PHYSIOLOGICAL ANATOMY OF THE LARGE INTESTINE. 261 



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

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

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

 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 faecal matters. The simple follicles are particularly 

 abundant in the rectum, and 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 vascular ity, the veins, espe- 

 cially, being very abundant. 



The rectum terminates in the anus, a button-hole orifice, situated a little 

 in front of the coccyx, which is kept closed and somewhat retracted, except 

 during the passage of the faeces, by the powerful external sphincter. This 

 muscle is composed entirely of striated fibres, which are arranged in the form 

 of an ellipse, its long diameter being antero-posterior. 



It is now almost universally admitted that the digestion of all classes of 

 alimentary substances is completed either in the stomach or in the small in- 

 testine, and that the mucous membrane of the large intestine does not secrete 

 a fluid endowed with any well marked digestive properties. The simple fol- 

 licles, the closed follicles, and the utricular glands, produce a glairy mucus, 

 which, as far as is known, serves merely to lubricate the canal. This has 

 never been obtained in sufficient quantity to admit of any accurate investiga- 

 tion into its properties. 



In studying the changes which the alimentary mass undergoes in its pas- 

 sage through the small intestine, it has been seen that in this portion of the 

 canal, the greatest part of all the nutritive material is not only liquefied 

 but is absorbed. Sometimes fragments of muscular fibre, oil-globules, and 

 other matters in a state of partial disintegration, may be detected in the 

 faeces ; but generally this is either the result of the ingestion of an excessive 

 quantity of these substances or it depends upon some derangement of the 

 digestive apparatus. When intestinal digestion takes place with regularity, 

 the transformation of the alimentary residue into faecal matter is slow and 

 gradual. As the contents of the stomach are passed little by little into the 

 duodenum, the mass becomes of a bright-yellow color, and its fluidity is in- 

 creased, from the admixture of bile and pancreatic fluid. In passing along 

 the canal, the consistence of the mass gradually diminishes on account of 

 absorption of its liquid portions, and the color becomes darker ; and by the 

 time that the contents of the ileum are ready to pass into the caecum, the 

 greatest part of those substances recognized as alimentary has become 



