n6 THE ENGINES OF THE HUMAN BODY 



A single muscle, like the biceps, has two or three hundred 

 terminal arteries, and each has its constrictor muscle or 

 stopcock. When the biceps is set in action, the arterial 

 stopcocks are also turned on. Every muscle or organ 

 can somehow command its vascular supply — can have its 

 turn-cocks turned on and off. We are not certain how a 

 muscle succeeds in getting access to the nerve centres 

 which control the stopcocks ; we simply know that it is 

 so. In walking or in hard exercise like digging, when all 

 the muscles are in action, it is plain that half or more of 

 the vascular stopcocks of the body must be turned on 

 full, and that the head of pressure in the aorta will quickly 



'erve 



Circular' 



muscle fibres 



Fig. 30. — A terminal artery, surrounded by muscle fibres, ending in a 

 network of capillary vessels. 



fall unless the heart responds by increasing its output. 

 We know that the heart always responds ; it can increase 

 output to eight times or more the amount thrown out 

 when the body is at rest. 



There is, however, another mechanism, besides that of 

 increasing the output of the heart, which is employed 

 to keep up the head of blood-pressure in the aorta. The 

 terminal arteries leading to parts which are not required 

 in severe bodily exercise, such as the organs of digestion 

 and the brain, are constricted, thus economising the blood 

 supply for the benefit of the muscles. When the muscles 

 are set in motion their vascular stopcocks are turned on, 

 while others are turned off. If the human machine were 

 not supplied by this elaborate stopcock system, it would 



