372 



COMPARATIVE ANATOMY 



pressure in the right atrium is greater than in the left atrium and conse- 

 quently the septum on the left is pushed back like a swinging door into 

 the left atrium, and blood enters freely from the right atrium through 

 the foramen ovale in Septum II. (Fig. 321) 



When, however, the child is born and the pulmonary circulation is 

 augmented, the increase in the amount of blood in the left atrium equalizes 



right duct 



left duct 

 of Cuvier 



ndocardial cusViion 

 of 

 'fS!Sig5S\ atrioventricular 

 canal 



- trabecular 

 carneae 



Fig. 310. — Section through the heart of a 9.4 mm. pig embryo, at the level of the atrio- 

 ventricular canals. X45. (From Patten's "Embryology of the Pig.") 



the pressure in the two atria, the left septum (Septum I) is forced back 

 against the right (Septum II), and the foramen ovale is closed. The blood 

 in the two atria is therefore no longer mixed. After a few months or 

 years the two septa normally unite to form an imperforate septum. 



Development of Valves. The processes involved in the development 

 of the atrio-ventricular valves are complicated. The valves arise from 

 thickened folds of the heart- wall lying in the constricted passage between 

 atrium and ventricle. The folds from which the valves develop are lip- 



