538 



CIRCULATION OF BLOOD AND LYMPH. 



rupting, in some other way cooling, for example the connection 

 between the center and the heart. When the two vagi are cut the 

 heart rate increases greatly and the blood-pressure rises on account 

 of the greater output of blood in a unit of time (Fig. 222). Section 

 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. 222. To show the effect of section of the two vagi in the dog upon the rate of 

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

 2, section of the 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. 



