DEVELOPMENT OF THE ALIMENTARY CANAL. 859 



THE ANUS AND ITS MUSCLES. 



The anus, or lower opening of the alimentary canal, is a dilatable orifice, 

 surrounded internally by the mucous membrane, and externally by the skin, 

 which two structures here become continuous with and pass into each other. 

 The skin around the borders of the anus, which is thrown into wrinkles or 

 folds during the closed state of the orifice, is covered with numerous sensi- 

 tive papilla?, and is provided with hairs and sebaceous follicles. 



The lower end of the rectum and the margin of the anus are, moreover, 

 embraced by certain muscles, which serve to support the bowel, and to close 

 its anal orifice. These muscles, proceeding from within outwards, are, the 

 internal sphincter, the levatores ani, the coccygei, and the external sphincter. 

 The three last muscles have already been described (pp. 262, 263). 



The internal sphincter muscle (sphincter ani internus) is a muscular ring 

 or rather belt, surrounding the lower part of the rectum, an inch above the 

 anus, and extending over about half an inch of the intestine. It is two 

 lines thick, and is paler than the external sphincter. Its fibres are con- 

 tinuous above with the circular muscular fibres of the rectum, and, indeed, 

 it consists merely of those fibres more numerously developed than elsewhere, 

 and prolonged farther down than the external longitudinal fibres. 



Kohlrausch describes a thin stratum of fibres between the mucous membrane and 

 the internal sphincter, these fibres having a longitudinal direction. Henle thinks 

 this is nothing more than the stratum of fibres belonging to the proper mucous 

 coat ; but Kohlrausch gives it a distinct name, the sustentator tunicae mucosse. (Kohl- 

 rausch, Anat. and Phys. d. Beckenorgane, Leipzig. 1854.) 



DEVELOPMENT OF THE ALIMENTARY CANAL AND PERITONEAL CAVITY. 



It has been already casually stated (p. 15) that the epithelial lining of the alimen- 

 tary canal is derived from the deepest of the three layers into which the germinal 

 membrane divides, while the rest of its walls are derived from a part of the middle 

 layer. To make this clear, it is necessary to state that, while those parts of the middle 

 layer of the embryo which lie next to the chorda dorsalis form the dorsal plates from 

 which the bones, nerves, and muscles of the trunk are derived, the lateral parts lying 

 beyond form, as described by Remak, the visceral plates, which on each side divide into 

 a deep and a superficial part, and, at the same time growing inwards, unite together on 

 the ventral aspect of the chorda dorsalis, forming by their union the mesial plate. 

 The superficial divisions of the two visceral plates, remaining in contact with the 

 outer epithelial layer of the embryo, form the cutis ; the deep division is the mus- 

 cul.o-intestinal layer, which forms the walls of the alimentary canal, with the exception 

 of its epithelial lining ; and the space between the superficial and deep divisions is 

 the common pleuro-peritoneal cavity, which becomes separated into the pleural and 

 peritoneal cavities in a subsequent stage of development. 



The alimentary canal commences in the form of a groove which opens towards the 

 yelk-cavity of the ovum ; and the internal epithelial and musculo-intestinal layers in 

 which this groove is formed, nre continued round the yelk, constituting the walls of 

 the vitelline sac. The open groove is soon changed into a tube at each end, but is left 

 open in the middle upon the ventral aspect, and communicates at first by a wide 

 aperture, but later by means of a tube, named the omphalo-enteric canal or vitelline 

 duct, with the vitelline sac. This duct is soon obliterated, and the vitelline sac becomes 

 the umbilical vesicle, which is thereafter connected for a time with the embryo only 

 by a slender elongated pedicle, which enters at the umbilicus and is accompanied by 

 the omphalo-mesenteric vessels ; this pedicle is finally atrophied and disappears. 



The alimentary canal, when it first assumes the tubular form, constitutes a simple 

 straight cylinder closed at each end, and placed along the front of the vertebral column, 

 to which it is closely attached at each extremity, whilst in the middle of its course it 

 is connected to the rest of the embryo by a median membranous fold, or rudimental 



