CHAP. II.] 



THE CATS GENERAL FORM. 



31 



§ 19. The SUBSTANCE of cacli tooth consists of a dense tissue of three 

 kinds, called (1) Dentine, (2) Enamel, and (3) Cement, investing a 

 small soft and sensitive mass called the pulp. The great hody of 

 each tooth is formed of dentine, and it is this which immediately 

 surrounds the pulp. The cement coats the fang of each tooth only, 

 while its crown is invested with a covering of enamel, which is the 

 hardest kind of tooth substance. 



The pulp consists of areolar tissue with cells and nuclei, and is in 

 fact a modified portion of the corium — a large dermal papilla. It is 

 highly vascular, and supplied with a nerve also. 



Dentine is an animal substance impregnated with 72 per cent, of 

 earthy matter, of which nearly 67 per cent, is phosphate of lime. 



V\s. 14.— Tooth Substances. 



A. Vertical section of secoiul upper premolar. 



B. Horizontal section of riglit upper sectorial. 



d. Dentine, c. Cement, e. Enamel. 

 IK. Pulp cavityi 



Instead, however, of presenting the lacunae and canaliculi of ordinary 

 osseous tissue, dentine only exhibits a number (but an enormous 

 number) of very minute and very close-set tubes, which radiate 

 fi'om the wall of the pulp cavity on every side and with slight 

 undulations ; they become smaller towards the outer part of each 

 tooth, while at their inner ends their diameter is about the t-Vo of 

 an inch. Each tube, as it proceeds, gives off exceedingly minute 

 branches, which appear to anastomose, and the tubules end distally 

 by forming loops or by opening into minute cavities (dentinal cells) 

 which are disposed around the dentine close to its surface, forming 

 what is called its granular layer. 



The greater part of the earthy matter is contained in the matrix, 

 between the tubules, which do not in fact proceed from the j)ulp, 

 but advance upon it, the outermost layer being that first calcified. 



The Cement closely resembles bone, since it contains both lacunae 



