1877.] 



Action of the Sphincter Ani. 



83 



entirely dependent on the ano-spinal centre in the cord, and also by what 

 mechanism the encephalic centre exerts its influence. 



I would, however, draw especial attention to the points of resemblance 

 between this reflex action of the internal sphincter and that of the 

 middle coat of the intestine in peristaltic action, which suggest the proba- 

 bility that the action of the sphincter, apart from the will, is under the con- 

 trol of a similiar mechanism, and is indeed only, so to speak, a concentrated 

 and more specialized instance of the action of the transverse fibres of the 

 intestine. The action of the intestine, as well as that of the sphincter, 

 is under central control, being inhibited by the vagus, intensified by the 

 splanchnics. The deliberate character of the reflex action of the sphincter 

 resembles closely the deliberate character of the intestinal reflex. 



Increased intestinal contraction, like that of the sphincter, is excited 

 most readily by irritation of the mucous membrane. Moreover Goltz 

 believed that in dogs the muscular coat of the rectum participated in 

 the rhythmical contraction which he observed in the sphincter ; and my 

 own observations have shown that the reflex action I have described is 

 not confined to the thickened extremity of the bowel, but can be obtained 

 in a modified form as high as two inches from the lower extremity. 



The power of reflex action which is possessed by the whole internal 

 sphincter must be possessed by each bundle of muscular fibres of which 

 it is composed. As the sphincter may be regarded as an aggregation of 

 bundles of fibres, such as are contained in the transverse muscular coat of 

 the intestine, so the latter may be regarded as a serial arrangement of the 

 bundles of which the internal sphincter is composed. An action of each 

 bundle of fibres so arranged, such as we have seen to occur in the 

 sphincter as a whole, must result in peristalsis, in the movement of a 

 contained and stimulating body along the intestine. If each bundle of 

 fibres passes through the same series of successive contractions and re- 

 laxations as the sphincter ani, then the curve traced by the action of the 

 latter will represent not merely the condition of one bundle of fibres in 

 successive intervals of time, but also the condition of successive bundles 

 at the same time, and two such curves in apposition will represent a 

 diagrammatic longitudinal section of the intestinal wall. The effect, 

 therefore, of the presence in the intestine of a mass of fasces or other 

 contents would be to cause, first, in the moderately contracted intestinal 

 wall in front of it, an increased contraction, the effect of which would 

 be to prevent the diffusion of the contents along the intestine (which 

 would materially interfere with their movement); secondly, complete 

 relaxation of the next portion of the intestinal wall into which the contents 

 of the intestine could pass ; and thirdly, a strong contraction behind, 

 sustained, and moving on the stimulating body, as the initial contraction 

 gave place to relaxation. The process would no doubt be materially 

 modified by the contraction of the longitudinal fibres of the bowel, which 

 would prevent the undue distension of the relaxed portion, and thus assist 



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