THE VASCULAR MECHANISM. 209 



sometimes the weakening is the more prominent. When the inhibitory 

 impulses, by reason of particular fibres being affected or otherwise, are 

 brought to bear chiefly on those parts of the heart, such as the sinus, which 

 possessing higher rhythmic potentiality (see 142) determine the sequence 

 and set the rate of rhythm, it is the rate which is most markedly affected. 

 When, on the other hand, the inhibitory impulses fall chiefly on the parts 

 possessing lower rhythmic potentiality, the most marked effect is a diminu- 

 tion in the force of the contractions. 



There is no adequate evidence then that the cardiac ganglia act as an 

 inhibitory mechanism in the sense that they produce important changes in 

 the nature of the impulses reaching them along vagus inhibitory fibres before 

 those impulses pass on to the muscular tissue. We may add that there is 

 similarly no adequate evidence that any of the ganglia act as an " aug- 

 menting " mechanism. We have previously seen ( 141, 142) reasons 

 for thinking the ganglia are not centres for the origination or regula- 

 tion of the spontaneous beats. The question then arises, What are their 

 functions? To this question we cannot at present give a wholly satisfac- 

 tory answer. 



The inhibitory fibres remain, as we have seen, medullated fibres until they 

 reach the heart, but it would appear that they lose their medulla somewhere 

 in the heart before they actually reach the muscular tissue, and it is probable 

 that the loss takes place in connection with some of the cardiac ganglia 

 much in the same way that the augmenting fibres lose their medulla in the 

 ganglia of the sympathetic chain ; but we do not know what is the physi- 

 ological effect or the purpose of this loss of the medulla, and we cannot 

 suppose that this is the sole or even chief use of the ganglia. Coincident 

 with the loss of the medulla an increase of fibres frequently takes place, 

 more than one non-medullated fibre leaving a nerve cell into which one 

 medullated fibre enters ; and we may suppose that this mode of branching 

 has purposes not fulfilled by the mere division of a fibre. Then again, 

 bearing in mind the nutritive or "trophic" function of the spinal gan- 

 glia alluded to in 96, we may suppose that the cardiac ganglia are in 

 some way concerned in the nutrition of the cardiac nerve fibres. But 

 our knowledge is not yet sufficiently ripe to allow exact statements to 

 be made. 



Other Influences Regulating or Modifying the Beat of the Heart. 



148. 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 disappear ; and their disappearance is greatly hastened by 

 washing out the heart with a 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 with a perfusion canula, in the manner 

 described ( 141), with diluted blood (of the rabbit, sheep, etc.), 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 



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