VILLI. 227 



To appreciate this preparation we must understand the 

 action and office of the Liver. 



This organ is employed in abstracting from the blood 

 that secretion called Bile, which is necessary to complete 

 the process of digestion, though its precise mode of acting is 

 still unknown. The liver is situated at the right side of 

 the stomach, and at its lower end a large vein, called the 

 portal vein, enters, charged with blood that it has received 

 from the intestinal veins, returning with impure blood from 

 various parts of the body. This portal vein is injected blue, 

 and has spread into those innumerable capillaries which 

 distribute the blood throughout the liver, and in a most 

 unusual way reunite and coalesce into a large cavity at 

 the upper end of the liver, called the vena cava. Through 

 this channel the cleansed blood is propelled into the heart, 

 to be afterwards sent through the lungs for further 

 purification. 



The substance of the liver itself is made up of secreting 

 cells and passages, called hepatic ducts (colour injected with 

 carmine), with branches terminating in enlargements, called 

 lobes, round which the capillaries spread. The bile with- 

 drawn is poured forth through a third passage into the 

 duodenum, or small intestine nearest the stomach. Here it 

 mingles with the food, performs the office assigned to it, 

 and the superabundance passes on and away from the 

 system altogether. 



VILLI. 



Small Intestines of Man Monkey PigDog Cat Rabbit. 



Any of these will show the structure which absorbs the 

 nutritive part of our food immediately after its leaving the 

 stomach. We see in this preparation a crowd of papillae, 

 covered with a network of vessels injected either with vermi- 

 lion or chromate of lead. These are the villi, or minute pro- 

 cesses of the mucous membrane of the small intestines ; two 

 or more arteries are distributed to each villus, and from 

 their capillaries proceed one, two, or more veins, which pass 

 out at the base of each villus. Also there are one or more 

 lacteal vessels in each of these minute villi, spreading in a 



