100 ELEMENTS OF PHYSIOLOGY 



its beats. When an animal requires a greater supply of 

 blood, as in running, these impulses for a time cease, and 

 the heart beat is quicker and stronger (Fig. 202). 



164. There are other nerves, called sympathetic nerves, 

 connected with the spinal cord below the point at which 

 the vagus branches, that bring impulses to the heart which 

 are opposite in effect to those brought by the vagus. 

 These impulses also start in the spinal bulb ; they cause a 

 quickening and strengthening of the beats (Fig. 200). 

 Do the vagus nerve fibers or the sympathetic nerve fibers 

 resemble the whip which a driver uses in driving a horse ? 

 Which kind corresponds to the reins ? Thus the need of 

 the body for a greater or less active blood supply is regu- 

 lated by controlling the rate and strength of the heart 

 beats. If the nerves are all in order, the heart beats more 

 slowly when the tissues of the body need little blood, and 

 more rapidly when the tissues need more food or more 

 oxygen. But quickening of the heart beat cannot send 

 more blood through one organ without sending more 

 blood through all the organs, hence it is not so delicate a 

 means of regulating the blood supply as the vasomotor 

 system. 



