THE SIGNIFICANCE OF HUMAN ANOMALIES, yi-j 



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 dis- 

 tinct 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 condi- 

 tion 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-vesBels 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 dis- 

 tributes blood to the trunk and 

 lower extremities. These branches 

 are now known to be derived from 

 certain of the original gill-arches 



R.C 



Pig. 5.— Norkal Aortic Arch in Maw. 

 R. C , L. C, ri^ht and left carotid arteries 



foiaff to the head ; R. S., L. S., right aud 

 eft subclavian arteries going to the arms ; 

 I, innominate artery. 



Fig. 6.— GiMj Arches— the dark lines show the 

 ones which normally persist in man. 



which persist (see Fig. 6), and when any variation in their arrange- 

 ment 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 



