204 THE VITAL FUNCTIONS. 



fectly known, it is consequently impossible 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 which are 

 performed by structures so large and so compli- 

 cated, as those composing the alimentary canal 

 and its various appendages. We plainly per- 

 ceive that provision is made, in the interior of 

 that canal, for subjecting its contents to the 

 action, first, of an extensive vascular and nervous 

 surface ; and secondly, of various fluid secretions, 

 derived from different sources, and exercising 

 powerful chemical agencies on the digested 

 aliment ; that a muscular power is supplied, by 

 means of the layers of circular and longitudinal 

 fibres, contained between the outer and inner 

 coats of the intestine,* for exerting a certain 

 pressure on their contents, and for propelling 

 them forwards by a succession of contractions, 

 which constitute what is termed their peristaltic 

 motion; and lastly, that contrivances are at the 

 same time resorted to for retarding the progress 

 of the aliment in its passage along the canal, so 

 that it may receive the full action of these several 

 agents, and yield the utmost quantity of nutri- 

 ment it is capable of affording. 



* See vol. i. p. 137. 



