344 THE DIGESTIVE APPARATUS IN MAMMALIA. 



enlarge the isthmus and allow solids or liquids to pass through. The descrip- 

 tion given of this septum permits us to understand how it plays the part of a 

 valve in risin" freely while the alimentary bolus or mouthful of fluid passes 

 from the mouth into the CBSophagus, across the pharyngeal vestibule, but 

 never allows the matters which have once entered the oesophageal canal to 

 return into the buccal cavity. Also why, when any obstacle is opposed to the 

 descent of aliment into the cesophagus, after it has cleared the isthmus of 

 the fauces, or even when the animal vomits, the matters arrested in their 

 passage or expelled from the stomach are ejected by the nasal cavities, 

 after iiowing over the posterior surface of the soft palate. This disposition 

 of the pendulous curtain, in forming a complete partition which hermeti- 

 cally seals the orifice of communication between the mouth and pharynx, 

 likewise sufficiently explains why, in normal circumstances, Solipeds respire 

 exclusively by the nostrils. 



6. The Teeth. 



Passive agents in mastication, the teeth are hard organs, bony in 

 appearance, implanted in the jaws, and projecting into the interior of the 

 mouth to bruise or lacerate the solid alimentary substances. 



Identical in all our domesticated animals, by their general disposition, 

 their mode of development, and their structure, in their external con- 

 formation these organs present notable differences, the study of which offers 

 the greatest interest to the naturalist. For it is on the form of its teeth 

 that an animal depends for its mode of alimentation ; it is the regime, in 

 its turn, which dominates the instincts, and commands the diverse modifica- 

 tions in the apparatus of the economy ; and there results from this law of 

 harmony so striking a correlation between the arrangement of the teeth 

 and the conformation of the other organs, that an anatomist may truly say, 

 " Give me the tooth of an animal, and 1 will tell you its habits and 

 structure." 



Compelled by the limits of our task to confine ourselves to the purely 

 descriptive part of the dental apparatus, we cannot stop to notice the 

 interesting physiological considerations on which this principle is founded; 

 but will begin at once the anatomical study of the teeth by indicating their 

 general characters, before examining them successively in all the domestic- 

 ated species. 



A. General Chaeaotbes op the Teeth — General Disposition. — The 

 teeth are fixed in the jaws, and ranged one against the other in such a way 

 as to form two parabolic arches opening behind, and interrupted on each 

 side by what is called the interdental space. Distinguished into superior 

 and inferior, like the jaws to which they belong, these arches come in contact 

 with one another in a more or less exact manner when the mouth is perfectly 

 closed. 



Those teeth which are placed altogether in front, at the middle of the dental 

 arches, are named incisors or incisive teeth ; the others, situated behind these, 

 and always numbering two for each jaw, are called the canine teeth or tushs; 

 while the designation of molars is given to those which occupy, in the more 

 retired portion of the buccal cavity, the lateral parts and extremities of the 

 dental arches. 



ExTEENAL Conformation. — Each tooth represents, when completely 

 developed, an elongated polyhedron, which has sometimes a pyramidal 

 form, and at others that of a cone or a parallelopiped. 



