108 



ANATOMT AND PHYSIOLOGY OP 



longitudinal fibres are abundant in the rec- 

 tum, but they only form bands in the ante- 

 rior two-thirds, as posteriorly to this they 

 uniformly smTound the gut. The inner 

 layer of fibres encircles the whole of the 

 gut, being tliickest towards the apex of the 

 caecum, as well as in the single colon and 

 rectum ; at the end of the latter the inter- 

 nal sphincter-ani is formed by an accumu- 

 lation of the circular fibres. The circular 

 fibres of the colon are engaged in forming 

 the ileo-colic valve, hereafter to be described. 



The cellular coat of the large intestine 

 resembles that of the small, only not so 

 abundant, except at the terminating portion 

 of the rectum, where it is much more de- 

 veloped. 



The mucus lining of the large intestine 

 is continuous anteriorly with that of the 

 ileum, posteriorly with the common integu- 

 ment. It is thin, more or less coated with 

 mucus, scantier in glands than the one of 

 the small intestine ; but the orifices of the 

 Lieberkuehnian crypts are more apparent, 

 owing to the surface here being destitute of 

 villi. Saccular recesses, more or less capa- 

 cious, exist in the membrane lining the 

 large intestine. The difference in degree 

 of vascularity gives rise to difference in the 

 color of the mucus coat in various portions 

 of the gut : thus, that lining the caecum is 

 generally more deeply colored than that of 

 the colon, whilst the rectal mucus mem- 

 brane is more vascular, and hence redder 

 than the colic or csecal one. 



At the termination of the ileum is the 

 ileo-colic or ileo-caecal valve, which is con- 

 stituted of two folds of mucus membrane, 

 almost parallel to each other, and horizontal, 

 leaving between them an eliptical orifice 

 when partially drawn asunder. The folds 

 consist of the circular fibres of the intes- 

 tine, lined on the inner or ileac side by the 

 villous membrane of the small, whilst on 

 the caecal and coUc side they are covered by 

 the mucus membrane proper to the large 

 intestine. It is worthy of notice, that 

 though muscular fibres partly enter into the 

 construction of the valve, its efficiency is 

 explicable on purely mechanical grounds, as 



proved by the fact, that it is competent in 

 the dead body. 



The anus is the outlet of the intestine, 

 which is perfectly closed, except during the 

 evacuation of feculent matters, and is made 

 perceptible externally by the elevation of the 

 tail, being situated in a space bounded su- 

 periorly by the sacrum and coccyx, laterally 

 by the ischial tuberosities, and inferiorly by 

 the urethra in the male and vulva in the 

 female. 



It is lined within by the mucus mem- 

 brane of the rectum, which is loose and of 

 a marked red color. Its external covering 

 is of common integument, destitute of hairs. 

 Lying between the skin and mucus mem- 

 brane are two circular muscles, whose office 

 is to keep the anus closed and prevent con- 

 stant evacuation of faeces, whilst there are 

 other muscular appendages situated exter- 

 nally to these, destined either to elevate or 

 retract the anus, being evidently antagonis- 

 tic to the sphincters. 



The internal sphincter-ani is in contact 

 with the attached surface of the intestinal 

 mucus membrane, and separated from the 

 integument by the external one. It is con- 

 stituted of the pale circular fibres of the gut, 

 but towards its free edge certain colored 

 fibres are apparent on it. 



The external sphincter is situated outside 

 the internal one, and within the anal integu- 

 ment: it is circular, and composed of red 

 fibres, attached superiorly under the first 

 coccygeal bone, and inferiorly its fibres 

 blend in the male subject in the accelerator 

 urinee and triangularis penis, and in the 

 female with the constrictor vaginae. 



The levatores-ani are two pale muscles, 

 attached on each side of the first coccygeal 

 bones, and, spreading downward and for- 

 ward on to the rectum, form an attachment 

 for the internal sphincter, and blending with 

 the longitudinal fibres, so as to increase the 

 thickness of the muscular coat of the rec- 

 tum. The action of these muscles must 

 be that of elevating the anus, and shorten- 

 ing the rectum from before backward. 



The retractors proper to the anus are one 

 on each side attached to the inner surface of 



