THE TEETH. 483 



last-mentioned lacunae, in fact, it is easy to demonstrate, by the aid of 

 acetic acid, a special wall, which is at first thick, but subsequently 

 becomes thinner ; and occasionally such lacunae, with walls which give 

 off a few processes externally, may be isolated. These lacunae are fre- 

 quently empty, but in other cases they contain a substance which at 

 first also resists hydrochloric acid, wherein, however, I could discover no 

 nucleus.* 



141. The soft parts of the teeth are : the periosteum of the alveolus, the 

 dental pulp, and the gum. The periosteum of the alveolus is very inti- 

 mately connected with the fangs of the tooth, and has the same structure 

 as any other periosteum, except that it is softer, contains no elastic ele- 

 ment, and possesses an abundant nervous network, containing many thick 

 fibres. 



The dental pufy the remains of the foetal dental papilla arises from 

 the periosteum at the bottom of the alveolus, enters the fangs, and, as a 

 continuous, soft, reddish, very vascular and nervous substance, fills their 

 canals and the pulp cavity in such a manner as to be everywhere in close 

 adherence to the inner surface of the dentine. The pulp consists of an 

 indistinctly fibrous connective tissue, totally destitute of the elastic ele- 

 ment, but containing many dispersed, round and elongated nuclei ; and 

 except that it occasionally contains narrow bundles, almost like imper- 

 fect foetal connective tissue. A fluid may be expressed from it which, 

 like mucus, is coagulated by acetic acid and is not dissolved in an excess ; 

 and, similarly, the entire pulp is rendered whitish by acetic acid, never 

 becoming transparent like perfect connective tissue. This substance 

 constitutes the principal mass of the pulp, so far as the vessels and 

 nerves extend ; but upon its surface we find, immediately beneath a deli- 

 cate structureless membrane, a layer of 0-02-0-04 of a line thick, com- 

 posed of many series of cells, 0-012 of a line long, 0-002-0-003 of a line 

 broad, cylindrical or pointed at one end, with long narrow nucleo- 

 lated nuclei of 0-005 of a line, arranged perpendicularly to the 

 surface of the pulp like a cylinder epithelium. More internally these 

 regular series are no longer recognizable, but the cells, without losing 

 their close radial arrangement, are more intermixed, and pass, finally, 



* [The structure of the cement on the fang of the still uncut molar of the calf, is very 

 peculiar and instructive. It is here a white, friable substance, about J ff of an inch thick, 

 bounded externally by a delicate Nasmyth's membrane. Its outer three-fourths are com- 

 posed of straight parallel fibres, resembling those of the enamel, but ^ of an inch long. The 

 inner fourth consists of similar fibres inextricably interlaced, cemented into a mass by a cal- 

 careous deposit, and containing here and there, spaces or lacunae, y^ 1 ^ of an inch in length, 

 as it were left among the fibres. This structure appears to become obliterated with age, as 

 the cement on the fang of the molar immediately in front of this, which had cut the gum, 

 had the ordinary appearance. (Huxley, 1. c.) TRS.] 



