112 VERTEBRATE RESPIRATION 



Despite the convincing evidence which has now been brought 

 forward to support the view that the blood from the right and 

 left auricles is not distributed at random into the aortic arches, 

 there seems little doubt that the separation achieved by the 

 dipnoan heart is far greater. The complete absence of physio- 

 logical work on the heart of the Dipnoi makes this a difficult 

 comparison, however, because future investigations may well 

 show that the separation, at least in Protopterus and Neocera- 

 todus, is less than might be supposed on morphological evidence. 

 Nevertheless, it must also be appreciated that the modem 

 Amphibia have departed considerably from the habit and mode 

 of life of their ancestors which were the progenitors of the main 

 tetrapod stock. In attempting to deduce the type of heart and 

 arterial circulation of the early land vertebrates it may be prefer- 

 able to take the dipnoan heart with an almost completely divided 

 ventricle as a model. On this view the single ventricle of modern 

 amphibians represents a form in which the inter-ventricular 

 septum of its crossopterygian ancestor has become reduced in 

 relation to its more specialised respiratory habits, chiefly in the 

 greater use of cutaneous respiration. On the other hand it must 

 equally be remembered that the heart of modern dipnoans also 

 shows specialisation in its structure indicative of a departure 

 from the main line of evolution. The atrio-ventricular opening 

 is guarded by a fibrous or partly cartilaginous plug which arises 

 from the posterior margin of the opening. It is moved by muscles 

 attached to the inter-auricular and inter-ventricular septa and 

 is a structure peculiar to the Dipnoi. 



(e) The heart and aortic arches of modern reptiles 



The heart of all reptiles has two auricles and the ventricle is 

 partially divided, but details of the inter-ventricular septum are 

 complex and vary between diff'erent groups. The conus of all 

 reptiles is divided from its origin at the ventricle into three 

 sections, namely the pulmonary, right systemic, and left sys- 

 temic arches. The division of the ventricle is complete in croco- 

 diles but incomplete in all other groups. The lizard (Lacerta) 

 heart (fig. 33) may be taken as an example which immediately 



