DIGESTION 149 



Here and there in the mucous membrane, in connection 

 with the lymphatic vessels, arc found small nodules consisting 

 of leucocytes held in a fine mcshwork of connective tissue, 

 that is, nodules having essentially the same structure as 

 lymphatic glands. They are, in fact, minute lymphatic glands. 

 They are called solitary lymphoid nodules. Here and there 

 M-veral of these nodules arc collected together forming patches, 

 called Peyer's patches, and just over these the villi are 

 absent. 



The blood is brought to the wall of the intestine by branches 

 from the aorta, the arteries passing to it along the mesentery. 

 The blood leaves the intestine by veins, lying in the mesentery ; 

 these gradually unite with one another, and form, with the 

 veins from the stomach, pancreas, and spleen, the portal vein, 

 which carries its blood to the liver. 



Functions of the Mucous Membrane of the Small 

 Intestine. The chief function of the mucous membrane of 

 the small intestine is to absorb the digested food. This is 

 carried out by the villi. The form of the villi and the presence 

 of the valvula? conniventes which they beset, enormously 

 increase the extent of surface in contact with the contents. The 

 epithelial cells covering the villi have the power of taking up 

 from the chyme in the cavity of the intestine, peptone, sugar 

 and salts, and also finely-divided fat,' as well as water. They 

 have also the power of passing these substances, more or less 

 altered, on to the blood-vessels and lymphatics. The power by 

 which the cells thus transfer material from the cavity of the in- 

 testine to the vessels in the wall of the intestine is somewhat akin 

 to the power, which, as we have seen, the cells of a gland have, 

 of allowing only certain substances to pass out of the blood 

 into their secretion. The more diffusible the subtances are, the 

 more readily can they, generally speaking, pass through the 

 epithelial membrane ; but what substances shall be absorbed 

 and at what rate they shall be absorbed depends mainly on 

 the living epithelial cells. Of these several substances, the 

 peptone, sugar, most of the salts, and a great deal of the water 

 into the blood capillaries in the substance of the villi, anil 

 "> i liter the blood stream. ( )n the other hand the finely-divided 

 fat taken up by the epithelial cells passes into the lacteals and i^ 

 carried away along the lymphatic vessels. Owing to the presence 



