PHYSIOLOGICAL ANATOMY OF THE LARGE INTESTINE. 289 



shallow concavity, which is provided with a horizontal, button-hole slit opening into 

 the caecum. The surface of the valve which looks toward the small intestine is cov- 

 ered with a mucous membrane provided with villi and in all respects resembling the 

 general mucous lining of the small intestine. Viewed 

 from the ca3cum, a convexity is observed corresponding 

 to the concavity upon the other side. The caecal surface 

 of the valve is covered with a mucous membrane identi- 

 cal with the general mucous lining of the large intestine. 

 It is evident, from an examination of these parts, that 

 pressure from the ileum would open the slit and allow 

 the easy passage of the semifluid contents of the intes- 

 tine ; but pressure from the ca3cal side approximates 

 the lips of the valve, and the greater the pressure the 

 more firmly is the opening closed. The valve itself is 

 composed of folds formed of the white fibrous tissue of 

 the intestine (the cellular tunic of some anatomists), and 

 circular muscular fibres from both the small and the 

 large intestine, the whole being covered with mucous 

 membrane. The lips of the valve unite at either extrem- 

 ity of the slit and are prolonged on the inner surface of 

 the cascum, forming two raised bands or bridles; and 

 these become gradually effaced and are thus continuous FIG. 80. Opening of the small intes- 

 with the general lining of the canal. The posterior bridle , tt the ccecum (Le Bon.) 



1, small intestine; 2, ileo-ca?cal valve; 



is a little longer and more prominent than the anterior. 3, caecum; 4, opening of the appen- 

 These assist somewhat in enabling the valve to resist gj^^JSi j^TS! ?2r 

 .pressure from the csecal side. The longitudinal layer of SofcjJ' ? ' f ld8 f th mU OU3 

 muscular fibres and the peritoneum pass directly over the 



attached edge of the valve and are not involved in its folds. These give strength to the 

 part, and, if they be divided over the valve, gentle traction will suffice to draw out and 

 obliterate the folds, leaving a simple and unprotected communication between the large 

 and the small intestine. 



Peritoneal Coat. Like most of the other abdominal viscera, the large intestine is 

 covered by peritoneum. The ca3cum is covered by this membrane only anteriorly and 

 laterally. It is usually bound down closely to the subjacent parts, and its posterior sur- 

 face is without a serous investment; although sometimes it is completely covered, and 

 there may be even a short mesocoecum. The ascending colon is likewise covered with 

 peritoneum only in front and is closely attached to the subjacent parts. The same ar- 

 rangement is found in the descending colon. The transverse colon is almost completely 

 invested with peritoneum ; and the two folds forming the transverse mesocolon separate 

 to pass over the tube above and below, uniting again in front to form the great omentum. 

 The transverse colon is consequently quite movable. In the course of the colon and the 

 upper part of the rectum, particularly on the transverse colon, are found a number of 

 little sacculated pouches filled with fat, called the appendices epiploi'csB. The sigmoid 

 flexure of the colon is invested with peritoneum, except at the attachment of the iliac 

 mesocolon. This division of the intestine is capable of considerable motion. The upper 

 portion of the rectum is almost completely covered by peritoneum and is but loosely IK-!U 

 in place. The middle portion is closely bound down, and is covered with peritoneum only 

 anteriorly and laterally. The lowest portion of the rectum has no peritoneal covering. 



Muscular CWf. The muscular fibres of the large intestine have an arrangement quite 

 different from that which exists in the small intestine. The external, longitudinal layer, 

 instead of extending over the whole tube, is arranged in three distinct bands, which com- 

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