1310 



THE ORGANS OF DIGESTION 



The Anal Canal (pars emails recti) (Figs. 1052 and 1053). The anal canal is 

 the terminal portion of the large intestine. It begins at the level of the apex of 

 the prostate, is directed downward and backward, and ends at the anus. It forms 

 an angle with the lower part of the rectum and measures an inch to an inch and 

 a half (2.5 to 3.75 cm.) in length. It has no peritoneal covering, but is invested 

 by the Internal sphincter, supported by the Levatores ani muscles, and surrounded 

 at its termination by the External sphincter; in the empty condition it presents 

 the appearance of a longitudinal slit. Behind is a mass of muscle and fibrous 

 tissue, the anococcygeal body; in front of it, in the male, are the membranous por- 

 tion and bulb of the urethra, and the base of the triangular ligament, and in the 

 female it is separated from the lower end of the vagina by a mass of muscle and 

 fibrous tissue, named the perineal body. 



LONGITUDINAL 



MUSCLE FIBRES 



OF RECTUM 



PART OF 



LEVATOR ANI 



INTERNAL 



SPHINCTER 



ANAL CANAL 



RUG/E OF 

 MUCOUS 

 MEMBRANE 



COLUMNS OF 

 MORGAGNI 



ANAL VALVES 



FIG. 1053. The interior of the anal canal and lower part of the rectum, showing the columns of Morgagni and 

 the anal valves between their lower ends. The columns were more numerous in the specimen than usual. 

 (Cunningham.) 



The Anal Orifice or Anus is the external opening of the anal canal, formed 

 by pigmented skin containing numerous sebaceous and sudoriparous glands 

 glaridulae circumanales} and furnished with hairs. While the anus is closed, 

 the skin around it is thrown into radial folds by the contraction of the External 

 sphincter. 



Structure of Large Intestine. The large intestine has four coats serous, muscular, sub- 

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

 portions of the large intestine to a variable extent. The cecum is completely covered by the 

 serous membrane, except in a small percentage of cases (5 or 6 per cent.), where a small portion 

 of the upper end of the posterior surface is uncovered. The ascending and descending colon 

 are usually covered only in front an 1 at the sides; a variable amount of the posterior surface 

 is uncovered. 1 The transverse colon is almost completely invested, the parts corresponding to 

 the attachment of the great omentum and transverse mesocolon being alone excepted. The 

 sigmoid flexure is completely surrounded, except along the line to which the sigmoid mesocolon 

 is attached. The upper two-thirds of the rectum is covered in front and laterally by the peri- 

 toneum, but not posteriorly, between the two posterior folds of peritoneum, the so-called meso- 

 rectum; later it is covered only on its anterior surface; and the lower portion is entirely devoid 

 of any serous covering. In the course of the colon the peritoneal coat is thrown into a number of 

 small pouches filled with fat, called appendices epiploicae. They are chiefly appended to the 

 transverse colon, and are particularly numerous along the anterior band. 



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

 muscle tissue. 



The longitudinal fibres do not form a uniform layer over the whole surface of the large intes- 

 tine. In the cecum and colon they are especially collected into three flat longitudinal bands 



1 See footnote, page 1304. 



