COURSE OF CIRCULATION IN MAMMALS 



147 



cistern viz., those placed in the common femoral, the subclavian, 

 and jugular veins (Fig. 55). The cistern, when filled, may hold some 

 400 c.c. of blood. In the liver there may be some 500 c.c. of blood. 

 This can be expressed into the cistern by abdominal pressure. In 

 the portal venous system, when distended, may be held another 



int nia. ueu 



It uentitcTt 



FIG. 55. DIAGRAM OF THE VENOUS CISTERN FROM WHICH THE HEART is FILLED. 



(Keith.) 



The abdominal or infra-diaphragmatic part of the cistern is indicated in black; the 

 thoracic or supra-diaphragmatic is stippled. The heart is compressed upwards 

 and backwards against its attachments. 



500 c.c., which can be expressed through the portal veins and the 

 liver into the cistern. There is thus a large volume of blood for the 

 heart to draw upon during diastole, and this may be of particular 

 value during the performance of a great muscular effort. Respiration 

 by the aspirating action of the thorax sucks this blood into the heart, 

 while the inspiratory descent of the diaphragm squeezes the abdominal 



