Apparatus of the circulation. Heart, arteries, veins, capillaries.

is the case with the hydra, represented in Fig. 1. And, what is very singular, the outside of the body of this animal is just as capable of acting as a stomach as its inside. For you may turn it inside out, as you can a stocking, and yet it will go on to catch and digest its food as usual. But, wide as the variations are in the digestive apparatus of animals, the same com mon object is aimed at in all-the assimilation (§ 10) of nutrient substances to the animal, to produce a material from which its structure can be built and kept in repair. There is, therefore, much that is common to them all in the modes in which this object is accomplished. And even the analogy

which exists between the animal and plant, in regard to assimilation, does not relate to the fact alone, but in some measure to the modes in which the process is effected.

CHAPTER VI.

CIRCULATION OF THE BLOOD.

97. In the last chapter I described the manner in which the blood is made from the food. The blood, thus prepared, is circulated in every part of the body, that it may be used for the purposes of construction and repair. The apparatus by which this is done acts, as I have before said, as the common carrier of the material which is used everywhere in the body by the laborers, the builders, to whom it is thus brought.

98. This apparatus consists of several parts—a great central organ, the heart, situated in the chest; the arteries, the tubes by which the blood is conducted to all parts of the body; the veins, other tubes, which bring the blood back to the heart; and capillaries, a network of exceedingly minute vessels, through which the blood passes as it goes from the extreme arteries into the beginnings of the veins. The blood goes from the heart through a large artery, called the aorta, which sends forth branches; and these divide and subdivide, so that the extreme arteries, through which the blood flows into the capillary network, are very minute. And the veins which receive the blood from this network to carry it back to the heart, are equally minute; but joining together more and more, as they proceed

Heart a forcing and suction pump. Arteries firm tubes. Why.

toward the heart, they are at length all united into two great venous trunks, one from above and the other from below, which pour their contents into this organ. The capillaries, taking their name from the Latin word, capilla, a hair, are so small that they can not be seen by the naked eye. In any small cut, the blood which oozes out comes from multitudes of these vessels. They serve to hold the blood, while the formative vessels, that construct and repair the body, may select from it such materials as they need for their purposes.

99. The heart is a great central forcing and suction pump, in the midst of this circulating apparatus. When it contracts, it forces the blood out through the aorta and its branching arteries into all parts of the system. And when it enlarges or dilates itself, it, by suction, as it is termed, receives the blood returning from the system through the veins. The blood never ceases to go these rounds. The necessity for this continual motion you will perceive as I proceed with the development of the subject.

100. The arteries differ from the veins in their structure and arrangement. The arteries are firm tubes, while the veins are lax in their structure. The object of the difference is obvious. As the blood is forced into the arteries by the powerful action of the heart, it is necessary that they should be strong and firm, else, they would be liable to dilatation and rupture, and death would frequently result. As it is, it is not a common event to have an artery dilate and burst, though it does occasionally happen. When dilatation does occur in an artery, it is called an aneurism. But the arteries need to be firm, not only for the sake of security against rupture, but also that the force of the heart may propel the blood to the extremities of the arterial system. If the arteries were lax tubes, like the veins, the impulse would soon be lost in the yielding tubes, and the blood would move very sluggishly in the small arteries at a distance from the heart. What we call the pulse, is caused by this impulse. If the arteries were lax tubes, the pulse would not be felt at any great distance from the heart. Instead of being distinct, as it now is, with every beat of the heart almost to the very extremities of the arterial system, it would be rendered confused by the yielding of the tubes, even quite near the heart, and at a distance from that organ it would be entirely lost.

101. Besides the firmness of the arteries, there is another circumstance which favors the freeness of the flow of blood

Different arrangement of arteries and veins.

through them. It is their mode of division. The branch of an artery leaves the main trunk at a sharp angle, making thus only a slight deviation from the direction of the current; while, on the other hand, in the veins where the current flows in an opposite direction, the branch unites with the trunk at nearly a right angle. This difference is represented in Fig. 22; 1 being the artery, and 2 the vein.

