60 PROFESSOR A. BIRMINGHAM. 



Action of the Levator Ani and of the Sphincters. 



Levator ani. — The fibres of the levator which arise from the 

 pubes (pubo-coccygeus or sphincter recti portion) pass back- 

 wards on each side of the upper part of the anal canal, and in 

 great part meet behind the passage. These two muscular bands, 

 which are quite close to one another at their origin, and are 

 actually united behind the bowel, during the contraction of the 

 muscles are more closely approximated, like the limbs of a clamp, 

 and, pressing on the sides of the anal canal, they assist in closing 

 the upper part of that orifice, whilst at the same time drawing 

 it slightly towards the pubes. There is little doubt that this 

 part of the levator ani, in this way, acts as one of the chief — if 

 not the chief — sphincters of the bowel; and it should be noticed 

 that it is placed where its action would be most effective, 

 namely, opposite the point at which the rectum is narrowed or 

 'pinched in' to form the anal canal. It is probably relaxed 

 during defa3cation, except perhaps at the completion of the act. 



The external sphincter forms a muscular cylinder around the 

 lower two-thirds of the anal canal, with (except in the case of 

 some of its inner fibres) an anterior and a posterior attachment. 

 When the muscle contracts, its fibres are tightly stretched 

 between their anterior and posterior attachments, and the space 

 between them is reduced to a narrow antero-posterior slit. By 

 this action the anal canal is flattened from side to side and 

 closed, so that whilst the levator ani is the sphincter of the 

 upper aperture of the anal canal, the external sphincter closes 

 its lower and greater part. 



The internal sphincter is continuous with the circular fibres 

 of the gut, not only in structure but probably also in action, its 

 chief use being to empty the anal canal completely after the 

 passage of each faecal mass. Owing to the fact that the canal is 

 an antero-posterior slit, not a circular orifice, and that the 

 internal sphincter forms a muscular ring around it, acting alone, 

 it is scarcely competent to keep the sides of the canal in ap- 

 position, and probably it acts rather as a detrusor than a true 

 sphincter of the anal passage. 



