1040 THE ORGANS OF DIGESTION. 



double fold called mesorectum. This is continued above with the sigmoid meso- 

 colon, is triangular, and ends below in an apex at the third sacral vertebra. Some 

 convolutions of the ileum, or a loop of the sigmoid flexure, usually lie in front 

 of this part of the rectum. They separate it from the bladder in the male and 

 posterior surface of the uterus in the female when the rectum is empty. If dis- 

 tended, one of these organs, according to sex, rests on its anterior surface, the 

 intestine being pushed up. Posteriorly is the mesorectum, left Pyriformis muscle, 

 left sacral plexus of nerves, branches of left internal iliac vessels, left portion of 

 anterior surface of two and a half sacral vertebrae. To the left side are the left 

 ureter and left internal iliac vessels. If this part of the rectum come down in the 

 middle line or from the right, these relations will differ. 



The middle or second part of the rectum is three or four inches long, and ex- 

 tends from the middle of the third sacral vertebra to a point opposite the apex of 

 the prostate gland. Here the course of the rectum changes to a posterior one, and 

 that is one inch below the tip of the coccyx. It is only partially covered by 

 peritoneum. It has no mesorectum and its posterior surface has no peritoneal 

 covering. At first it is covered anteriorly and laterally, but gradually the peri- 

 toneum leaves the sides, and finally, about one inch above the prostate or at the 

 length of an index finger above the anus, never more than 8 cm., it is reflected 

 from the anterior surface of the rectum to the bladder or to the upper fifth of the 

 posterior wall of the vagina, making the pouches as above noted. Distention of 

 bladder or rectum would diminish the depth of these pouches. This part of the 

 rectum is in relation anteriorly in the male with the recto-vesical pouch, with the 

 triangular portion of the base of the bladder, the vesiculae seminales and vasa 

 deferentia, and beyond them with the under surface of the prostate. In the female 

 it is related anteriorly to the posterior wall of the vagina, with which it is adherent, 

 with the recto-vaginal and recto-uterine pouches and small intestines therein. 



The posterior wall lies upon the lower part of the sacrum, middle sacral artery, 

 origin of Pyriformis muscles, coccyx and ano-coccygeal body, and Coccygei mus- 

 cles. The ano-coccygeal body is a dense mass of musculo-fibrous tissue situated 

 between the tip of the coccyx and anus. 



The lower portion or anal canal is about one inch long when the rectum is 

 empty; it is shorter when the rectum is distended. It turns downward and back- 

 ward at the lower part of the prostate gland and ends at the anal orifice. It has 

 no peritoneal covering. It is invested by the sphincter muscles and supported by 

 the Levatores ani. Behind it is in relation to the ano-coccygeal body and Coc- 

 cygei muscles; on the sides to the fat of the ischio-rectal fossae and the Levatores 

 ani muscles. 



Anteriorly in the male is the bulb of the urethra and its membranous por- 

 tion ; in the female it is separated from the lower end of the vagina by the peri- 

 neal body. 



The skin about the anus is provided with a ring of sweat-glands called circum- 

 anal glands. The skin is also thrown into minute corrugations by means of little 

 dermal muscles, the Corrugator cutis ani. The anal orifice is not situated alike 

 in the sexes ; it is farther forward in the female, and less concealed between the 

 ischial tuberosities. It is 3 cm. in front of the coccyx or just at the bi-ischial 

 line, a little elongated, and the skin is destitute of hair. In the male it is 2.5 cm. 

 in front of the coccyx just behind the bi-ischial line and deeply placed. The skin 

 is covered by hair more or less abundantly, and the orifice is circular and pre- 

 sents little skin folds vertically arranged like rays toward a centre. Between 

 these, where they continue into the mucous membrane, linear excoriations may 

 occur fissure of the anus. 



Structure of the Rectum. 



Four coats are again met, but the muscular and mucous ones differ from those 



yet seen. The walls are 3 to 4 mm. thick, while those of the colon are 1 to 1.5 mm. 



The peritoneal coat surrounds the first portion only and forms a mesorectum. 