102. The venous system has a much greater capacity than the arterial. That is, all the veins of the body are together capable of holding more blood than all the arteries are. And the blood moves very rapidly and directly from the heart through the arteries, but it comes back to the heart quite slowly through the veins. Every thing is arranged to promote this rapid circulation through the arteries, while the venous system is calculated for a slow but sure progress of the blood back to the heart. To secure this, valves, made of folds of the inner lining of the veins are so arranged as to prevent the blood from flowing in the wrong direction. Fig. 23 represents a vein cut open so as to show these valves. A shows the valves as they appear when the vein is laid open and spread out; B, as they appear when the vein is simply laid open; and C represents the appearance of the outside of the vein where there are valves.

Valves in veins. Dangerous to wound an artery. Therefore well guarded

The need which there is of this help to the circulation through the veins is obvious. The suction power of the heart is not competent, unaided, to move the blood throughout all the lax venous system. These pocket-like valves, therefore, are made in the veins to assist the circulation there. They do so in this way. Every motion of the muscles or other parts about the veins tends to keep the blood in motion, and the valves serve to prevent this motion from being in the wrong direction. The difference in force and velocity with which the blood moves in the arteries and in the veins, is made manifest when they are wounded. The blood flows from a wounded vein in a slow and steady stream. From an artery it flows rapidly, showing the impulse of the heart in its jets, which correspond exactly with the pulse. Hence comes the danger in wounding an artery, while the wound of a vein is ordinarily attended with no danger. Accordingly, we find that the "Maker of our bodies" has so placed the arteries that they cannot easily be wounded, while many of the veins are quite freely exposed. The arteries are deeply seated, except in some few cases where this is impossible; but the veins are often superficially situated. You can see this, for example, in the bend of the arm. Some large veins appear there just under the skin, while the artery which supplies the arm is imbedded among the muscles and tendons. In every part of the body, the most secure spot is chosen for an artery. Thus, at the knee joint, the artery, instead of running over the surface of bone, where it would be liable to be

Few arteries superficial. Mode of stopping the bleeding of an artery. wounded, lies deep in the ham at the rear of the joint. The same is true of the elbow joint, just alluded to, and of other parts of the body. Although there are arteries everywhere, they are so uniformly deeply seated, that it is only in a few localities that you can readily find one. You can feel one pulsating at the wrist, and also on the temple. Here the arteries are superficial, only because it is impossible that it should be otherwise.

103. When the physician bleeds a patient, he commonly does it at the bend of the arm, as being the most convenient place for the operation. A ligature of some sort, as a ribbon, is tied around the arm above the elbow, with sufficient tightness to interrupt the flow of blood toward the heart in the superficial veins, but not so tightly as to prevent the free supply of blood to the arm by the artery. It is commonly tied as tightly as it can be without stopping the pulse at the wrist. An opening is then made in one of the veins; and, as the blood flows freely into the arm from the heart through the artery, on its return, so much of it as passes through the opened vein is discharged at that point.

104. It will be proper here to give some practical instruction, in regard to stopping the flow of blood from a wounded artery, as many lives have been lost from the ignorance of bystanders when such accidents have happened. Enveloping the part in cloths, which is so commonly done at such times, does no good, but only serves to catch and conceal the blood as it flows. Pressure upon the artery, on that side of the wound which is toward the heart, will of course interrupt the supply of blood from this organ to the wound. Firm pressure with the thumb will do it. But the pressure must be made at the right point, that is, directly upon the artery. You may not, in all cases, press upon the right spot at once. If you do not, the blood will continue to flow. In this case, press at different points, until you find the point at which you see that pressure stops the flow of blood from the wound. But you may not be able to find the right spot. If you can not, you can tie a slip of strong cloth or a handkerchief around the limb, above the wound, and twist a stick in it until the bleeding stops. In one or the other of these ways, you can prevent the loss of blood until the surgeon arrives to take charge of the case.

105. Although there is no such free communication between arteries as exists between the capillaries, there is some amount of communication, and particularly in certain parts of the body.