i 4 GENERAL ANATOMICAL CHARACTERS 



giims or fibro-mucous membrane covering the alveolar borders of 

 the upper and lower jaws, or, in other words, the premaxillary 

 and maxillary bones and the mandible. In the process of develop- 

 ment, for the purpose of giving them that support which is needful 

 for the performance of their functions, they almost always become 

 implanted in the bone, — the osseous tissue growing up and mould- 

 ing itself around the lengthening root of the tooth, so that 

 ultimately they become apparently parts of the skeleton. In no 

 mammal, however, does ankylosis or bony union between the 

 tooth and jaw normally take place, as in many fishes and reptiles, 

 — a vascular layer of connective tissue, the alveolo-dental mem- 

 brane, always intervening. 1 The presence of two or more roots, 

 frequently met with in the cheek-teeth of mammals, implanted in 

 corresponding distinct sockets of the jaw, is now peculiar to animals 

 of this class. 2 



Structure. — The greater number of mammalian teeth when fully 

 formed are not simple and homogeneous in structure, but are com- 

 posed of several distinct tissues, which are enumerated below. 



The pulp, a soft substance, consisting of a very delicate 

 gelatinous connective tissue, in which numerous cells are imbedded, 

 and abundantly supplied with blood-vessels and nerves, constitutes 

 the central axis of all the basal part- of the tooth, and affords the 

 means by which the vitality of the whole is preserved. The 

 nerves which pass into the pulp and endow the tooth with 

 sensibility are branches of the fifth pair of cranial nerves. The 

 pulp occupies a larger relative space, and performs a more important 

 purpose, in the young growing tooth than afterwards, as by the 

 calcification and conversion of its outer layers the principal hard 

 constituent of the tooth, the dentine, is formed. In teeth which 

 have ceased to grow the pulp occupies a comparatively small space, 

 which in the dried tooth is called the pulp-cavity. This communi- 

 cates with the external surface of the tooth by a small aperture at 

 the apex of the root, through which the branches of the blood- 

 vessels and nerves, by which the tooth receives its nutrition and 

 sensitiveness, pass in to be distributed in the pulp. In growing 

 teeth the pulp-cavity is widely open, while in advanced age it often 

 becomes obliterated, and the pulp itself entirely converted into 

 bone-like material. 



The dentine or ivory forms the principal constituent of the 

 greater number of teeth. When developed in its most character- 

 istic form, it is a very hard but elastic substance, white, with a 

 yellowish tinge, and slightly translucent. It consists of an organic 



1 The lower incisors of some species of Shrews are, however, said to become 

 aukyloaed to the jaw in adult age. 



- The teeth of the extinct Dinosaurian reptile Triceratqps have two distinct 

 roots, placed transversely to the axis of the jaws. 



