PHYSIOLOGY. 33 



called duodenum. Here it meets with two fluids, one secreted 

 by the liver, called gall or bile, and the other secreted by the 

 pancreas or sweetbread, called pancreatic juice. The fluids 

 from both are conveyed into the duodenum by small tubes or 

 canals. Immediately after the admixture of the chyme with 

 the fluids, it begins to separate into two distinct portions, one 

 consisting of the nutritious portion, called the chyle, and the 

 other the refuse portion, part of which forms the excrement. 



Absorption of the Chyle. — The intestines are composed, in 

 a great measure, of muscular fibres, some of which run in a 

 circular, .and others in a longitudinal direction. When the 

 fibres contract, the diameter of the canal is diminished ; and 

 when the longitudinal fibres are in action, the canal is short- 

 ened ; and by the combined action of these fibres, the food 

 is gradually propelled through the whole length of the canal. 

 The internal surface of the intestines consists of a nervous 

 and vascular membrane, which is constantly forming a mu- 

 cous substance for its own protection. In cases of constipa- 

 tion and colic, this secretion is so abundant as to be discharged 

 in considerable quantity with the dung. There is also, on the 

 inner surface of the intestines, an immense number of minute 

 vessels, called lacteals, by which the chyle is sucked up or 

 absorbed. The lacteals terminate on the inner surface of the 

 intestines by small tubes, with open mouths. These tubes 

 are full of little valves, which prevent the return of chyle. 

 In passing through these canals, the fluid traverses small 

 bodies, called mesenteric glands. Eventually, the whole of 

 the lacteals terminate in a vessel called the thoracic duct, 

 from thence into one of the jugular veins,* into which it 

 empties. The chyle thus mixes with the venous blood in the 

 veins, and goes with it to the lungs, where, coming in contact 

 with atmospheric air, it is deprived of its carbon, and assumes 

 the character of pure arterial blood. There are other vessels 

 that empty into the thoracic duct, called lymphatics ; these, 

 with the lacteals, form what is termed the absorbent system. 



* It enters the subclavian vein in the human being. 

 5 



