THE CARDIAC NERVES. 579 



of one vagus gives usually a partial effect, — that is, the heart-rate 

 is increased somewhat, — but it is still further increased by section 

 of the second vagus. The exact result obtained when the nerves are 



Fig. 242. — To show the effect of section of the two vagi in the dog upon the rate of 

 heart beat and tlie blood-pressure : 1 marks the section of the vagus on the right side ; 

 2, section of tlie second vagus. The numerals on the vertical mark the blood-pressures ; 

 the numerals on the blood-pressure record give the rate of heart beats. (Dawson.) 



severed separately varies undoubtedly with the conditions, — for 

 instance, with the intensity of the tonic activity of the center. 

 Throughout life, speaking in general terms, the cardio-inhibitory 

 center keeps the "brakes" on the heart rate, and the extent of its 

 action varies under different conditions. When its tonic action is 

 increased the rate becomes slower; when it is decreased the rate 

 becomes faster. In all probability, this tonic action of the center, 

 like that of the motor centers generally, is in reality a reflex tonus. 

 That is, it is not due to automatic processes generated within the 

 nerve cells by their own metabolism or by changes in their 

 liquid environment, but to stimulations received through sensory 

 nerves. The continuous though varying inflow of impulses into 

 the central nervous system through different nerve paths keeps 

 the center in that state of permanent gentle activity which we 



