230 INTESTINAL DIGESTION 



external sphincter. This muscle is composed of striated fibres, 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 intestine, and that the mucous membrane of the large intestine 

 does not produce a secretion endowed with well-marked digestive prop- 

 erties. The simple follicles, the closed follicles and the utricular glands 

 produce a glairy mucus, which, so far as is known, serves merely to lubri- 

 cate the canal. This has never been obtained in sufficient quantity to 

 admit of any accurate investigation into its properties. 



In the human subject those portions of the food which resist the 

 successive and combined action of the different digestive secretions are 

 derived chiefly from the vegetable kingdom. Hard vegetable seeds, 

 the cortex of the cereals, spiral vessels, and, indeed, all parts composed 

 largely of cellulose pass through the intestinal canal without much 

 change. These substances form, in the feces, the greatest part of 

 what can be recognized as the residue of matters taken as food. It 

 is well known that an exclusively animal diet, particularly if the nutri- 

 tious matters are taken in a concentrated and readily assimilable form, 

 leaves very little undigested matter to pass into the large intestine, and 

 gives to the feces a character quite different from that which is observed 

 in herbivorous animals or in man when subjected to an exclusively vege- 

 table diet. The characters of the residue of the digestion of albuminous 

 substances are not very distinct. As a rule the proteids can not be 

 recognized in the healthy feces by the ordinary tests. 



Absorption of various articles of food in a liquid form may take place 

 with great activity in the large intestine, although it has not been shown 

 that the secretions in this part of the alimentary canal have any distinct 

 digestive properties ; still, as is seen in rectal alimentation, eggs, milk 

 and meat-extracts may be taken up by the mucous membrane, and they 

 enter the circulation in such a form that they contribute to nutrition. 



Processes of Fermentation in the Intestinal Canal. The processes 

 of fermentation in the intestines are not properly digestive and are to a 

 great extent due to the action of micro-organisms, which exist here in 

 great numbers and variety. Some of the substances resulting from 

 intestinal fermentations have already been described. Indol (C 8 H 7 N), 

 skatol (C 9 H 9 N), phenol (C 6 H 5 OH) and cresol (C 6 H 4 .OH.CH 3 ) probably 

 result from the action of micro-organisms ; but their production may be 

 arrested or retarded by the action of certain drugs, such as calomel, 

 salicylic acid and other so-called antiseptics. The fermentative changes 

 in the intestines involve the formation of certain gases, which will be 

 described at the close of this chapter. 



