118 VERTEBRATE RESPIRATION 



An alternative hypothesis which also entails their function as 

 shunts between the pulmonary and systemic circuits is suggested 

 by the recent work on Xenopus and more especially by analogy 

 with the circulation of the mammalian foetus at birth. This 

 latter system has been worked out in great detail and is discussed 

 below. In both the adult Xenopus and the mammal shortly after 

 birth, it appears that, despite the relative inefficiency of the lung, 

 the volume of blood passing through the pulmonary capillaries 

 exceeds the volume passing round the rest of the body. In this 

 way compensation is made for the relative inefficiency of the 

 respiratory organ at taking up oxygen. In the young mammal 

 blood is shunted from the systemic circuit through the ductus 

 arteriosus and into the pulmonary circulation, rather than the 

 reverse which might be expected and is the case before birth. 

 There appear to be no observations on the path of the blood 

 through the ductus arteriosus of reptiles nor is there any 

 information available about the relative volumes of blood 

 flowing round the pulmonary and systemic circuits. It would 

 be a possible solution to their respiratory problems if a greater 

 volume of blood passed to the lung in order to compensate for 

 the relative inefficiency of their gaseous exchange. On this view 

 the evolution of a respiratory organ capable of an efficient 

 gaseous exchange is stressed rather than the change in the pul- 

 monary capillary resistance involved in this changed lung struc- 

 ture. Of course it is very probable that both a decrease in 

 resistance to blood flow and an increase in its efficiency for 

 gaseous exchange would have gone hand in hand. 



(f) The mammalian double circulation 



It is apparent from the considerations given above that the 

 mammalian circulation cannot be derived from any known 

 reptihan type, for completion of the inter-ventricular septum 

 would lead to a pair of very unequal ventricles as one would 

 receive blood from both auricles whereas the other would 

 become a closed chamber. A further difficulty is that in all 

 modern reptiles both carotid arches arise from the right sys- 

 temic whereas in all mammals they take their origin from the 



