122 PRINCIPLES OF PHYSIOLOGY 



supply of blood to the capillary districts even 

 between the heart beats. When the heart 

 ceases to beat, as at death, the arterioles 

 become at the same time widely dilated, the 

 pulse disappears, and by the elastic recoil of 

 the walls of the great arteries the blood 

 passes through the capillaries into the veins. 

 Hence, after death, in ordinary circumstances, 

 the blood is found in the veins, while the 

 heart and arteries are empty It will be 

 seen, also, that the contractile coat of the 

 arteries regulates the supply of blood to various 

 capillary districts, according to their physio- 

 logical necessities. 



63. The movements of respiration also 

 assist the circulation. During inspiration, 

 when the chest is dilated, pressure is removed 

 from the surface of the heart and of the great 

 vessels springing from it ; these tend to dilate 

 and thus the blood is as it were sucked towards 

 the heart by the great veins and by the 

 right auricle and right ventricle. During 

 expiration there is increased pressure on the 

 heart, more especially when expiration is 

 forced, and the blood does not flow towards 



