STRUCTURE OF THE LARGE INTESTINE. 



295 



serous coat has the same structure as that of the small intestine. The muscular coat has 

 external longitudinal fibres occurring all round the gut, but they form three flat ribbon-like 

 longitudinal bands in the caecum and colon (fig. 214). Inside this coat are the circular fibres. 

 The sub-mucosa is practically the same as that of the small intestine. The mucosa is distin- 

 guished by negative characters. It has no villi and no Peyer's patches, but otherwise it resembles 

 structurally the small intestine, consisting of a basis of adenoid tissue with the simple tubular 

 glands of Lieberkuhn (fig. 199). These glands are very numerous and somewhat longer than 

 those of the small intestine, and they always contain far more goblet-cells about ten times as 

 many. The cells lining them are devoid of a clear disc. Solitary glands occur throughout 

 the entire length of the large intestine. At the bases of Lieberkiihn's glands is the muscularis 

 mucosae. The blood-vessels and nerves have a similar arrangement to those in the stomach.] 



Epi- 

 thelium 





Solitary 

 follicle. 



Sub- 

 mucous 

 coat. 



Circular 

 fibres. 



Musculo,) 

 coat. 



Longitudi 

 nal fibres. 



Fig. 214. 

 Longitudinal section of the large intestine. 



[Blood- Vessels. On looking down on an opaque injection 'of the mucous membrane of the 

 stomach, one sees a dense meshwork of polygonal areas of unequal size, with depressions here 

 and there. The orifices are the orifices of the gastric glands, each surrounded by a capillary. 

 A somewhat similar appearance is seen in an opaque injection of the mucous membrane of the 

 large intestine, but in the latter the meshwork is uniform, all the orifices (of Lieberkiihn's 

 glands) being of the same size.] 



191. ABSORPTION OF THE DIGESTED FOOD. The physical forces con- 

 cerned are : endosmosis, diffusion, and nitration. 



All the constituents of the food, with the exception of the fats, which in part are changed 

 into a fine emulsion, are brought into a state of solution by the digestive processes. These 

 substances pass through the walls of the intestinal tract, either into the blood-vessels of the 

 mucous membrane or into the beginning of the lymphatics. In this passage of the fluids two 

 physical processes come into play endosmosis and diffusion as well as filtration. 



I. Endosmosis and diffusion occur between two fluids which are capable of forming an inti- 

 mate mixture with each other, e.g., hydrochloric acid and water, but never between two fluids 

 which do not form a perfect mixture, such as oil and water. If two fluids capable of mixing with 

 each other, but of different compositions, be separated from each other by means of a septum 

 with physical pores (which occur even in a homogeneous membrane), an exchange of the constitu- 



