THE HEART AND CIRCULATORY SYSTEM 119 



left systemic. These difficulties are due, of course, to the fact 

 that among modern reptiles there are only representatives of the 

 diapsid line of evolution which was distinct from that which 

 gave rise to the mammals. It seems, therefore, that in attempting 

 to look for a common ancestral pattern from which to derive 

 the mammalian circulation we must look at something like the 

 dipnoan type of heart and aortic arches. From such a beginning, 

 it is probable that the first stage in evolution was the separa- 

 tion of the pulmonary arch from the rest of the conus ; such a 

 primitive type could lead directly to the mammalian system via 

 reptiles in which the carotids came off the left systemic arch. 

 Unfortunately, there are no living descendants of the synapsid 

 line of reptiles which gave rise to the mammals and it is for this 

 reason that such a large gap exists in our comparative morpho- 

 logical story. One important feature which both birds and 

 mammals have in common, however, is that the systemic arch 

 which persists in both groups is that which takes its origin from 

 the left side of the ventricle. This is of fundamental importance 

 because in all vertebrates where the oxygenated and deoxy- 

 genated bloods are separated, the left side of the heart receives 

 the blood which is returned from the lung. 



(g) Structural and physiological changes in the mammalian heart 

 and circulation at birth 



All six embryonic arches are represented during the develop- 

 ment of mammals, but not all appear at the same time. Those 

 persisting in the adult are the 3rd, 4th, and 6th arches which, as 

 in other tetrapods, respectively become the carotid, systemic, 

 and pulmonary arteries. In the development of the venous 

 system, a similar recapitulatory series is present, for the posterior 

 cardinal veins appear early and later give way to the inferior 

 vena cava which has complex homologies. 



It is, however, in the later stages of development just prior and 

 immediately following birth that the most interesting features 

 are found. The circulation of a foetal lamb is shown diagram- 

 matically in fig. 36. The four-chambered heart functions as two 

 parallel pumps which serve to circulate the whole foetus, as was 



