174 THE FROG CHAP. 



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 epithelium, 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 feebler 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 accom- 

 panied by a branch of the sympathetic which has an exactly 

 opposite effect ; t.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 muscular 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 a result, the 

 arteries are ordinarily in a state of slight contraction. By 

 various circumstances these stimuli may be diminished for any 

 given artery and at the same time stimuli pass down another 

 kind of vaso-motor fibres : the artery will then dilate and 

 the blood-supply of the organ to which it is distributed will 



