412 



THE DIGESTIVE APPARATUS IN MAMMALIA. 



Fig. 225. 



physiological considerations on which this principal 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 domesticated species. 



A^ General Characters of the Teeth— General Arrangement. — 

 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 together in front, at the 

 middle of the dental arches, are named iticisors, or incisive 

 teeth ; the others, situated behind these, and always number- 

 ing two for each jaw, are called the canine teeth, or tusks ; 

 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. 



External Conformation. — Each tooth represents, 

 when completely developed, an elongated polyhedron, which 

 has sometimes a pyramidal form, and at others that of a 

 cone or parallelepiped. 



A portion of the tooth is buried and solidly implanted 

 in one of the alveolar cavities of the maxillary bones ; this 

 is the roof, or embedded fortion (or fang). The other por- 

 tion, circumscribed at its base by the gum, leaves the 

 alveolus to project into the interior of the mouth, forming 

 the crown, ox free portion. The narrow constriction between 

 the crown and root is named the 7ieck. 



The fang is perforated at its inferior extremity by one 

 or more excavations {caintas pulpce), which penetrate deeply 

 into the substance of the tooth, and admit into their 

 interior the vasculo-nervous papilla, simple or ramified, 

 known by the name of the huJb, or dental pulp. 



The crown, the portion submitted to friction during 

 mastication, and, consequently, to wear, offers the most 

 varied forms : sometimes it is shaped like a very acute cone ; 

 at others, it is divided into several tubercles more or less 

 salient ; and sometimes, again, it bears at the extremity of 

 the tooth a wearing surface more or less plane and regular. 

 Structure. — Three essentially different substances enter 

 into the structure of all the teeth — the ivory, enamel, and 

 cement; to which ought to be added the soft parts — the 

 pulp, gum, and alveolo-dental periosteum. 



Ivory. — The ivary, or dentine, has the hardness of bone, 

 is of a whitish-yellow colour, and is rendered brilliant in places by its nacrous 

 reflection. It forms the principal mass of the tooth, enveloping everywhere the 

 pulp cavity. 



Examined by aid of the microscope, this substance is found to be channeled 

 by a multitude of minute, undulating, and branching canals {dental canaliculi or 

 tubuli) embedded in amorphous matter (the fundamental substance). 



The tubuli, or canaliculi, extend from the dental cavitv to the inner face of 



MAGNIFIED SECTION OF 

 A CANINE TOOTH, 

 SHOWING ITS INTI- 

 MATE STRUCTURE. 



1, Crown ; 2, 2, neck ; 

 3, fang, or root; 4, 

 cavitas pulpac ; 5, 

 opening by which the 

 vessels and nerves 

 communicate with 

 the pulp; 6, 6, den- 

 tine, showing fibrous 

 structure ; 7, 7, 

 enamel ; 8, 8, cement. 



