THE CARDIAC NERVES. 



519 



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. 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 severed sepa- 



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Fig. 215. To show the effect of section of the two vagi 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.) 



rately 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 



