XIII MAMMALIA 



443 



bird the unpaired dorsal aorta receives its blood-supply entirely from 

 one side of the body : only in the mammal this is the left side whereas 

 in the bird it is the right. A comparison of Figs. i86 and 185 will show 

 however that the arrangement in the mammal is by no means a simple 

 reversal, as regards right and left, of that in the bird. For in the bird 

 the entire fourth aortic arch on the left side has disappeared together 

 with the whole of the aortic root lying on the tailward side of that arch, 

 while in the mammal the fourth arch of the right side and a large part 

 of the right aortic root have been saved from disappearance by the 

 presence of the (" primary ") subclavian artery branching off from the 

 aortic root. In the bird on the other hand the primary subclavian is 

 no longer present having been replaced by a new (" secondary ") sub- 

 clavian which branches out from the region of the ventral end of Arch III. 



Inspection of the two figures mentioned also teaches another interesting 

 evolutionary lesson for it is seen that the arrangement in the bird is 

 clearly a development of that seen in the typical reptile, brought about 

 by the disappearance of the left systemic aorta, while the arrangement 

 characteristic of the mammal seems equally clearly to have evolved out 

 of a condition less advanced than that of the typical reptile, a condition 

 in which the systemic portion of conus and ventral aorta had not yet 

 become split longitudinally into a right and a left half. 



In the descriptive anatomy of man and other mammals the whole 

 extent of vessel from the left ventricle to the dorsal aorta is spoken of 

 as the " arch of the aorta," and in a typical mammal this gives off in 

 succession three branches — the innominate, the left common carotid and 

 the left subclavian arteries — the relations of which to the primitive scheme 

 are made clear by Fig. 186. In various members of the group Mammalia 

 departures are made in detail from the typical arrangement. Thus the 

 left subclavian may take its origin further forward and have a portion 

 in common with the left common carotid so that one may speak of a 

 left innominate artery in addition to the normal (right) innominate. Or 

 the right subclavian may arise directly from the main systemic aorta so 

 that there is no innominate artery but a pair of symmetrical common 

 carotids. Again in such a case the two common carotids may for a 

 considerable distance form an unpaired so-called primary carotid, and 

 finally the two subclavian arteries may originate symmetrically from 

 this unpaired trunk. All such departures from the more typical arrange- 

 ment are of interest as illustrating the ways in which blood-vessels are 

 liable to vary. 



The venous system of the mammals is characterised by the well- 

 developed posterior vena cava draining the blood from both kidneys, by 



