MCSCLES OF THE ANUS. 339 



Muscles of the Anus. 



The anus has a single muscle, and one pair. 

 The single muscle is 



Sphincter Ani. 



Arises from the skin and fat that surrounds the verge of the 

 anus on both sides, nearly as far as the tuber of the os ischium ; 

 the fibres are gradually collected into an oval form, and surround 

 the extremity of the rectum. 



Fig. 81* 



Inserted, before, by a narrow point, into the perineum, accel- 

 eratores urinae, and transversi perinei ; behind, by an acute ter- 

 mination, into the extremity of the os coccygis. 



Use. Shuts the passage through the anus into the rectum ; 

 pulls down the bulb of the urethra, by which it assists in ejecting 

 the urine and semen. The sphincter ani is always in a con- 

 tracted state, except at the time of the evacuation of the faeces. 

 When the sphincter is in a healthy state, it may by made by an effort 

 of the will to contract more strongly, but it cannot be made to relax. 



* The muscles of the perineum. 1. The acceleratores urinae muscles ; the 

 figure rests upon the corpus spongiosum penis. 2. The corpus cavernosum of 

 one side. 3. The erector penis of one side. 4. The transversus perinei of one 

 side. 5. The triangular space through which the deep perineal fascia is seen. 

 6. The sphincter ani ; its anterior extremity is cut off. 7. The levator ani of 

 the left side ; the deep space between the tuberosity of the ischium (8) and the 

 anus, is the ischio-rectal fossa ; the same fossa is seen upon the opposite side. 

 9. The spine of the ischium. 10. The left coccygeus muscle. The boundaries 

 of the perineum are well seen in this engraving. 



