STRUCTURE OF TEETH. 56 



constitute the cavitas pulpse, and contain a soft cellulo-vascular organ, 

 the pulp, which receives its supply of vessels and nerves through the 

 small opening at the apex of each root. Mr. Nasmyth, to whose inves- 

 tigations science is so much indebted for our present knowledge of the 

 intimate structure and development of the teeth, has observed with 

 regard to the pulp, that it is composed of two different tissues, vascular 

 and reticular, the former being an intricate web of minute vessels ter- 

 minating in simple capillary loops, the latter a network of nucleated 

 cells in which calcareous salts are gradually deposited, and which by 

 a systematic continuance of that process are gradually converted into 

 ivory. This process naturally takes place at the surface of the pulp, 

 and as the pulp is thus robbed of its cells, new cells are produced by 

 the capillary plexus to supply their place, and be in their turn simi- 

 larly transformed. 



A tooth is composed of three distinct structures, ivory or tooth-bone, 

 enamel, and a cortical substance or cementum. The ivory consists of 

 very minute, tapering, and branching fibres embedded in a dense 

 homogeneous, interfibrous substance. The fibres commence by their 

 larger ends at the walls of the cavitas pulpse and pursue a radiating 

 and serpentine course towards the periphery of the tooth, where they 

 terminate in ramifications of extreme minuteness. These fibres, 

 heretofore considered to be hollow tubuli, have been shewn by Mr. 

 Nasmyth to be rows of minute opake bodies, arranged in a linear 

 series (baccated fibres, Nasmyth), to be, in fact, the nuclei of the 

 ivory cells, the interfibrous substance being the rest of the cell filled 

 with calcareous matter. In the natural state of the tooth all trace of 

 the parietes or mode of connection of the cells is lost, but after steeping 

 in weak acid the cellular network is perfectly distinct. 



The enamel forms a crust over the whole exposed surface of the 

 crown of the tooth to the commencement of its Toot ; it is thickest 

 over the upper part of the crown, and becomes gradually thinner as it 

 approaches the neck. It is composed of minute hexagonal crystalline 

 fibres, resting by one extremity against the surface of the ivory, and 

 constituting by the other the free surface of the crown. The fibres 

 examined on the face of a longitudinal section have a waving arrange- 

 ment, and consist, like those of ivory, of cells connected by their sur- 

 faces and ends and filled with calcareous substance. When the latter 

 is removed by weak acid the enamel presents a delicate cellular net- 

 work of animal matter. 



The cortical substance, or cementum, (substantia ostoidea,) forms a 

 thin coating over the root of the tooth, from the termination of the 

 enamel to the opening in the apex of the fang. In structure it is 

 analogous to bone, and is characterized by the presence of numerous 

 calcigerous cells and tubuli. The cementum increases in thickness 

 with the advance of age, and gives rise to those exostosed appear- 

 ances occasionally seen in the teeth of very old persons, or in those 

 who have taken much mercury. In old age the cavitas pulpse is often 

 found filled up and obliterated by osseous substance analogous to the 



