THE TEETH. 45 
becomes narrower by the continued calcification 
of the pulp, leaving only a central pulp cavity, 
into which blood vessels and nerves pass. A 
portion of the involuted epithelium becomes 
separated off from the original sac, and a new 
papilla rises at its side; from this the perma- 
nent tooth arises. 
Structure. Each tooth consists of three tissues, 
enamel, dentine and cement, covering a soft 
vasculated tissue, the pulp. 
Enamel forms the outermost part of each tooth. 
It is the hardest of all the tissues, containing 
from 95 to 97 per cent. of mineral substances 
(calcium phosphate and fluoride, calcium car- 
bonate, magnesium phosphate, &c.), and the 
smallest amount of organic matter. It consists 
of a series of slender prismatic fibres. 
Dentine constitutes the greater part of a tooth. 
Although not identical with, it is not unlike, 
bone. It consists of a matrix, largely impreg- 
nated with calcium phosphate, and permeated 
by a series of fine branched tubes—dentinal 
tubes—arising from the pulp cavity. In each 
is a soft uncalcified fibre, which is continuous 
with a cell on the surface of the pulp (Tomes). 
There are a number of varieties of dentine, 
known as Plici-dentine, Vaso-dentine, Osteo- 
dentine, &c. 
Cement. The cement forms a coating over the 
fangs of each tooth. It is very closely allied 
