THE DIGESTIVE SYSTEM 103 



understand the mechanism of intestinal absorption the diagram of 

 a villus (Fig. 26) should be carefully studied. A mechanical factor 

 of importance in absorption is the rhythmic increase in pressure 

 in the intestines due to the contraction of the diaphragm with 

 each inspiration. 



The extent to which absorption takes place in the stomach of 

 the horse is not positively known, but is limited to water and 

 water-soluble substances in small amounts. Easily diffusible 

 substances like alcohol and ether are readily absorbed. Colin 

 conducted a series of experiments to determine the part played by 

 the stomach in absorption. He ligated the pylorus and then 

 placing strychnine in the stomach found that no matter how 

 long it was left there poisoning did not occur; but when the liga- 

 ture was removed and the chyme was allowed to pass into the 

 intestines, symptoms of strychnine poisoning rapidly appeared. 

 On the strength of this work and from lack of more conclusive 

 evidence it may be assumed that practically no absorption takes 

 place from the stomach of the horse, or if it does, the process must. 

 be extremely slow. 



The greatest amount of absorption is through the millions 

 of minute villi that stud the small intestine. The process is 

 more pronounced here than in the large intestine, not because 

 the ingesta remain longer, but because of the greater length 

 of this part of the canal. The experiment of Colin in which he 

 injected hydrocyanic acid into the small intestines of a horse and 

 caused death in less than one and one-half minutes proves that 

 intestinal absorption is very rapid. 



Absorption is complicated from the fact that there are two 

 paths by which the materials of digestion may enter the general 

 circulation; these are: (1) the numerous small veins which collect, 

 the blood from the stomach and intestines; (2) the lacteal vessels 

 which collect the fluid absorbed by the lacteals of the intestines. 



The very large portal vein (Fig. 27) is formed by the confluence 

 of the small veins just mentioned. It discharges its flow into the 

 liver. This organ is most advantageously situated between the 

 digestive and circulatory systems (Fig. 31). It performs the 

 highly important work of separating the nutrient from the poison- 

 ous substances in the portal blood. This function of the liver 

 may be shown by connecting the portal vein of an experimental 

 animal with the posterior vena cava so that the portal blood will 



Digitized by Microsoft® 



