258 INHIBITION AND AUGMENTATION. [BOOK i. 



ing, by rendering incapable of activity, and muscarin its effect by 

 exciting, stimulating into activity, this same inhibitory mechan- 

 ism. It has further been suggested that some of the ganglia in the 

 heart furnish the mechanism in question. And it has been sup- 

 posed that there is a corresponding augmenting mechanism. But 

 objections may be urged against this view, and it is safer to leave 

 as an open question the exact manner in which inhibition and 

 augmentation are brought about. 



One point is perhaps worthy of mention. We have seen 

 that inhibition may be followed by a phase of increased activity, 

 and that on the whole the heart is strengthened rather than 

 weakened by the process, while on the other hand augmentation is 

 followed by depression and the process is distinctly an exhausting 

 one. Hence whatever be the exact mechanism of inhibition and of 

 augmentation, whatever be the particular elements of the cardiac 

 structures which are concerned in the one or the other, augmenta- 

 tion means increasad expenditure, inhibition means a lessened ex- 

 penditure, of energy on the part of the muscular tissue of the 

 heart. Whatever the manner in which the respective fibres act, 

 the effect of the activity of the augmentor fibres is to hurry on 

 the downward, catabolic changes of the cardiac tissue, while that 

 of the inhibitory fibres is an opposite one, and we may probably 

 say that the latter assists the constructive, anabolic, changes. 



Other Influences regulating or modifying the Beat of the Heart. 



140. Important as is* the regulation of the heart by the 

 nervous system, it must be borne in mind that other influences 

 are or may be at work. The beat of the heart may for instance 

 be modified by influences bearing directly on the nutrition of the 

 heart. The tissues of the heart, like all other tissues, need an 

 adequate supply of blood of a proper quality ; if the blood vary 

 in quality or quantity the beat of the heart is correspondingly 

 affected. The excised frog's heart, as we have seen, continues to 

 beat for some considerable time, though apparently empty of blood. 

 After a while however the beats diminish and eventually disappear ; 

 and their disappearance is greatly hastened by washing out the 

 heart with normal saline solution, which when allowed to flow 

 through the cavities of the heart readily permeates the tissues on 

 account of the peculiar construction of the ventricular walls. If 

 such a ' washed out ' quiescent heart be fed by means of a perf u- 

 sion cannula, in the manner described ( 133), with diluted blood 

 (of the rabbit, sheep, &c.), it may be restored to functional activity. 

 A similar but less complete restoration may be witnessed if serum 

 be used instead of blood ; and a heart fed regularly with fresh 

 supplies of blood or even of serum may be kept beating for a 

 very great length of time. 



