THE MUSCLES AND FASCIA OF THE RECTUM AND ANUS. 1677 



The internal sphincter ani (Fig. 1421), composed of involuntary muscular 

 fibres, is a thickening of the circular layer of the rectum, which becomes marked at 

 the beginning of the anal canal. It surrounds the latter for a distance of from 

 2.5-3 cm., and is about 4 mm. thick. 



Nerves reach the internal sphincter through the sympathetic system. Very 

 probably some of them come directly from spinal nerves. 



Accessory Muscular Bundles. — As they reach the anal canal, the longi- 

 tudinal fibres of the rectum send bundles to the skin, which gain their destination by 

 coursing through those of the external sphincter ; the longitudinal muscle-fibres of 

 the mucous coat, becoming enlarged, pass in the same way between the bundles of 

 the internal sphincter. No important accessions are received from the levator ani. 

 The longitudinal muscular fibres of the rectum, moreover, enter into connection with 

 the areolar tissue of the pelvic fascia between the peritoneum and the levator ani, and 

 perhaps with the latter also. Various bundles of muscle-fibres, apparently arising 



Fig. 1425. 



External %. 



iliac vessels 



Pelvic 

 fascia 



White line 



Recto-vesi-^ 



cal fascia 



Obturator 

 fascia 



Obturator/ 

 membrane 



Cowper's 



gland 



Iliacus 



Iliac fascia 



Peritoneum 



Ilio-pecti- 

 neal line 



Obturatoi 



, Jl Anterior wall 

 of rectum 

 Levator ani 



Prostate gland 



Obturator 

 externus 



Pubic ramus 



Corpus cavernosum 

 Ischio-cavernosus 

 Triangular ligament, inferior layer 

 Bulbo-cavernosus 



Colles's fascia 

 Triangular ligament, superior layer 

 Trans, perinei superf. 

 Bulb of penis 



Frontal section oi pelvis passing just behind the bladder, posterior surface. 



from the pelvis, mingle with those of the rectum. The redo-coccygeus of Treitz 

 arises from the anterior surface of the coccyx above the pelvic floor and mingles with 

 both the longitudinal and circular fibres at the back of the rectum. It is said to con- 

 sist of striated fibres at its origin. Bundles of fibres are described as arising from 

 the fascia on the deep surface of the transversus perinei profundus muscle and pass- 

 ing to the front of the gut. 



The corrugator cutis ani is a small system of muscular fibres radiating from the 

 submucous tissue at the anus to the deep side of the skin, which it tends to pucker. 



Actions.— The function of the sphincters is to keep the anal canal closed. 

 They differ, inasmuch as the external, although mostly acting automatically, is 

 under the control of the will and the internal is not. The levator ani has a more 

 complicated and in part an apparently inconsistent action, since it may pull the anus 

 upward and probably dilate it, as it pulls its borders apart under the resistance of 



