most powerful of machines. "Its daily work is equal to one third that of all the muscles. If it should expend its entire force in lifting its own weight vertically, it would rise twenty thousand feet in an hour." [Footnote: "The greatest exploit ever accomplished by a locomotive, was to lift itself through less than one eighth of that distance." Vast and constant as is this process, so perfect is the machinery, that there are persons who do not even know where the heart lies until disease or accident reveals its location.] Its vitality is amazing. The most tireless of organs while life exists, it is one of the last to yield when life expires. So long as a flutter lingers at the heart, we know the spark of being is not quite extinguished, and there is hope of restoration. During a life such as we sometimes see, it has propelled half a million tons of blood, yet repaired itself as it has wasted, during its patient, unfaltering labor. The play of its valves and the rhythm of its throb have never failed until, at the command of the great Master Workman, the "wheels of life have stood still." [Footnote: Our brains are seventy-five- year clocks. The Angel of Life winds them up once for all, then closes the case, and gives the key into the hand of the Angel of the Resurrection. Ticktack! Ticktack! go the wheels of thought; our will can not stop them, they can not stop themselves; sleep can not stop them; madness only makes them go faster; death alone can break into the case, and, seizing the ever-swinging pendulum which we call the heart, silence at last the clicking of the terrible escapement we have carried so long beneath our wrinkled foreheads.--HOLMES.]

FIG. 43.

[Illustration: _Lymphatics of the Head and Neck, showing the Glands, and,_ B, _the thoracic duct as it empties into the left innominate vein at the junction of the left jugular and subclavian veins._]

THE LYMPHATIC CIRCULATION is intimately connected with that of the blood. It is, however, more delicate in its organization, and less thoroughly understood. Nearly every part of the body is permeated by a second series of capillaries, closely interlaced with the blood capillaries already described, and termed the Lymphatic system. The larger number converge into the thoracic duct--a small tube, about the size of a goose quill, which empties into the great veins of the neck (Fig. 43). Along their course the lymphatics frequently pass through _glands_,--hard, pinkish bodies of all sizes, from that of a hemp seed to an almond. These glands are often enlarged by disease, and then are easily felt.

_The Lymph_, which circulates through the lymphatics like blood through the veins, is a thin, colorless liquid, very like the serum. This fluid, probably in great measure an overflow from the blood vessels, is gathered up by the lymphatics, undergoes in the glands some process of preparation not well understood, and is then returned to the circulation.

FIG. 44.

[Illustration: _Lymphatics in the Leg, with Glands at the Hip_.]

OFFICE OF THE LYMPHATICS.--It is thought that portions of the waste matter of the body capable of further use are thus, by a wise economy, retained and elaborated in the system.

The _lacteals_, a class of lymphatics which will be described under Digestion (p. 166), aid in taking up the food; after a meal they become milk white. In the lungs, the lymphatics are abundant; sometimes absorbing the poison of disease, and diffusing it through the system. [Footnote: Persons have thus been poisoned by tiny particles of arsenic which evaporate from green wall paper, and float in the air.]

The lymphatics of the skin we have already spoken of as producing the phenomena of absorption, [Footnote: Pain is often relieved by injecting under the cuticle a solution of morphine, which is taken up by the absorbents, and so carried through the system.] Nature in her effort to heal a cut deposits an excess of matter to fill up the breach. Soon, the lymphatics go to work and remove the surplus material to other parts of the body.

Animals that hibernate are supported during the winter by the fat which their absorbents carry into the circulation from the extra supply they have laid up during the summer. In famine or in sickness, a man unconsciously consumes his own flesh.

DISEASES, ETC.--l. _Congestion_ is an unnatural accumulation of blood in any part of the body. The excess is indicated by the redness. If we put our feet in hot water, the capillaries will expand by the heat, and the blood will set that way to fill them. The red nose and purplish face of the drunkard show a congestion of the capillaries. Those vessels have lost their power of contraction, and so are permanently increased in size and filled with blood. Blushing is a temporary congestion. The capillaries being expanded only for an instant by the nervous excitement, contract again and expel the blood. [Footnote: Blushing is a purely local modification of the circulation of this kind, and it will be instructive to consider how a blush is brought about. An emotion--sometimes pleasurable, sometimes painful--takes possession of the mind; thereupon a hot flush is felt, the skin grows red, and according to the intensity of the emotion these changes are confined to the cheeks only, or extend to the "roots of the hair," or "all over." What is the cause of these changes? The blood is a red and a hot fluid; the skin reddens and grows hot, because its vessels contain an increased quantity of this red and hot fluid; and its vessels contain more, because the small arteries suddenly dilate, the natural moderate contraction of their muscles being superseded by a state of relaxation. In other words, the action of the nerves which