182 THE VITAL FUNCTIONS. 



its cavity. These folds are often of considerable 

 breadth ; contributing not only to the extension of 

 the surface for 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 ; 

 the inner coat of the large intes- 

 tine being expanded into a broad 

 fold, which, as is seen in Fig. 3 Hi, 

 representing this structure in the 

 interior of the intestine of the 

 Shark, takes a spiral course ; and 

 this is continued nearly the whole 

 length of the canal, so that the 

 internal surface is much augmented, without any 

 increase in the length of the intestine.* 



When the nature of the assimilatory process is 

 such as to require the complete detention of the 

 food, for a certain time, in particular situations, we 

 find this object provided for by means of cceca, or 

 separate pouches, opening laterally from the cavity 

 of the intestine, and having no other outlet. Struc- 

 tures of this kind are described by Ehrenberg as 

 existing in the polygastric Infusoria;! and they 

 are met with, indeed, in animals of every class, 

 occurring in various parts of the alimentary tube, 

 sometimes even as high as the pyloric portion of 

 the stomach, and frequently at the commencement 

 of the small intestine. Their most usual situation, 



* Structures of this description have a particular claim to attention 

 from the light they throw on the nature of several fossil remains, 

 lately investigated with singular success by Dr. Buckland, 



f Page 85, of this volume. 



i\ 



