THE HEART AND CIRCULATORY SYSTEM 



123 



saturation of the blood in the carotid artery is lowered by 10- 

 20%, which indicates to what extent the patency of the ductus 

 arteriosus compensates for the relative inefficiency of the 

 respiratory exchange. Occlusion of the ductus arteriosus nor- 

 mally takes place by differential growth and its constriction is 

 complete within a day of birth, but it already begins one-quarter 

 of an hour after the dehvery of the new-born lamb. In other 

 mammals its complete occlusion may take a much longer period, 



FOETAL 



[body] I L. HEART | 



DA 



-0- 



|r heart I 



NEONATAL 



iW] |l heart [ 



ADULT 



Fig. 37. 



Diagrams of the changes in circulation which occur at birth of a 

 lamb (from Born, Dawes, Mott, and Widdicombe, 1954). 



notably some aquatic mammals such as seals and whales where 

 it may be patent for as long as 20 days after birth. 



The knowledge obtained about this circulation during the 

 change from an incomplete to a complete double circulation 



