( 14S ) 



CIIx\PTEll VIII. 



Chrjl'ificalion. 



The formation of Chyle, or the Huid which is the imme- 

 diate and cxchisivc source of nutriment to the system, takes 

 ])lace in the intestinal tube, into which the chyme ])repared 

 by the stomach is receiv'ed, and where farther chemical 

 changes are effected in its composition. The mode in which 

 the conversion of chyme into chyle is accomplished, and 

 indeed the exact nature of the changes themselves, being, 

 as yet, very imperfectly known, it is consequently impos- 

 sible to trace distinctly the correspondence which, in all 

 cases, undoubtedly exists between the objects, to be answered 

 and the means employed for their attainment. No doubt 

 can be entertained of the importance of the functions that 

 are performed by structures so large and so complicated as 

 are those composing the alimentary canal, and its various 

 appendages. We plainly perceive that provision is made 

 in the interior of that canal, for subjecting its contents to 

 the action, first, of an extensive vascular and nervous sur- 

 face; and secondly, of various fluid secretions, derived from 

 different sources, and exercising powerful chemical agencies 

 on the digested aliment; that a muscular power is su])plied, 

 by means of the layers of circular and longitudinal fibres, 

 contained between the outer and inner coats of the intes- 

 tine,* for exerting a certain pressure on their contents, and 

 for propelling them forwards by a succession of contractions, 

 which constitutes what is termed their pcrisiallic motion; 

 and lastly, that contrivances arc at the same time resorted 

 to for retarding the progress of the aliment in its passage 



• See vol. i. p. 106. 



