LARGE INTESTINE. 673 



ttie lower part of the sacrum and coccyx ; and, near the extremity of the latter 

 bone, inclines backwards to terminate at the anus, being curved both in the lateral 

 and antero-posterior directions. The rectum is, therefore, not straight, the upper 

 part being directed obliquely from the left side to the median line, the middle 

 portion being curved in the direction of the hollow of the sacrum and coccyx, the 

 lower portion presenting a short curve in the opposite direction. The rectum is 

 cylindrical, not sacculated like the rest of the -large intestine ; it is narrower at its 

 upper part than the sigmoid flexure, gradually increases in size as it descends, 

 and immediately above the anus presents a considerable dilatation, capable of 

 acquiring an enormous size. The rectum is divided into three portions, upper, 

 middle, and lower. 



The upper portion, which includes about half the length of the tube, extends 

 obliquely from the left sacro-iliac symphysis to the centre of the third piece of the 

 sacrum. It is almost QOmpletely surrounded by peritoneum, and connected to the 

 sacrum behind by % duplicature of this membrane, the mesorectum. It is in 

 relation behind with ^the Pyriformis muscle, the sacral plexus of nerves, and the 

 branches of the internal iliac artery of the left side, which separate it from the 

 sacrum and sacro-iliac symphysis ; in front, it is separated, in the male, from the 

 posterior surface of the bladder, in the female, from the posterior surface of the 

 uterus and its appendages, by some convolutions of the small intestine. 



The middle portion of the rectum is about three inches in length, and extends 

 as far as the tip of the coccyx. It is closely connected to the concavity of the 

 sacrum, and covered by peritoneum only on the upper part of its anterior surface. 

 It is in relation, in front, with the triangular portion of the base of the bladder, 

 the vesicula3 seminales, and vasa deferentia ; more anteriorly, with the under sur- 

 face of the prostate. In the female, it is adherent to the posterior wall of the 

 vagina. 



The lower portion is about an inch in length ; it curves backwards at the fore 

 part of the prostate gland, and terminates at the anus. This portion of the intestine 

 receives no peritoneal covering. It is invested by the Internal sphincter, supported 

 by the Levatores ani muscles, and surrounded at its termination by the External 

 sphincter. In the male, it is separated from the membranous portion and bulb of 

 the urethra by a triangular space ; and, in the female, a similar space intervenes 

 between it and the vagina. This space forms by its base the perineum. 



Structure of the large intestine. The large intestine has four coats ; serous, rnus 

 cular, cellular, and mucous. 



The serous coat is derived from the peritoneum, and invests the different portions 

 of the large intestine to a variable extent. The caecum is covered only on its 

 anterior surface and sides ; more rarely, it is almost completely invested, being 

 held in its position by a duplicature, the mesocaecum. The ascending and 

 descending colon are usually covered only in front. The transverse colon is 

 almost completely invested, excepting at the points corresponding to the attach- 

 ment of the great omentum and transverse mesocolon. The sigmoid flexure is 

 nearly completely surrounded, excepting at the point corresponding to the attach- 

 ment of the iliac mesocolon. The upper part of the rectum is almost completely 

 invested by the peritoneum ; the middle portion is covered only on its anterior 

 surface ; and the lower portion is entirely devoid of any serous covering. In the 

 course of the colon, and upper part of the rectum, the peritoneal coat is thrown 

 into a number of small pouches filled with fat, called appendices epiplo'icse. They 

 are chiefly appended to the transverse colon. 



The muscular coat consists of an external longitudinal and an internal circular 

 layer of muscular fibres. 



The longitudinal fibres are found as a uniform layer over the whole surface of 

 the large intestine. In the ca3cum and colon, they are especially collected into 

 three flat longitudinal bands, each being about half an inch in width. These bands 

 commence at the attachment of the appendix vermiformis to the caecum : one, the 

 posterior, is placed along the attached border of the intestine ; the anterior band, 

 43 



