PHYSIOLOGICAL ANATOMY OF THE LARGE INTESTINE 229 



cular fibres beginning at the rectum are situated between them, so that 

 this part of the canal, especially in its lower portion, is covered with 

 longitudinal fibres in a nearly uniform layer. 



Mucous Coat. The mucous lining of the large intestine presents 

 several points of difference from the corresponding membrane in the 

 small intestine. It is paler, somewhat thicker and firmer, and is more 

 closely adherent to the subjacent parts. In no part of this mem- 

 brane are there any folds, like those which form the valvulae con- 

 niventes of the small intestine; and the surface is smooth and free 

 from villi. 



Throughout the entire mucous membrane, from the ileo-caecal valve 

 to the anus, are orifices that lead to simple follicular glands. These 

 structures resemble in all respects the follicles of the small intestine, 

 except that they are a little longer, owing to the greater thickness of the 

 membrane, are wider and rather more abundant. Among these small 

 follicular openings are found, scattered irregularly throughout the 

 membrane, larger openings which lead to utricular glands, resembling 

 the closed follicles in general structure, except that they have an 

 opening into the cavity of the intestine, that is sometimes so large 

 as to be visible to the naked eye (see Plate V, Fig. 5). The num- 

 ber of these glands is variable, and they exist throughout the intestine, 

 together with the closed follicles, except in the rectum. In the caecum 

 and colon, isolated closed follicles usually are found, which are identical 

 in structure with the solitary glands of the small intestine. These are 

 variable both in number and size. 



The mucous membrane of the rectum, in the upper three-fourths of 

 its extent, does not differ materially from that of the colon. In the 

 lower fourth, the fibrous tissue by which the lining membrane is united 

 to the subjacent muscular coat is loose, and the membrane, when the 

 canal is empty, is thrown into a great number of irregular folds. At 

 the site of the internal sphincter, five or six little semilunar valves have 

 been observed, with their concavities directed toward the colon. These 

 form an irregular festooned line which surrounds the canal ; their folds, 

 however, are small and have no tendency to obstruct the passage of 

 fecal matters. The simple follicles are particularly abundant in the 

 rectum, arid the membrane is constantly covered with a thin coating of 

 mucus. Another peculiarity to be noted in the mucous membrane of 

 the lower portion of the rectum is its great vascularity, the veins, espe- 

 cially, being very abundant. 



The rectum terminates in the anus, a buttonhole orifice, situated 

 a little in front of the coccyx, which is kept closed and somewhat 

 retracted, except during the passage of the feces, by the powerful 



