260 



COMPARATIVE PHYSIOLOGY. 



control through the vagus nerve by the cardio-inhibitory cen- 

 ter ; second, through the sympathetic (motor) fibers in the 

 vagus itself ; and, finally, through fibers with similar action 

 in the sympathetic system, usually so called. 



The share taken by these factors is certainly variable in dif- 

 ferent species of animals, and it is likely that this is true of the 

 same animals on different occasions. It is also conceivable, 

 and indeed probable, that they act together at times, the inhibi- 

 tory action being diminished and the augmentor influence in- 

 creased. 



THE HEART IN RELATION TO BLOOD-PRESSURE. 



It is plain that all the other conditions throughout the cir- 

 culatory system remaining the same, an increase in either the 

 force or the frequency of the heart-beat must raise the blood- 

 pressure. But, if the pressure were generally raised when the 

 heart beats rapidly, it would fare ill with the aged, the elasticity 

 of their arteries being usually greatly impaired. As a matter of 

 fact any marked rise of pressure that would thus occur is pre- 



X«<'^ 



Fig. 218.— Tracing from a rabbit, showing the influence of cardiac Inhibition on blood- 



Eressare. The fall m this caae was very rapid, owing to sudden cessation of the 

 eart-beat. The relative emptiness of the vessels accounts (or the peculiar char- 

 acter of the curve of rising blood-pressure (Foster). 



vented as a rule, and in different ways, as will be seen ; but, so 

 far as the heart is concerned, its beat is usually the weaker the 

 more rapid it is, so that the cardiac rhythm and the blood-press- 

 ure are in inverse proportion to each other. 



By what method is the heart's action tempered to the condi- 



