378 PRACTICAL ANATOMY. 



continuous layer, thicker at the ridges between the 

 pouches and in the lower part of the rectum; re-inforced 

 by some voluntary fibres, they form a band one and one- 

 half inches broad, one-third of an inch thick, which 

 forms the internal, the main sphincter of the rectum. 

 The cellular coat consists of loose connective tissue. 



The mucous coat is of a grayish color, and thrown into a 

 great number of transverse semilunar wrinkles ; it has no 

 villi. It is lined by columnar epithelium. In the rec- 

 tum it is thick and vascular, and loosely connected with 

 the muscular coat. When empty it is thrown into nu- 

 merous longitudinal wrinkles, which disappear as disten- 

 sion occurs. Three, sometimes four, folds of mucous 

 membrane at right angles to the axis of the rectum form 

 a series of shelves for the support of the weight of the 

 faecal matter. 



The anus is the external opening of the rectum. It 

 forms an antero-posterior slit-like opening between the 

 buttocks, posterior to a line across the tuberosities of the 

 ischia. The skin surrounding this orifice is pigmented, 

 and presents numerous short, stiff hairs. Beneath the 

 skin are a series of delicate bundles of muscular fibres, 

 called the corrugator cutis ani, which throw the skin 

 into a number of wrinkles, and which radiate from the 

 anal aperture. The external sphincter consists of an 

 elliptical plane of voluntary fibres, which surround the 

 anus. It is about a millimetre in thickness, and lies 

 beneath the skin. A critical examination of the anus 

 shows that, when contracted, it is puckered into from 

 seven to twelve slightly nodular elevations, the ex- 

 tremities of the anal columns. In the intervals between 

 the nodules are minute pouches, which are not infre- 

 quently subject to excoriations and ulceration. The 

 large intestine is studded with mucous follicles. It also 



