124 VERTEBRATE RESPIRATION 



reveals the many factors which are involved in the functioning 

 of such systems with incomplete separation of the oxygenated 

 and deoxygenated bloods. The vital importance of the shunts 

 between the two main circuits is easily appreciated. Before birth 

 both the foramen ovale and the ductus arteriosus perform vital 

 functions as does the ductus following the tying of the placenta. 

 One interesting feature is the evidence that the inefficient func- 

 tioning of the lungs is overcome by an increase in the flow of 

 blood in the pulmonary circuit. As has been mentioned earher it 

 is usually assumed, when discussing incomplete circulations of 

 lower vertebrates, that the lung capillaries are unable to take a 

 sufficient volume of blood to enable the separation of the two 

 circuits to have evolved. It now seems probable that under 

 these conditions the shunts in the circulation function by 

 allowing more blood to pass to the lungs from the systemic 

 circuit, especially during increased activity of the animal. 



