Page:Popular Science Monthly Volume 25.djvu/745

Rh valves, and is placed far forward in the neck. The gill-arches now partly disappear, and, though the circulation still remains single as in reptiles, the heart-cavities are beginning to be separated into two distinct systems. Soon a double circulation is acquired by a complete separation of the heart into right and left. The right heart propels the venous and the left the arterial blood. At this period the condition is identical with that of birds; at last the true mammalian type of heart and blood-vessels develops and remains permanent. The arrangement of the great blood-vessels going to and from the heart varies considerably in different mammals. In man the rule is for the great artery, carrying the blood from the heart to the general system, to give off three main branches, named the innominate, left carotid, and left subclavian (see Fig. 5). These are distributed to the head and the two arms; the main vessel or aorta curves downward and distributes blood to the trunk and lower extremities. These branches are now known to be derived from certain of the original gill-arches

which persist (see Fig. 6), and when any variation in their arrangement takes place it always occurs in the line of some of these gill-arches; that is, some of the arches persist which usually are obliterated. Nearly all the variations occurring in these large vessels in man are found to be the regular condition in animals lower in the scale; for instance, sometimes only two branches are given off instead of three; each of these, again, dividing into two, one for the head and one for the arm of that side (see Fig. 7, B). This is the usual arrangement in the bat, porpoise, and dolphin. The commonest variation of the aortic arch is where the innominate gives off the left carotid, and so