94 



STRUCTURE OF TEETH. 



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 terminating 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 pro- 

 cess 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 similarly transformed. 



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

 r ^ enamel, and a cortical substance or cementum. 



The ivory consists of very minute, tapering, and 

 branching fibres embedded in a dense homogene- 

 ous, interfibrous substance. The fibres Commence 

 by their larger ends at the walls of the cavitas 

 pulpaB and pursue a radiating and serpentine course 

 towards the periphery of the tooth, where they ter- 

 minate in ramifications of extreme minuteness. 

 These fibres, heretofore considered to be hollow 

 tubuli, have been shown by Mr. Nasmyth to be 

 rows of minute opaque bodies, arranged in a linear 

 series (baccated fibres, Nasmyth), to be, in fact, 

 the nuclei of the ivory cells, the interfibrous sub- 

 stance being the rest of the cell filled with calca- 

 reous matter. In the natural state of the tooth all 

 trace of the parietes or mode of connexion of th 

 cells is lost, but after steeping in weak acid the 

 cellular network is perfectly distinct. 



The enamel forms a crust over the wliole exposed surface of the crown 

 of the tooth to the commencement of its root ; 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 arrangement, and consist, like those 

 of ivory, of cells connected by their surfaces and ends and filled with cal- 

 careous substance. When the latter is removed by weak acid, the ena- 

 mel 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 appearances occasionally seen in the 

 teeth of very old persons, or in those who have taken much mercury. In 

 old age the cavitas pulpaB is often found filled up and obliterated by 



* Microscopic section of a molar tooth. 1. Enamel with its columns and latr.inated 

 atructure. 2. Cortical substance or cementum on the outside of the fang. 3. Ivory, 

 showing tubuli. 4. Foramen entering the dental cavity from the end of the fang. This 

 fang has a bulbous enlargement in consequence of a hypertrophy of the cementum. 

 5 Dental cavity. 6. A few osseous corpuscles in the ivory just under the enamel. 



