CHYLIFICATION. 181 



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 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 contriv- 

 ances 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 

 nutriment it is capable of affording. 



The total length of the intestinal tube differs 

 much in different animals ; being, in general, as 

 already stated, smaller in the carnivorous tribes 

 than in those which feed on substances of difficult 

 digestion, or affording but little nourishment. In 

 these latter animals, the intestine is always of great 

 length, exceeding that of the body many times ; 

 hence it is obliged to be folded into a spiral or ser- 

 pentine course ; forming many convolutions in the 

 abdominal cavity. Sometimes, probably for greater 

 convenience of package, instead of these numerous 

 convolutions, a similar effect of increasing the sur- 

 face of the inner membrane is obtained by raising 

 it into a great number of folds, which project into 



* See vol. i. p. 126. 



