PHYSIOLOGICAL ANATOMY OF THE LARGE INTESTINE. 291 



membrane, larger openings which lead to utricular glands, resembling the closed follicles, 

 in general structure, except that they have an orifice opening into the cavity of the in- 

 testine, which is sometimes so large as to be visible to the naked eye. The number of 

 these glands is very variable, and they are irregularly disseminated throughout the intes- 

 tine, in company with the closed follicles, except in the rectum, where they are absent. 

 In the caecum and colon, numerous isolated, closed follicles are generally found, which 

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

 ceedingly 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 vascularity, the veins, especially, being very numerous. 



Finally, 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 pas- 

 sage of the faeces, by the powerful external sphincter. This muscle is composed entirely 

 of red, or 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 sub- 

 stances is completed either in the stomach or in the small intestine, and that the mucous 

 membrane of the large intestine does not secrete a fluid endowed with any well-marked 

 digestive properties. The simple follicles, the closed follicles, and the utricular glands, 

 produce a glairy mucus, which, as far as we know, serves merely to lubricate the canal. 

 This has never been obtained in sufficient quantity to admit of any accurate investigation 

 into its properties. 



In studying the changes which the alimentary mass undergoes in its passage through 

 the small intestine, we have 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 mus- 

 cular fibre, oil-globules, and other matters in a state of partial disintegration, are to be 

 detected in the faeces by the microscope ; but generally this is either the result of taking 

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

 digestive apparatus. When intestinal digestion takes place with regularity, the trans- 

 formation of the alimentary mass into faecal matter is slow and gradual. As the con- 

 tents of the stomach are passed little by little into the duodenum, the chymous mass be- 

 comes of a bright-yellow color, and its fluidity is increased, from the admixture of bile 

 and pancreatic fluid. In passing along the canal, the consistence of the mass gradually 

 diminishes, from the 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 crccum, the 

 greatest part of those substances which we have recognized as alimentary principles 

 have become changed and absorbed. The various forms of starchy and saccharine prin- 

 ciples, unless they have been taken in excessive quantity, soon disappear from the in- 

 testine ; and the glucose, which is the result of their digestion, may be recognized in the 

 blood of the portal system. As a rule, fatty matters are not found in the lower part of 

 the ileum, having passed into the lacteals in the form of an emulsion. Neither fibrin, 

 albumen, nor caseine, can be detected in the ileum; and, as we have seen, the muscular 

 substance, as recognized by its microscopical characters, becomes gradually disintegrated 



