608 DIGESTION. 



alternate contraction and dilatation of the organ, proceeding generally 

 from above to below, -so as to propel the chyme downwards. When it 

 reaches any point of the intestine, its contact excites the contraction 

 of the circular fibres of the part; so that it is sent forwards to another 

 portion of the canal; the circular fibres of which contract, whilst the 

 former are relaxed; and this occurs successively through the whole 

 tract of the intestines. The longitudinal fibres, by their contrac- 

 tion, shorten the intestine, and in this manner meet the chyme, so 

 as to facilitate its progress ; but their effect cannot be considerable. 

 When digestion is not going on, the peristaltic action occurs only at 

 intervals; always slowly and irregularly; and, perhaps, as has been 

 suggested, only when sufficient mucous secretion has collected on the 

 inner coat of the intestine to provoke it. During digestion, it is much 

 more energetic and frequent, and more marked in the duodenum and 

 small intestine than in the large; occurring not continuously, but at 

 intervals, as the chyme arrives and excites it. When the small intes- 

 tine is surcharged, it may take place in several parts of the canal at 

 once; and, at times, the action is inverted. 



The secretions poured into the intestinal canal lubricate it, and 

 facilitate the progress of the chyme. This is aided by the free and 

 floating condition of the intestine; and by the agitation of the diaphragm 

 and abdominal muscles in respiration. Yet its course along the small 

 intestine is slow. The chyme is not transmitted from the stomach 

 continuously; and the peristaltic action of the intestines occurs only at 

 intervals. Moreover, owing to the convolutions of the intestinal canal, 

 the chyme must, in many cases, proceed against its own gravity ; and 

 be retarded by the numerous valvulse conniventes, which bury them- 

 selves in it, when the canal is contracted by the action of the circular 

 fibres. All these circumstances must cause it to proceed slowly along 

 this part of the tube, a point of some importance, when we reflect, 

 that an essential change is effected on it through the influence chiefly 

 of the bile and pancreatic juice, and that its nutritive portion is here 

 absorbed. In the duodenum, the course of the chyme is slow. In the 

 jejunum it is more rapid, hence the name, which indicates, that it is 

 almost always found "empty:" in the ileum again it is slower on account 

 of the greater consistence acquired by the absorption of the chylous 

 portion. Whilst the food is in progress along the small intestine, it 

 experiences the change in its physical properties, which enables the 

 chyle to be separated from it by absorption. These two actions have 

 been termed respectively chylification and the absorption of chyle; 

 although by some the former term has been applied to both processes. 



Above the point, at which the common choledoch and pancreatic 

 ducts open into the duodenum, no change is observable in the chyme. 

 It preserves its colour, semi-fluid consistence, sour smell, and slightly 

 acid taste; having been simply mixed with the exhaled and follicular 

 secretions of the lining membrane; but, immediately after it has passed 

 the part, at which the hepatic and cystic bile and the pancreatic juice 

 are poured into the intestine, it assumes a different appearance; its 

 colour is found to be changed; it becomes yellowish; of a bitter taste; 

 its sour smell diminishes; and chyle can now be separated by the lac- 



