290 



THE ORGANS OF ABSORPTION. 



Scheme of intestinal absorption. LAC. 



Physiology of Absorption. 



189. THE ORGANS OF ABSORPTION. [As most substances in the state 

 in which they are used for food are either insoluble, or diffuse but imperfectly 

 through membranes, the whole drift of the complicated digestive processes is to 

 render these substances soluble and diffusible, and thus fit them for absorption; 

 while most of the fats are emulsionised.] 



The mucous membrane of the whole intestinal tract, as far as it is covered by a 



single layer of columnar epithelium, i.e., from the 

 cardiac orifice of the stomach to the anus is 

 adapted for absorption. The mouth and oeso- 

 phagus, lined as they are by stratified squamous 

 epithelium, are much less adapted for this pur- 

 pose. Still, poisoning is caused by placing potas- 

 sium cyanide in the mouth. The channels of 

 absorption in the intestinal tract are (fig. 206) 

 (1) the capillaries [direct], and (2) the lacteals 

 [indirect] of the mucous membrane. Almost the 

 whole of the substances absorbed by the former 

 lacteals ;*T.D., Ihoracic "duct ; P. v! P ass i nto tne rootlets of the portal vein, and tra- 

 and H. V., portal and hepatic veins ; verse the liver, while those that enter the lacteals 

 INT., intestine. really pass into lymphatics, so that the chyle 



passes through the thoracic duct and is poured by it into the blood, where the 

 thoracic duct joins the subclavian vein. 



Watery solutions of salts, grape-sugar, peptone, poisons, and in a still higher 

 degree alcoholic solutions of poisons, are absorbed in the stomach. The empty 

 stomach absorbs more rapidly than one filled with food; gastric catarrh delays 

 absorption. After a copious diet of milk, fatty granules have been found in the 

 protoplasm of the goblet-cells ; so that according to this view, the goblet-cells have 

 a double function, to secrete mucus and to absorb nutriments. 



The greatest area of absorption is undoubtedly the small intestine, especially 

 its upper half, owing to the presence of the valvulae conniventes and the villi. 



190. STRUCTURE OF THE SMALL AND LARGE INTESTINES. [The 

 wall of the small intestine consists of four coats ; which, from without inward, are 

 named serous, muscular, sub-mucous, and mucous (fig. 207). 



(1) The serous coat has the same structure as the peritoneum, i.e., a thin basis of fibrous 

 tissue covered on its outer surface by endothelium. 



(2) The muscular coat consists of a thin outer longitudinal and an inner thicker circular 

 layer of non-striped muscular fibres (fig. 207). 



(3) The sub-mucous coat consists of loose connective-tissue containing large blood-vessels 

 and nerves, and it connects the muscular with the mucous coat. 



(4) The mucous coat is the most internal coat, and its absorbing surface is largely increased 

 by the presence of the valvulae conniventes and villi. [The valvule conniventes are permanent 

 folds of the mucous membrane of the small intestine, arranged across the long axis of the gut. 

 They pass round a half or more of the inner surface of the gut. They begin a little below the 

 commencement of the duodenum, and are large and well marked in the duodenum, and remain 

 so as far as the upper half of the jejunum, where they begin to become smaller, and finally dis- 

 appear about the lower part of the ileum.] The villi are characteristic of the small intestine, 

 and are confined to it ; they occur everywhere as closely-set projections over and between the 

 valvulae conniventes (fig. 207). When the inner surface of the mucous membrane is examined 

 in water, it has a velvety appearance owing to their presence. [They vary in length from ^ to 

 ^ of an inch, are largest and most numerous in the upper part of the intestine, duodenum, and 

 jejunum, where absorption is most active, but they are less abundant in the ileum. Their total 

 number has been calculated at four millions by Krause.] Each villus is a projection of the 

 entire mucous membrane, so that it contains within itself representatives of all the tissue- 



