302 HUMAN PHYSIOLOGY 



good posture, to give the heart a clear space in which 

 to work and a desirable traction outward to assist its 

 diastole. It also widens the cross-section of the pul- 

 monary vessels so that the blood need not be hurried 

 through them, lengthens its time of exposure to the 

 alveolar air, and lightens the labor of the right ventricle. 

 There are various involved relations of a mechanical 

 sort between the breathing movements and the blood- 

 flow which are too difficult to discuss in a work like the 

 present. 



