344 THE DIGESTIVE APPARATUS l\ U.I M W.i/./.i. 



enlarge tho isthmus and allow solids or liquids to pass through. The desn-ip 

 tion given of this septum permits us to understand lio\v it plays tin part <>l'i 

 valve in rising freely while the alimentary bolus or mouthful of fluid pi 

 from tho mouth into tho oesophagus, across tho pharyngeal vestibule, but 

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

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

 descent of aliment into the oesophagus, after it has cleared the isthmiK of 

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

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

 after flowing over tho posterior surface of the soft palate. This disposition 

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

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

 likewise sufficiently explains why, in normal circumstances, Solipeds re.^pue 

 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 tho 

 mouth to bruise or lacerate tho 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 tho 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 tho teeth 

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

 " Give mo tho tooth of an animal, and I 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 CHARACTERS OF THE TEETH GENERAL DISPOSITION. Tho 

 teeth are fixed in tho jaws, and ranged one against tho 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, arc called the canine teeth or /MX/.-X ; 

 while tho designation of molars is given to tin-so which occupy, in the more 

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

 dental arches. 



EXTKUNAL CONFORMATION. Kaeh tooth represents, when completely 

 developed, an elongated polyhedron, which has sometimes a pyramidal 

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



