116 VERTEBRATE RESPIRATION 



mentally the same as that found in birds. This relationship is 

 supported by developmental studies which reveal that the 

 ventricular septa in these two groups are homologous. Both 

 these septa develop in a way which is completely different from 

 the inter- ventricular septum of the Dipnoi, the horizontal septum 

 of lizards and other reptiles, and the inter- ventricular septum of 

 mammals. 



Further evidence for the close relationships between croco- 

 diles and birds which may be derived from the circulatory 

 system is shown when the part of the carotid system which has 

 become elongated in the neck is examined. In the crocodile and 

 bird the elongation occurs mainly after the third arch has passed 

 round to the dorsal side whereas in the lizards and snakes the 

 elongation occurs ventrally. The inter-ventricular septa of croco- 

 diles and birds are in fact homologous with the dorsal ridge of the 

 lizard ventricle (fig. 33b). All these features are to be expected 

 from the palaeontological evidence which shows conclusively 

 that birds and crocodiles are both derived from the ruling rep- 

 tiles or Archosauria whereas the lizards and snakes arose from 

 a different stock, the Lepidosauria. 



The persistence in many modern reptiles of the left systemic 

 arch, ductus arteriosus, and ductus caroticus, together with an 

 incomplete inter-ventricular septum in all except crocodiles, 

 provides many possibilities for the inter-mingling of streams of 

 blood which are separate in birds and mammals. The presence 

 of these short-circuits (fig. 35), especially between the pulmonary 

 and systemic circulations, is indicative of some 'inefliiciency' in 

 the mechanism as a whole. If, as seems probable, the inter- 

 ventricular septum was almost complete in the earliest Amphibia 

 or Crossopterygii, then such an incomplete separation must have 

 persisted for a long time. The possibility that it may have some 

 definite functional significance has been considered many times 

 but no fully satisfactory reason has been given for its presence, 

 largely because details of the volumes circulating in different 

 parts of the vascular system are unknown for any reptile. 

 One reasonable suggestion, however, is that these persistent 

 portions of the ancestral system represent 'shunts' which make 



