THE TONGUE. 



S2i 



on the inner surface of the dentine, so that the central cavity of a tooth becomes 

 gradually diminished in size, whUst the pulp slowly shrinks or disappears. This 

 additional substance, formerly regarded as an exten- 

 sion of the cement into the interior of the tooth, — 

 has been shown to have a distinct structure, in part 

 resembling dentine, and in part bone. It is traversed 

 by canals, which contain blood-vessels, and are sur- 

 rounded by concentric lamellae like the Haversian 

 canals of bone. From these canals, numerous tubules 

 radiate, larger than the canaliculi of bone, resem- 

 bling, in this respect, and also in their mode of rami- 

 fication, the tubes of the dentine. This new growth 

 may indeed be compared to a collection of miniature 

 pulps, each siuTOunded by its appertaining dentine, 

 IDierced by radiating tubules. It may or may not 

 coalesce with the previously f onned dentine ; it appeal's 

 to be produced by a slow conversion of the dental pulp. 



2. Dentine of Repair (Salter). — When the outer 

 siu-face of the dentine becomes denuded at any place, 

 so that the peripheral ends of the tubules are there 

 exposed, as may ha^jpen in the crown from injiuy or 

 wear of the enamel, or at the cervix from continued 

 friction and abrasion of the cement, a deposition 

 of dental matter occurs on the inner surface of the 

 dentine exactly corresponding in position and extent 

 with the area occupied by the central ends of the 

 exposed tubules. Many of the affected tubules become 

 subsequently filled up by a deposit of hard matter 

 A\'ithin them, so that on section both the secondary 

 dentine and the corresponding part of the primary 

 dentine appear clearer and more transparent than 

 the remainder of the dentinal substance (see fig. 22.5). 



AVhen the surface-injmy has been considerable, the 

 dentine of repair is largely in excess, and may in such 

 cases completely fill up the pulp-cavity. 



THE TONGUE. 



Fig. 225. — Longitudinal 

 Section of Incisor 

 Tooth showing den- 

 tine OP REPAIR. 

 Slightly Magnified. 

 (Reduced from Salter. ) 



cl, d', denuded surfaces 

 of dentine ; r, r', corre- 

 sponding deposits of se- 

 condary dentine. Two 

 or three incremental lines 

 are observed in the den- 

 tine. 



The tongue is a muscular organ covered with 

 mucous membrane. Bj its muscular structure 

 it takes part in the processes of mastication and 

 deglutition, and in the articulation of speech, 

 while its mucous membrane is endowed with 

 common and tactile sensibility and is the seat of 



the sense of taste. The tongue occupies the concavity of the arch of 

 the lower jaw ; posteriorly it is connected with the hyoid bone, and the 

 back part of its upper surface forms the floor of the arch of the fauces ; 

 inferiorly it receives from base to apex the fibres of the genio-glossus 

 muscle, and through the medium of that muscle is attached to the 

 lower jaw. 



A.— Mucous Membrane.— On the under surface of the tongue the 

 mucous membrane is smooth and thin. It forms a fold in the middle 

 line called the frccnum Unguce, placed in fi-ont of the anterior border of 

 the genio-glossi muscles. On each side below, as the mucous membrane 

 passes from the tongue to the inner surface of the gums, it is reflected 

 over the sublingual gland. Not ftir from the line continued forwards 

 from the frajnum, the ranine vein may be distinctly seen through the 

 mucous membrane, and close to it lies the ranine artery. Further out 



