414 THE DIGESTIVE APPARATUS IN MAMMALIA. 



of magnesia, and other salts. The rods are directed vertically on the summit 

 of the crown of the tooth, and horizontally at the sides.) 



Cemext {cementum, substantia ostoidea, cortical substance, or criista 

 petrosa). — The cement is spread in a non-continuous layer over the external 

 surface of the enamel and dentine. It is accumulated in large quantity in the 

 substance of some teeth, as will be noticed when speaking of the incisors in 

 the Horse and the molars of the Herbivora. 



The structure and properties of this substance differ in nothing from the 

 structure and properties of the spongy tissue of bone. In a physiological con- 

 dition, the cement does not contain any Haversian canals. (It contains, 

 sparingly, the lacunae and canaliculi which characterize true bone : those placed 

 near the surface have the canaliculi radiating from the side of the lacunae 

 towards the periodontal membrane ; and those more deeply placed join with the 

 adjacent dental tubuli. In the thicker portions of the crusta petrosa, the lamellae 

 and Haversian canals peculiar to bone are also found. As age advances, the 

 cementum increases in thickness, and gives rise to those bony growths, or 



exostoses, so common in the teeth of 



^ ig. 227. ^ ^YiQ aged ; the pulp cavity also be- 



jj comes partially filled up by a hard sub- 



J^^L 1;$4^ n B H stance, intermediate between dentine 



^^^^£/^^^ fjt^^^BH ^^^ ^^^^ — osfeo-dentine, or secondary 



^^W^^Jf^ i^^&r^nd <?(?»/w?e. It appears to be formed by 



^7^> ■''V?'^^" ^^^ffrPf'Q ^ ^^^^^ conversion of the dental pulp, 



^l^fr^^r^ Js^^S^^i which shrinks or even disappears.) 



jj|^ -Mj/^ pg^^^^0 Dented pidp. — The jndp, or papilla, 



C^^^ is formed by a fibrillar and nuclear 



mass that fills the internal dental 



A, TRANSVEKSE SECTION OF ENAMEL, SHOWING . t. • i i i i J 



ITS HEXAGONAL PRISMS ; B, SEPARATED PRISMS, cavity. Itreccivcs blood-vcssels aud 



nerves, and is enveloped in a very 

 thin membrane which is entirely composed of several layers of beautiful cyhn- 

 drical or prismatic cells, the most superficial of which send fibrillar prolonga- 

 tions into the dental tubuli. Towards the base of the papilla, this membrane 

 assumes the texture of connective tissue, and is reflected upwards on the fang of 

 the tooth to line the alveolus, and join the gum at the origin of the crown. 



Gum. — The gum is a portion of the buccal mucous membrane surrounding 

 the neck of the tooth ; it concurs in consolidating it in the alveolar cavity. 

 Its structure is that of the membrane to which it belongs, being a thick dermis 

 furnished with papillae and tesselated epithehum. It does not contain any 



Alveolo-dental periosteum. — This scarcely differs from ordinary periosteum, 

 except in being a little softer. It lines the alveolus and covers the cementum of 

 the fang. 



Development. — Each tooth is developed in the interior of a closed sac named 

 the dented follicle or sac,^ and lodged in an excavation in the maxillary bones. 

 The sac presents, according to the species of animal and kind of teeth, numerous 

 variations, which we cannot stay to consider here : but must confine ourselves 

 merely to a brief sketch of the general and constant characteristics of its 



' Two adjoining follicles and the papillae they contain, sometimes join each other, for we 

 have seen a two-years-old Ass in which the central and lateral incisors in the inferior maxilla 

 were united, so as to form only two teeth, instead of four. 



