340 MUSCLES OF THE ANUS. 



The irritation induced by the accumulation of faeces in the 

 rectum, causes it at first to contract more strongly, and the 

 contraction continues till it is overcome, by the increasing 

 effort of the muscular fibres of the rectum, and the action of 

 the diaphragm and abdominal muscles. It acts also as an anta- 

 gonist to the levator ani muscles. 



-V. B. The sphincter interhus of Albinus and Douglas, is 

 only that part of the cellular fibres of the muscular coat of the 

 rectum which surrounds its extremity. 



Levator Ani : 



Arises from the os pubis within the pelvis, as far up as the 

 upper edge of the foramen thyroideum, and joining of the 

 os pubis with the os ischium ; from the thin tendinous mem- 

 brane that covers the obturator internus and coccygeus muscle, 

 and from the spinous process of the os ischium : its fibres run 

 down like rays from a circumference to a centre. 



Inserted into the sphincter ani, acceleratores urinse, and anterior 

 part of the two last bones of the os coccygis ; surrounds the 

 extremity of the rectum, neck of the bladder, prostate gland, and 

 part of the vesicula seminalis ; so that its fibres behind and 

 below the os coccygis joining it with its fellow, they together 

 very much resemble the shape of a funnel. 



Use. To draw the rectum upwards after the evacuation of 

 the fasces, and to assist in shutting it ; to sustain the contents of 

 the pelvis, and to help in ejecting the semen, urine, and contents 

 of the rectum ; and, perhaps by pressing upon the veins, to 

 contribute greatly to the erection of the penis. 



The muscular funnel, formed by the levator ani muscles of 

 the two sides is antagonized by the action of the sphincter ani, 

 which, by its connexion with the coccyx and perineal centre 

 prevents its lower extremity from being drawn upwards. 

 When the sphincter is inflamed, and a fluid effused among its 

 fibres, as is an occasional occurrence in the bowel complaint of 

 children, the sphincter loses its power, and the levator ani muscles, 

 unopposed, retract ; and thus by everting the lower margin of the 

 rectum, contribute mainly to the formation of prolapsus ani. 



