A MANUAL OF PHYSIOLOGY 



28' 5 

 S30 



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standstill. The question cannot fail to press itself upon the mind of 

 anyone who has ever witnessed this most beautiful of physiological 

 experiments ; but as yet there is no answer except ingenious specula- 

 tions. The most plausible of these is the trophic theory of Gaskell, 

 who sees in the vagus a nerve which so acts upon the chemical changes 

 going on in the heart as to give them a trophic, or anabolic, or con- 

 structive turn, and thus to lessen for the time the destructive changes 

 underlying the muscular contraction. The augmentor nerves, on 

 the other hand, are supposed to exert a katabolic influence, and to 

 favour these destructive changes. And while, according to Gaskell, 

 the natural consequence of inhibition is a stage of increased efficiency 

 and working power when the inhibition has passed away, the natural 



complement of aug- 



jpj mentation is a tem- 



porary exhaustion. 

 But it must be re- 

 membered that this 

 distinction is not as 

 yet based upon any 

 very solid founda- 

 tion of actually ob- 

 served and easily 

 interpreted facts, 

 while to some of the 

 facts brought for- 

 ward in its favour 

 undue importance 

 has been given. 



Whatever the ex- 

 act mechanism of 

 augmentation may 

 be, there is no foun- 

 dation for the state- 

 ment that the car- 

 dio-aug mentor 

 nerves have an ac- 

 tion on the heart so 

 fundamentally dif- 

 ferent from the ac- 

 tion of motor nerves 

 on skeletal muscle 

 that they cannot 

 originate contrac- 

 tions in a heart entirely at rest. Excitation of the cardio-aug- 

 mentor nerves can cause rhythmical contractions in the perfectly 

 quiescent heart of molluscs, and a sudden and prolonged outburst of 

 beats of great force in the frog's heart, which has been brought to a 

 standstill by cautiously heating it to 40 to 43 C. (Practical Exer- 

 cises, p. 178) for a minute or two, or to a considerably lower tempera- 

 ture, for a longer time (Fig. 65). A similar effect can be obtained on 

 the quiescent mammalian heart by stimulation of the nervi 

 aocelerantes. 



The Normal Excitation of the Cardiac Nervous Mechanism. 

 We have now to inquire how this elaborate nervous mechanism 

 is normally set into action. And we may say at once that, 



FIG. 65. EFFECT OF STIMULATION OF FROG'S CARDIAC 

 SYMPATHETIC DURING COMPLETE STANDSTILL OF THE 

 HEART AT 28-5 C. 



Upper tracing, auricle ; lower, ventricle. To be read 

 from right to left. Time-trace, two-second intervals. 



