THE TEETH DENTINE. 367 



cartilage, or of tbe white fibrous tissue. The ossification, so called, 

 of arteries, and the valves of the heart is not actually so, since no 

 true osseous tissue is formed. It is mere calcification, and consists 

 merely in a deposit of calcareous salts (mainly the carbonate and 

 phosphate of lime) among the histological elements of the part 

 affected. True ossification occurs in the permanent cartilages (of 

 the ribs and larynx, and, rarely, the epiglottis), in tendons, in the 

 dura mater and arachnoid (the latter is very doubtful) ; in the eye 

 ( Valentin) ; in the ovary; in fibrous membranes (the obturator mem- 

 brane); in enchondroma; in fibrous and carcinomatous growths, 

 and in the lungs. (Mohr.) On the other hand, no tissue is exempt 

 from a liability to calcification, except hair, nails, and epidermis. 



CHAPTEE VIII. 



SECTION I. 

 THE DENTAL TISSUES. 



THE solid portion of the teeth within which the soft portion, or 

 the pulp, is inclosed, consists of three distinct parts so far as the 

 structure is concerned. 



1. The Dentine, which constitutes by far the greatest part of the 

 whole solid portion. 



2. The Enamel, a peculiar structure which covers the body of the 

 tooth, or the part which is free and exposed to view. 



3. The Cementum, which is a layer of true bone, covering the 

 fang and the neck of the tooth, i. e. the part also covered by the 

 gum. Fig. 230 shows the relations of these three. The dentine is 

 sometimes not covered at the neck of the tooth by the cementum. 



I. THE DENTINE. 



Dentine is replaced by true bone in the teeth of some of the 

 lower animals; and its histological relationship to bone even in 

 human teeth, is shown by the fact that an Haversian rod is some- 

 times seen in sections of teeth. In other respects, however, the 

 analogy is not striking in man. A section of dentine presents 

 under the microscope but two elements. (Fig. 231.) 



