6l2 A MANUAL OF ANATOMY 



fourth sacral, and (2) the inferior hemorrhoidal nerve, which is a 

 branch of the pudic. The former supplies the integument between 

 the tip of the coccyx and the back of the anus, and the latter the 

 integument between the anus and tuber ischii. 



Fascia. — ^The fascia of this division is very thick, and is prolonged 

 into the ischio-rectal fossa, where it forms an elastic pad for the 

 lower part of the rectum and the anal canal. There is no deep fascia. 



Ano-coccygeal Body. — ^This is a collection of fibrous and muscular 

 tissues situated between the coccyx and anal canal, the muscular 

 element being connected with the levatores ani and sphincter ani 

 muscles (Symington). Its importance lies in the fact that it gives 

 support to the anal canal. 



Muscles. — ^The muscles in this division are the sphincter ani 

 externus, corrugator cutis ani, levator ani, and coccygeus. 



Sphincter Ani Externus — Origin. — ^The tip of the coccyx and the 

 integument over it. 



Insertion. — ^The greater part of the muscle is inserted into the 

 central tendinous point of the perineum. The more superficial 

 fibres, however, are inserted into the skin, and a certain amount 

 of decussation takes place across the middle line. 



Nerve-supply. — ^The part of the muscle between the coccyx and 

 the anus is supplied by the hemorrhoidal or perineal branch of the 

 fourth sacral, and the remainder by the inferior hemorrhoidal and the 

 deep branch of the perineal nerve, both of which are derived from 

 the pudic. 



Action. — ^To keep the anal aperture closed, at the same time 

 producing a wrinkled condition of the skin. 



The muscle is situated immediately beneath the skin, and is 

 elliptical. Behind and in front of the anus it is single, but around 

 that aperture it is arranged in two symmetrical halves, which are 

 intimately connected with the middle portions of the levatores ani. 



Corrugator Cutis Ani (Ellis). — ^This muscle is represented by a 

 very delicate sheet of involuntary muscular fibres, which pass in a 

 radiating manner from the submucous tissue of the anal aperture 

 to be inserted into the surrounding skin. 



Action. — (i) To throw the integument around the anus into 

 wrinkles ; and (2) to invert the mucous membrane of the lower 

 end of the anal canal after it has been everted during defaecation. 



For the levator ani and coccygeus, see pp. 862 and 863. 



Iscliio-rectal Fossa. — ^This is a deep fossa which is situated on 

 either side between the ischium and rectum. It measures nearly 

 2^ inches in depth, 2 inches from before backwards, and i inch 

 from side to side. In transverse section it is triangular, the base 

 being directed downwards and the apex upwards. The outer 

 wall, which is vertical, is formed by the lower part of the obturator 

 internus muscle covered by the obturator fascia. The inner wall, 

 which is oblique and longer than the outer, is formed by the lower 

 part of the rectum, the anal canal, the levator ani muscle covered 

 by the anal fascia, and the sphincter ani externus. The base is 



