BOOK II. 

 THE DIGESTIVE APPARATUS. 



CHAPTER I. 



General Considerations on the Digestive Apparatus. 



We have considered the animal as a machine composed of various levers and 

 capable of various movements ; but it will be easily understood that the working 

 of this machine will cause the wear or decomposition of the molecules which 

 enter into the construction of its organs, and that these springs or animated 

 wheels demand for their maintenance an incessant supply of new materials, 

 in order to repair their continual losses. Animals, therefore, are under the 

 necessity of taking aliment, from which they extract those reparative principles 

 that, distributed to all the organs, are assimilated to their proper substance. 



The organs in which this work of preparation and absorption of the organiz- 

 able material is carried on, are collectively named the digestive apparatus — one of 

 the most important of those which, as we shall see, successively complicate and 

 perfect the animal machine. This apparatus does not, properly speaking, 

 constitute an essentially distinctive characteristic of animality, as there are 

 animals without a digestive cavity ; but it is yet one of the most salient 

 attributes, for the exceptions just mentioned are extremely rare. Considered in 

 the Vertebrata, this apparatus appears as a long tube, most frequently doubled 

 on itself many times, dilated at intervals, and provided along its course with 

 several supplementary organs, the majority of which are of a glandular nature. 

 This tube extends the whole length of the animal's body, and opens externally 

 by two orifices, one of these serving for the introduction of aliment, the other 

 for the expulsion of the residue of digestion. These openings are at the 

 extremities of the alimentary canal. 



The conformation of this apparatus is not incidentally the same in all the 

 individuals composing the sub-kingdom of Vertebrata ; on the contrary, it 

 presents very numerous varieties, according to the habits and modes of life of 

 these individuals, and this makes its study interesting from two points of view : 

 in relation to the science of zoology, and to that of veterinary hygiene, which 

 derives from this study valuable indications concerning the regime of the 

 domesticated animals. 



But this diversity of characters does not suffice to establish sharply defined 

 limits between the conformations that are distinguished by it. There is, in 

 reality, but one typical form of digestive apparatus, and the same principle 

 'prevails in its construction throughout the entire series. Thus, whichever of 

 the Vertebrata we may be studying, its alimentary tube will be found composed 



