STOMACH AND INTESTINE. 



371 



still more frequent association of movement 

 is probably exemplified in the tenesmus pro- 

 duced by dysentery and other disorders, which 

 involve great irritation of the rectum. For 

 the sensations of the patient, and the uncon- 

 trollable impulse which follows them, seem 

 to indicate that the irritation of the mucous 

 membrane is often accompanied by violent 

 contraction of the muscular coat of the in- 

 testine, both of which unite to excite the 

 subsequent abdominal pressure. 



The nature of the contents of the rectum 

 greatly affects the degree in which the muscles 

 of the abdomen are made to assist in their 

 expulsion. When these contents consist only 

 of gases and liquids, they require so little of 

 this aid, as to be sometimes expelled without 

 it. But the extrusion of hard scybalous 

 evacuations often demands the help of ab- 

 dominal pressure, to an extent such as in- 

 volves all the viscera of the trunk, and 

 seriously obstructs the flow of blood in the 

 larger veins of the head and thorax. 



As regards the levator ani muscle, its origin 

 and insertion, together with the course taken 

 by its fibres, leave no doubt as to what must 

 be the direct effect of its contraction. It 

 raises the end of the rectum, together with 

 the ligamentous structures of the perinaeum 

 anteriorly, and the coccyx posteriorly. But 

 as the time of this action seems exactly to 

 coincide with the exertion of the abdominal 

 pressure just alluded to, the degree in which 

 it really raises these structures can only equal 

 the surplus of its force over that of the mus- 

 cles of the belly. Hence it may be doubted 

 whether the muscle generally does more than 

 fix the bowel : an effect which is, however, 

 of the highest importance to the mechanism 

 of defaecation. The influence of the neigh- 

 bouring perinasal muscles is still more obscure. 



Such being the known agents of the pro- 

 cess of defecation, we may next attempt to 

 sketch the course of its phenomena. 



The ordinary peristalsis of the large in- 

 testine propels into the rectum a variable 

 quantity of faeces. These, after a longer or 

 shorter sojourn in its first or second por- 

 tions, excite an active peristaltic contraction 

 of its muscular coat. In general it is only 

 when they reach the lower extremity of the 

 bowel, that the abdominal pressure adds to 

 this peristalsis its far more powerful expulsive 

 force. The combined effect of both these 

 actions urges the faecal mass against the ex- 

 ternal sphincter, which relaxes at this in- 

 stant, by a voluntary effort, so as to permit 

 the extrusion of the descending mass : a small 

 portion of the loose mucous membrane being 

 at the same time generally everted around it. 



A variable length of faecal substance thus 

 passes through the orifice of the anus. The 

 continuity of the descending mass being finally 

 interrupted, the act of respiration* isresumed ; 



namely, the application of a wet bandage tightly 

 around the bellv. 



The way in which this act is affected during 



the abdominal pressure ceases; and at the 

 same time, the contraction of the levator ani, 

 aided by that of the rectum itself, returns the 

 projecting extremity of the bowel into the 

 pelvis, by a kind of sudden and forcible re- 

 traction. The latter act, in which both of 

 the sphincters may be presumed to play an 

 important part, often subdivides a continuous 

 faecal mass ; returning the upper segment 

 thus cut off into the cavity of the rectum 

 which it was just leaving. The total du- 

 ration of the expulsive act appears to be 

 chiefly determined by the consistence of the 

 faeces, the velocity of their transit, and the 

 exigencies of the suspended process of respi- 

 ration itself. 



The mucous membrane of the rectum is con- 

 nected with its muscular coat much more 

 loosely than that of the colon. Owing to 

 this circumstance, it generally exhibits nume- 

 rous folds. Most of these are mere tempo- 

 rary results of the passive contraction of the 

 muscular coat. And in correspondence with 

 such an origin, they are very irregular in 

 size, number, and position. They are, how- 

 ever, more frequently found occupying the 

 dilated lower end of the bowel, where they 

 take what is usually a longitudinal direction. 



But in addition to such casual and tem- 

 porary folds, Mr. Houston * has described 

 others, which he states to have a definite 

 direction and situation, as well as a more 

 permanent character. According to this 

 anatomist, three is the number of these folds 

 or valves usually present. The largest and 

 most constant of the three projects from the 

 anterior wall of the rectum, opposite to the 

 base of the bladder, and about three inches 

 above the anus. The valve next above this 

 springs from the left wall of the bowel, about 

 midway between the last and the third or up- 

 permost fold. This latter projects from the 

 right wall of the upper end of the rectum. 

 The shape of all three is nearly semilunar : 

 their depth about half an inch ; and they are 

 fixed by a convex border to about half the 

 circumference of the intestine. And Kohl- 

 rauschf has described a fold which tolerably 

 answers to the fourth and least constant of 

 those mentioned by Mr. Houston. He states it 

 to be always present,as a transverse projection 

 from the posterior wall of the rectum opposite 

 the middle of the coccyx. In general it con- 

 tains no muscular fibres : but rarely these may 

 even encircle the bowel, as a continuous ring 

 or third sphincter, which forms the lower 

 boundary of a dilated and sacciform segment 

 of the rectum. 



As regards these folds, we may point out, 

 that their usual situation corresponds to 

 the most prominent parts of those three 

 curves of the rectum which we have already 

 alluded to. Thus the third answers to that 

 convex mucous surface which marks the 



* Dublin Hospital Reports, vol. v. p. 163. 

 j- " Zur Anatomic nnd 1'liysiologie der Beckenor- 

 gane," Leipzig, 1854 ; also Valentin's " liericht 



.V>s\i* A i 1-1 T i-kicif -it n rmr\ ill ili"j* 1 *ll 1' Cl f\ 1 1~\ (Tl 1 T1 I \ fl 11 



the exercise of abdominal pressure has already been ueber die Leistungen in der Fhysiologie " in (Jan- 

 explained at p. 316. 



stall's Jahresbericht, 185-4. 



B B 



