Fig. 11.
BLOOD-VESSELS.
The blood is the fluid into which our food is changed, and which is employed to minister nourishment to the whole body. For this purpose, it is carried to every part of the body, by the arteries; and, after it has given out its nourishment, returns to the heart, through the veins.
The subjoined engraving, (Fig. [12],) which presents a rude outline of the vascular system, will more clearly illustrate this operation, as we shall presently show.
Before entering the heart, the blood receives a fresh supply of nourishment, by a duct which leads from the stomach. The arteries have their origin from the heart, in a great trunk, called the aorta, which is the parent of all the arteries, as the spinal marrow is the parent of the nerves which it sends out. When the arteries have branched out into myriads of minute vessels, the blood which is in them passes into as minute veins; and these run into each other, like the rills and branches of a river, until they are all united in two great veins, which run into the heart. One of these large receivers, called the vena cava superior, or upper vena cava, brings back the blood from the arms and head, the other, the vena cava inferior, or lower vena cava, brings back the blood from the body and lower limbs.
In the preceding figure, H, is the heart, which is divided into four compartments; two, called auricles, used for receiving the blood, and two, called ventricles, used for sending out the blood. A, is the aorta, or great artery, which sends its branches to every part of the body. In the upper part, at a, a, a, are the main branches of the aorta, which go to the head and arms. Below, at a, a, are the branches which go to the lower limbs. The branches which set off at X, X, are those by which the intestines are supplied by vessels from the aorta. Every muscle in the whole body, all the organs of the body, and the skin, are supplied by branches sent off from this great artery. When the blood is thus dispersed through any organ, in minute vessels, it is received, at their terminations, by numerous minute veins, which gradually unite, forming larger branches, till they all meet in either the upper or lower vena cava, which returns the blood to the heart. V I, is the vena cava inferior, which receives the blood from the veins of the lower parts of the body, as seen at v, v. The blood, sent into the lower limbs from the aorta, is received by minute veins, which finally unite at v, v, and thus it is emptied through the lower vena cava into the heart: o, o, represent the points of entrance of those tributaries of the vena cava, which receive that blood from the intestines, which is sent out by the aorta at X, X. In the upper part, V S, is the vena cava superior, which receives the blood from the head and arms; v, v, v, are the tributaries of the upper vena cava, which bring the blood back from the head and arms; d, d, represents the course of the thoracic duct, a delicate tube by which the chyle is carried into the blood, as mentioned on page [89]; t, shows the place where this duct empties into a branch of the vena cava.