ABSORPTION. 



197 



FIG. 38. 



The muscular layer throughout the alimentary canal is double, consist- 

 ing of circular and longitudinal fibres. Their action may be excited 

 in the recently killed animal, by pinching the exposed intestine with 

 the blades of a forceps. A contraction takes place at the spot irritated, 

 the intestine is reduced in diameter, and its contents forced onward. 

 The local contraction then propagates itself to the neighboring parts, 

 while the portion originally contracted becomes relaxed ; and a slow, 

 continuous, creeping motion of the intestine is produced, by successive 

 waves of contraction and relaxation, following each other from above 

 downward. At the same time the longitudinal fibres have a similar 

 alternate action, drawing the narrowed portions of intestine up and 

 down, as they successively become contracted or relaxed. The effect 

 produced is a peculiar, writhing, worm-like, or " vermicular " motion, 

 among the coils of intestine. 

 During life, this action of the 

 intestine is excited by the food 

 undergoing digestion. By its 

 means the substances passing 

 from the stomach into the duode- 

 num are made to traverse the 

 entire length of the small intes- 

 tine, and brought in contact suc- 

 cessively with the whole of its 

 mucous membrane. During this 

 passage the absorption of the 

 digested food takes place, so that 

 its liquefied portions disappear, 

 and, at the lower end of the 

 small intestine, there remains 



Ollly the undigested part of the CHYLE FROM COMMKNCKMKXT OF THORACIC DUCT ; 



food, together with the refuse 



of the intestinal secretions. These pass through the ileo-caBcal orifice 



into the large intestine, and are there reduced to the condition of feces. 



The fluids of the intestine are first absorbed by the epithelial cells 

 of the villi, and thence transmitted to the deeper portions of the tissue. 

 This passage of the products of digestion through the substance of the 

 epithelial cells is difficult of demonstration for homogeneous liquids, 

 but it may be seen in the fatty matters of the chyle. Chyle, drawn 

 either from the lacteal vessels or from the thoracic duct, presents the 

 same appearance, containing fatty matter under the form of granules, 

 w r hich vary in size from 2.5 mmm. downward, and which have the 

 usual characters of oil in a state of minute subdivision. 



The emulsioned fat of the chyle has accordingly passed from the 

 cavity of the intestine into that of the lacteal vessels. Its transmis- 

 sion is facilitated by the alkaline condition of the blood and of the 

 intestinal juices. Oil by itself is non-diffusible. If a fluid containing 

 oil be placed on one side of an animal membrane, and pure water on 



