THE CIRCULATION 



275 



expiration. The blood is thus forced on into the arteries and 

 so to the muscles, and they, by their alternate contraction 

 and relaxation, still further help to drive it on and to accele- 

 rate the* circulation. The high arterial tension tends to 

 drive the blood through the cranial vessels. The benefit 



INTRACRANIAL CIRCULATION 



PULMONARY CIRCULATION 



[ABDOMINAL CIRCULATION 



CIRCULATION IN LIMBS 



FIG. 129. Scheme of the Circulation, modified from Hill, to illustrate the influence 

 of the various extra-Cardiac Factors which maintain the Flow of Blood. 



of intermittent muscular exercise on the circulation is thus 

 manifest. 



When, on the other hand, some sustained muscular strain 

 has to be undergone, the thorax, is fixed and hence the 

 pressure in the heart and thoracic organs is raised, and the 

 increased pressure in the thorax helps to support the heart 

 and to prevent over-distension. The abdominal vessels are 



