THE HEART AND CIRCULATORY SYSTEM 



117 



Fig. 35. 



Diagram of the circulation in reptiles, birds and mammals to 

 illustrate the different places where blood can be short-circuited 

 between the systemic and pulmonary circulations. These 'shunts' 

 are respectively: (1) The foramen ovale, (2) an incomplete inter- 

 ventricular septum, (3) foramen of Panizza, (4) the right systemic 

 of reptiles, (5) the ductus arteriosus. 



it possible for any excess blood in one part of the system to pass 

 across into another portion where the pressure is not so great. 

 With reference to the systemic and pulmonary circulation, this 

 would clearly have advantages in early tetrapods where the lungs 

 are relatively undeveloped and their capillary bed quite likely to 

 be unable to take sufficiently large volumes of blood. On this 

 view it is supposed that only when the lung had evolved suffi- 

 ciently did it become possible for the pulmonary circulation to 

 take the same amount of blood as the systemic circulation. For 

 it is a condition of a complete double circulation that both 

 circuits should be able to carry equal volumes of blood in unit 

 time. If these persisting portions of the system do function as 

 'shunts', they play an important role in the functioning of the 

 reptilian cardio-vascular system. Unfortunately, details of their 

 working are not available and this supposition remains un- 

 proven. The few blood pressure recordings that have been made 

 on reptile hearts suggest, however, that the resistance to flow 

 through the lung is not as great as has been generally supposed. 



