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 susceptible of various movements ; but it will be easily under- 

 stood 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, destined to repair their continual losses. 

 Animals, therefore, are under the necessity of taking aliment, from which 

 they extract those reparative principles which, distributed to all the organs, 

 are assimilated into their proper substance. 



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

 organisable material is carried on are collectively named the digestive 

 apparatus : one of the most important of those which, as we will see, suc- 

 cessively 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 ex- 

 tremely rare. Considered in the vertebra ta, this apparatus appears as a 

 long tube, most frequently doubled on itself many times, bulging 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 identically 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 mode 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 valuaHe 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 for the 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 of a collection of bulging or tubuliform cavities, which 

 succeed each other from before to behind in the following order : the mouth, 

 pharynx, oesophagus, stomach, and intestine. 

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