CHYLIFICATION. 



149 



i 



along the canal, so that it may receive tlie full action of 

 these several agents, and yield the utmost (quantity of nutri- 

 ment it is capable of affording. 



The total length of the intestinal tube differs much in dif- 

 ferent animals, being in general, as already stated, smaller in 

 the carnivorous tribes, than those which iced on substances 

 of difficult dige stion, 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 oi* serpentine course, forming many 

 convolutions in the abdominal cavity. Sometimes, probably 

 for greater convenience of package, instead of these nume- 

 rous convolutions, a similai' effect of increasing the suri\ice 

 of the inner membrane is obtained by raising it into a great 

 number of folds, which project into tl>c cavity. These folds 

 are often of consiflerable breadth, contributing not only to the 

 extension of the surface foi' secretion and absorption, but also 

 to the detention of the materials, with a view to their more 

 complete elaboration. Remarkable examples of this kind of 



structure occur in most of the cartilaginous 

 fishes, when the inner coat of the large in- 

 testine is expanded into a broad fold, which, 

 as is seen in fig. 316, representing this struc- 

 ture in the interior of the intestine of the 

 sliark, takes a spiral course; and this is con- 

 tinued nearly the whole length of tlie canal, 

 so that tlie internal surface is much augment- 

 ed without any increase in the length of the 



intestine.* 



When the nature of the assimilatory pro- 

 cess is such as to require the complete detention of the food, 

 for a certain time, in particular situations, we find this ob- 

 ject provided for by means of casca, or separate pouches 



• Structures of ttiis description have a particular claim to attention, from 

 the light they tlirow on the nature of several fossil remains, lately investi- 

 gated with singular success by Dr. Duckland. 



