SOME POINTS IX THE ANATOMY OF THE DICxESTIVE SYSTEM. 59 



mesial borders of the two levatores ani, as between the two 

 limbs of a clamp. When the distended rectum is cut across 

 near its lower end, and the anal canal examined from the interior 

 in a hardened body, a distinct projection formed by the inner 

 border of the levator ani (pubo-rectalis, or sphincter recti portion) 

 is seen on each side, indicating the upper limit of the canal 

 These ridges can also be felt during life by the finger introduced 

 into the rectum (Cripps). 



The columns of Morgagni, which are placed in the upper 

 and part of the middle thirds of the anal canal, are permanent 

 folds, not effaceable ones as often described. I have found 

 them present in the foetus of the third or fourth month, as are 

 also the valves of Houston. 



It is very difficult to say how much of the anal canal is 

 formed from the proctodasal invagination ; Ball's view that the 

 anal valves are the remains of the anal membrane, and that all 

 below them is proctodeeal, has much to commend it. He bases 

 this view largely on the fact that the anal valves have squamous 

 epithelium on their surface and columnar epithelium above. 

 The character of the epithelium alone must not be considered 

 conclusive, for the stomodteum, at the opposite end of the 

 intestinal canal, gives rise to the columnar epithelium of the 

 nose. Besides, Jonnesco states that whilst the epithelium 

 covering the grooves between the columns of Morgagni is 

 columnar, that of the columns themselves is squamous. 



Ball and others limit the term ' anal canal ' to the portion of 

 the passage below the anal valves, on the ground that only so 

 much is developed from the proctodseum. I think the division 

 given originally by Symington is more satisfactory, for two 

 reasons. In the first place, the term ' anal canal ' or passage is a 

 purely physiological, not an embryological one, and the correct- 

 ness of its application does not depend on the condition in the 

 embryo; and secondly, it has not yet been shown definitely 

 hov^ much of the canal is formed from the proctodeum. 

 Should it be proved that Ball's view is correct, then this part 

 might be known as the proctodseal canal, or the proctoda^al 

 portion of the anal canal. The term ' anal canal ' should be 

 applied to the whole canal or passage leading from the rectal 

 cavity to the surface. 



