THE CIRCULATION OF THE BLOOD AND LYMPH 143 



ligature cuts off the ventricle from the inhibitory impulses, while 

 leaving the auricle still under their influence. 



Nature of Inhibition and Augmentation. So far we have been 

 discussing the phenomena of inhibition and augmentation as ultimate 

 facts. We have not attempted to go behind them, nor to ask what it 

 is that really happens when inhibitory impulses fall into a heart, which 

 from the first days of embryonic life has gone on beating with a regular 

 rhythm, and in the space of a second or two bring it to a 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 speculations. 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 

 neart as to give them a 

 trophic, or anabolic, or 

 constructive 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 destruc- 

 tive changes. And while, 



FIG. 52. FROG'S HEART. 



Sympathetic stimulated (30 mm. between the coils). 



nf inhihitinn is a <jtao-F> Temperature 12*. Marked increase in force. Only 

 I IS a Stage auricular trad reproduced. Time trace, two-second 

 of increased efficiency intervals. 

 and working power when 



the inhibition has passed away, the natural complement of augmenta- 

 tion is a temporary exhaustion. 



But it must be remembered that this distinction is not as yet based 

 upon any very solid foundation of actually-observed and easily- 

 interpreted facts, while to some of the facts brought forward in its 

 favour undue importance has been given. For instance, a positive 

 electrical variation has been seen in the quiescent auricle of the 

 tortoise on stimulating the vagus, and a negative variation in the 

 quiescent frog's ventricle on stimulating the cardiac sympathetic, 

 neither of these variations apparently being accompanied with any 

 sensible mechanical change. It has been argued from this (on the 

 assumption that the negative variation observed when most excitable 

 tissues, muscle and nerve, for example, are stimulated, is the ex- 

 pression of destructive metabolic changes or katabolism), that the 

 vagus has the power of causing constructive (anabolic) changes, and 



according to Gaskell, 

 the natural consequence 



