364 



STOMACH AND INTESTINE. 



zontal slit or fissure, as it enters this 

 fold. 



Fig. 275. 



Caecum inflated, dried, and opened, to exhibit the 

 arrangement of its valve. 



a, termination of the ileum ; b, ascending colon ; 

 c, cajcum ; d, transverse constriction projecting into 

 the caecum from its inner surface; e f, valve sepa- 

 rating the small from the large intestine ; e, its ho- 

 rizontal ileo-colic lamina ; f, its more oblique ileo- 

 Cecal lamina ; g, the vermiform appendix of the 

 caecum. 



Such a description at once explains the 

 form of the valve : how each of its .seg- 

 ments, for example, constitutes a crescentic 

 membrane, the plane of which meets that of 

 its fellow at an acute angle, and the free 

 edge of which is directed outwards ; and 

 how both end anteriorly and posteriorly in a 

 commissure or fold, that gradually decreases 

 in depth as it passes either forwards or back- 

 wards round the intestine. 



As regards the details of -its construction, 

 each segment of the valve is chiefly formed 

 by the prolongation of a corresponding por- 

 tion of the circular muscular fibres of the 

 ileum, together with a few proper to the 

 large intestine. The assistance afforded to 

 these by the attachment of the peritoneum 

 and the longitudinal fibres of the ileum, to 

 the fixed margin of each segment, is well 

 shown by the effect of dividing the latter 

 structures. For after such an injury, mode- 

 rate traction draws out the valve into a sur- 

 face, which is directly continuous with the 

 lower end of the small intestine ; and at the 

 same time converts its horizontal slit into a 

 large elliptical aperture. The difference be- 

 tween the ileo-caecal and ileo-colic portions 

 consists chiefly in the fact, that the plane of 

 the former is more oblique, and its margin 

 more concave, than that of the latter. 



The mechanism of this valve may be easily 

 deduced from its structure. In all states short 

 of actual distention, the passive contraction 

 of its muscular walls no doubt insures their 



contact, and shuts off the cavity of the ileum 

 from that of the caecum. While any approach 

 towards a more active dilatation of the large 

 intestine whether of the caecum, or colon 

 at once brings about a close apposition of 

 the two portions of the valve. And what- 

 ever aid may be given to this mutual apposi- 

 tion of the surfaces of the valve by its own 

 active muscular contraction during life, no- 

 thing is more certain than that its closure is 

 essentially independent of any such vital pro- 

 cess. For the gradual and equable distention 

 of the caecum with liquid in the dead subject 

 can also produce this result. 



Nor is it difficult to understand how such 

 a closure is effected. The passage of the con- 

 tents of the large intestine, over either plane 

 of the valve, presses it against the opposite 

 one, so as at once to close its orifice. Be- 

 sides this, the free margins of the valve form 

 segments of a larger circle than its attached 

 ones. Hence they are disproportionately 

 tightened by the same distending force. In 

 this way, the double curve of each lamina is 

 soon reduced to a straight line, that brings it 

 into exact apposition with its antagonist. So 

 that, within all ordinary limits, the greater 

 the dilating force, the more closely are the 

 two lips of the valve applied to each other. 



The only valid exceptions to this rule may 

 be found in those cases in which the ileum 

 and caecum are filled simultaneously. Such a 

 process of distention necessarily occurs in all 

 cases of mechanical obstruction of the diges- 

 tive canal below this valve; as a result of the 

 downward flow of the contents of the small 

 intestines. And since it obviously distends 

 the aperture by the application of a counter- 

 force from the side of the ileum, its mecha- 

 nical action is so simple as to require no 

 further explanation. Its effect may indeed 

 be seen in the caecum, as usually inflated and 

 dried.* 



The function of this valve therefore offers 

 a complete contrast to that by which the 

 stomach opens into the small intestine. For 

 while it affords little or no obstacle to an 

 onward transit of the contents of the canal, 

 it resolutely bars the way to all regurgitation: 

 an action which we have already seen is 

 exactly reversed by the pylorus. And even 

 in the absence of information respecting the 

 details of its active contraction, its structure 

 entitles us to conjecture, that the greater 

 part of its efficiency depends upon a passive, 

 and therefore permanent mechanism ; and 

 not, as is the case with the pylorus, on an in- 

 termittent (and vital) shortening of its mus- 

 cular fibres. 



The use of the caecum is evidently that of 

 forming a receptacle, in which the contents of 

 the small intestine may sojourn for a certain 

 time, before passing onwards into the colon. 



* Hence it is scarcely a superfluous caution to 

 add, that in examining such a preparation we ought 

 always to recollect, that the patulous orifice thus 

 seen is in reality an abnormal one, which does not 

 illustrate the mechanism of the valve in the 

 healthy living body. 



