174 THE FROG CHAP. 



So far we have considered the nervous system only in 

 its relations to the skin or general surface of the body 

 and to the muscles or organs of movement. The other 

 parts of the body are, however, under nervous control. 



It has been mentioned that the heart continues to 

 beat in a frog when the brain has been removed : not 

 only so, but it pulsates with perfect regularity when 

 removed from the body. This is due to the fact that 

 the muscles of the heart, like the cilia of ciliated epithe- 

 lium, have the power of contracting rhythmically quite 

 independently of the nervous system, although the 

 heart contains nerve-cells which were formerly supposed 

 to serve as a special nervous system for this organ, 

 originating all its motor impulses. It is, however, 

 under the control of the central nervous system. We 

 have seen that it is supplied by a branch of the vagus ; 

 when this is stimulated, the heart stops in the dilated 

 state and begins to beat again only after a certain 

 interval. A feeble stimulus to the vagus will not 

 actually stop the heart, but will diminish the rate and 

 the strength of its contractions and consequently the 

 amount of blood propelled through the body. The 

 vagus is accompanied by a branch of the sympathetic 

 which has an exactly opposite effect, i.e., stimulation of 

 it accelerates the heart's action. In this way, the general 

 blood-supply of the body is regulated by the central 

 nervous system. 



The blood-supply of the various parts and organs is 

 regulated by the vaso-motor nerves. These are traceable 

 through the sympathetic into the spinal cord by the 

 ventral roots : distally they send branches to the mus- 

 cular coat of the arteries. Under ordinary circumstances 

 a constant succession of gentle stimuli pass along these 

 from a group of nerve-cells in the medulla oblongata 

 and, as the result, the arteries are ordinarily in a state 



