Page:Popular Science Monthly Volume 25.djvu/746

728 supplies both sides of the head (see Fig. 7, A), the artery supplying the left arm coming off as usual. This is the normal condition in apes, bears, dogs, and all the feline tribe. In some rare cases in man one branch only comes off from the aortic arch, and this, again, divides into the various arteries supplying the head and arms. In horses and other solipeds, we see this form of aortic arch (see Fig. 7, D). Again, the branches may all be given off separately from the arch, as is the arrangement in the walrus (see Fig. 7, C).

I have three times met with rather a rare anomaly of the great veins going to the heart from the upper part of the body. The usual



arrangement in man, on each side, is for the great vein of one arm and the corresponding side of the head to unite and form a single trunk (brachio-cephalic), so we have two large venous trunks, one on each side; these two trunks then join to form a single large vessel, called the superior vena cava, which empties its blood into the right side of the heart (see Fig. 8, A). It occasionally happens that the great venous trunks formed by the veins of the arm and head of each side do not unite to form the superior vena cava, but each continues its downward course and opens separately into the heart (see Fig. 8, B). On studying the development of the blood-vessels, we find that in early fetal life this condition of affairs exists, but after a time a transverse branch forms between the two trunks. This branch gradually enlarges, while the left trunk shrivels up, and at birth is only represented by a fibrous cord. This anomaly of the veins we find, then, is a persistence of a usually transient fetal condition in man, and also that in all birds and many of the lower mammals it is the permanent condition.