370 



STOMACH AND INTESTINE. 



of the levator passes around the tube, return- 

 ing upon itself in the similar portion from the 

 opposite side, so as to sling and sustain the 

 canal. Between these anterior and posterior 

 slips, is a median portion ; which descends to 

 be inseparably united with the upper and outer 

 rings of the external sphincter itself. And 

 besides interlacing with the striped fibres of 

 this muscle, it may generally be traced sending 

 off a certain number of its scattered bundles, 

 to decussate and disappear amongst the un- 

 striped fibres of the longitudinal and transverse 

 layers of the rectum just at their termination. 



Movements of the rectum. The contraction 

 of the muscular coat of the rectum, like that 

 of the other segments of the digestive tube, 

 has for its object the onward propulsion of its 

 contents. But since the rectum ends the intes- 

 tinal canal, it forms the portal by which these 

 contents are altogether dismissed from the 

 body. Health and comfort alike require that 

 this act of dismissal should be both inter- 

 mittent and infrequent. While the mere con- 

 sistence of the fasces is generally such as to 

 demand the application of a comparatively 

 powerful force in order to effect their rapid 

 removal. Hence the entire mechanical action 

 of the rectum is naturally divisible into two 

 stages : one which propels, and one which 

 expels, the various substances occupying its 

 interior. The latter of these two processes 

 is called defecation ; and, as we shall see, in- 

 volves the aid of various agents which are 

 strictly extraneous to the bowel itself. 



Those movements of the rectum which are 

 seen in living or newly-killed animals, can only 

 be regarded as confirming the conclusions that 

 might fairly be inferred from the much greater 

 thickness of the muscular coat in this particu- 

 lar segment of the large intestine. 



When empty, the bowel remains in a state 

 of rest, from which it can scarcely be aroused 

 to peristalsis by the application of any local irri- 

 tation. But when occupied by a moderate quan- 

 tity of faeces, it will often respond to a direct 

 mechanical stimulus ; and still more energetic- 

 ally to the electrical irritation of its nerves. 

 On applying the rapid and powerful shocks of 

 the electro-magnetic machine to those larsje 

 branches of the sympathetic which are distri- 

 buted in the muscular coat of the rectum, a 

 violent contraction of this tube frequently 

 occurs. In this intense but irregular move- 

 ment, we may generally observe a shortening 

 and a constriction: acts which no doubt re- 

 present the specific contractions that form the 

 functions of the longitudinal and transverse 

 fibres respectively. Thus the bowel suddenly 

 becomes straighter and shorter; and hence 

 appears as though it were jerked downwards 

 towards its most fixed point at the lower open- 

 ing ot the pelvis. This shortening is generally 

 accompanied by a less violent and more uniform 

 movement: in short, by a peristalsis; which 

 offers the ordinary progressive constriction, 

 and is evidently the principal agent in the pro- 

 pulsion of the faecal pellets. But both of these 

 movements are very irregular. The former 

 commonly alternates with a relaxation, which 



appears to pull the rectum upwards, and exag- 

 gerate its curves. While the latter occupies 

 various parts of the bowel with very unequal 

 intensity and duration; and, occasionally, even 

 seems to take a retrograde course. In most of 

 these details, we may observe the same close 

 analogy between the rectum and the esopha- 

 gus, which has already been remarked in their 

 median and terminal situation, and (to a 

 lesser degree) in the nature and thickness of 

 their muscular coat. 



The normal movement of the bowel differs 

 from the above in the fact of its being a more 

 exact and co-ordinate action ; and, therefore, 

 a much more effective one. But while obser- 

 vation and experiment both agree in repre- 

 senting it as a peristalsis, which is quite capable 

 of slowly expelling fasces of moderate consist- 

 ence, nothing is more certain than that it is 

 rarely called upon to exert such an inde- 

 pendent and unaided force. 



Under all ordinary circumstances, its influ- 

 ence is assisted by the action of various vo- 

 luntary muscles. These may be divided into two 

 classes,which differ in their situation and action. 



The first are those large planes of muscle 

 which form the anterior, lateral, and superior, 

 walls of the belly; and thus, by their contrac- 

 tion, exert a forcible pressure on the intestines 

 contained in this cavity. 



The second are the similar structures that 

 close the outlet of the pelvis. These have for 

 their office to support the end of the canal. 

 And their muscular nature enables them to 

 effect this support by a texture, the passive or 

 active contraction of which can always increase 

 the reaction or resistance to the abdominal 

 pressure in exact correspondence with the 

 varying demands made upon it. They thus fix 

 the end of the intestinal canal, while it is 

 being emptied of its contents by the pressure 

 of the muscles of the belly. 



The mechanism of the abdominal pressure 

 having already been described in treating of 

 the act of vomiting*, but little need here be 

 added with reference to the special conditions 

 under which it is called upon to aid that of 

 defalcation. Of course the chief of these con- 

 ditions consists in the application of a stimulus 

 to the large intestine itself. And though any 

 irritation of the mucous membrane of the 

 rectum seems capable of producing that vio- 

 lent straining which marks the exertion of 

 the abdominal pressure, still its strictly co- 

 ordinate character is well illustrated by the 

 preference apparently shown to a stimulus 

 which acts directly on the muscular fibres of 

 the bowel themselves. Thus a mechanical 

 distention of the rectum appears to be a more 

 efficient stimulus to the total expulsive act 

 than the application of any ordinary irritant. 

 While, vice versa, there are good reasons for 

 conjecturing, that mere distention of the 

 belly is capable of arousing a sluggish large 

 intestine to expel its contents. f And a 



* See p. 316. 



t Such a "reflex action," from the animal to the 

 organic muscle, may perhaps explain the operation 

 of one of the ordinary remedies against constipation ; 





