CHAP XXT1I.] THE ALVEOLI. 173 



A), and not well understood, and also larger cracks or fissures (fig. 

 149), often branched, running through a part or the whole of its 

 thickness. As Mr. Owen has pointed out, the enamel is the least 

 constant of the dental tissues, being absent in many fishes, in exist- 

 ing ophidian reptiles, and in the edentate and many cetacean mam- 

 malia. 



The tooth-bone or cement is disposed as a permanent thin layer 

 of osseous tissue on the roots of the teeth, and it also invests the 

 enamel with a delicate film on the first emergence of the tooth 

 from the gum. On the roots it is thickest towards the apex, and 

 often lines the pulp-cavity of the dentine for a little way in. It 

 contains sparingly the lacunae and canaliculi which characterise 

 bone ; and, when thick enough, it presents also the lamellae of that 

 structure. In general it is in too small a quantity to require spe- 

 cial Haversian canals ; but Mr. Tomes has shewn, that, between 

 the roots of the larger human teeth, the tooth-bone is often in 

 sufficient mass to be penetrated by a true canal of that nature; 

 and, in the teeth of many animals, the cement is as vascular as 

 ordinary bone. The canaliculi of the tooth-bone are, for the most 

 part, directed from the lacuna? towards the surface, where the ves- 

 sels are spread out ; but a few communicate with the peripheral 

 branches of the tubuli of the dentine. It is through this osseous 

 investment of the roots, that the teeth adhere so firmly to the 

 sockets in which they are implanted. 



The cavity of the teeth, containing the pulp, is in the fully formed 

 tooth the analogue of the Haversian canal of bone, by which the 

 organs of nutrition and sensation find access to the internal surface. 

 The blood-vessels of the pulp are branches of the internal maxillary, 

 and the nerves of the fifth pair, and they are both extremely abun- 

 dant, in proportion to the extent of surface with which they are in 

 relation. The capacious capillaries form numerous arches, and the 

 nerves likewise end in loops (vol. i. p. 221), which are best seen in 

 the young tooth. The white substance of the nerve fibres has 

 seemed to us to be often diminished or lost towards the convexity 

 of the loops. 



The alveoli, or sockets in which the teeth are set, are cavities in 

 the border of the jaws, corresponding in shape and direction to the 

 roots of the teeth, formed on the outer and the inner side by a firm, 

 compact plate of bone, which bounds the alveolar arch in front and 

 behind, and separated from one another by septa of less compact 

 material. The surface of these cavities is spongy, being perforated 

 by minute vessels, which pass across to the surface of the roots of 



