218 



THE BODY AT WORK 



for the first time, distended with air. Up to that particular 

 minute they have had no functional use. Nothing would be 

 gained by compelling all the blood of the body to traverse the 

 vessels of the embryo's lungs. Until birth, therefore, the inter- 

 auricular septum is perforate. The blood takes a short-cut, 

 through the foramen ovale, from right auricle to left. But by 

 birth-time a curtain has grown down on the left side of the 



a. Carotid A. 

 Artery to rlykt arm 



L. Carotid A. 



left arn 



To Lungs, 

 from Lungs 



from Liver 



FIG. 10. THE HEART OUT IN THE PLANE OF ITS LONG Axis, AND THE VESSELS WHICH OPEN 



INTO AND OUT OP IT. 



Chorda? tendinese attach the margins of the auriculo-ventricular valves to musculi papillares 

 which project from the inner aspect of each ventricle. 



foramen. When the lungs are expanded by the forcible 

 enlargement of the chest-cavity which contains them, their 

 bloodvessels are distended by the same extensile force. Blood 

 is sucked into them from the right side of the heart. A differ- 

 ence in pressure on the two sides is established. A condition is 

 set up which is favourable to what may almost be termed the 

 adherence of the flap which hangs down on the left side of the 

 foramen ovale. The growth of its margin very rapidly 

 obliterates the hole. Occasionally the closure of the foramen 



