CONNECTIVE TISSUES TEETH. 



75 



51. TKKTII. As soon as solid food is introduced into the mouth it is di- 

 vided and comminuted by the Teeth. Partly in accordance with the general 

 fact that epithelial growths die and are cast off to be replaced by others 

 formed beneath them, but chiefly to allow for that enlargement of the jaws 

 which occurs in the passage from infancy to adult age, two sets of these 

 organs are developed the first, temporary or milk teeth, which last up to 

 the seventh or eighth year, are 20, the second, or permanent set, are 32 in 

 number. Both are firmly imbedded in sockets or alveoli of the upper and 

 lower jaws. The front or incisive teeth (8 in number) are, as their name 

 implies, provided with a cutting edge for the prehension and division of 

 morsels of food of appropriate size for mastication. 

 The sharp-pointed canines (4 in number) pierce 

 and cut the firmer and tougher constituents of our 

 ordinary aliment, whilst the bicuspids (8) and the 

 molars (12) triturate and bruise the food, till, with 

 the aid of the saliva, it is reduced to the consistence 

 of pulp; it is then fitted for deglutition. Every 

 tooth consists of a crown, a neck, and one or more 

 fangs (Fig. 35), and in all teeth three structures 

 are found the dentine, which gives the general 

 form and size to the tooth ; the enamel, which caps 

 the crown ; and the crusta petrosa, which invests 



FIG. 35. 



the fang. On examining a thin longitudinal sec- 



Vertical section of Human 

 Molar Tooth: 1, enamel; 2, 7, 

 cementum or <:ru>tii petrosa; 3, 

 dentine or ivory ; 4, osseous ex- 

 cresci'iice, arising from hyper- 

 trophy of cementum ; 5, cavity, 

 subsequently filled with osteo- 

 dentiue; 6, osseous lacunae at 

 outer part of dentine. 



tion of a tooth, the dentine is found, like the shaft 

 of the long bones, to be hollowed out into a pulp- 

 cavity, containing a little connective tissue, with 

 bloodvessels and nerves. These enter by a'minute 

 orifice at the extremity of the fang. The dentine 

 which surrounds the pulp-cavity is composed of a 

 matrix, which is traversed by numerous tubuli that 

 radiate from the pulp-cavity towards the external 

 surface of the tooth, and which pursue a gently 

 undulating course, dividing dichotomously, and 

 sending off minute branches. In dry sections the 

 tubuli, having a diameter of about ^--yo^th of an 

 inch, are seen to open into the pulp-cavity by one 



extremity, and by the other to terminate either by exquisitely fine branches 

 or in similar cavities in the dentine, or overstepping the limits of this to 

 end in the enamel, or crusta petrosa. The tubuli are most closely set near 

 the crown of the tooth. In the living state they are occupied by prolonga- 

 tions of the protoplasmic substance which fills the pulp-cavity. These pro- 

 cesses are not in direct contact with the matrix, but are invested by sheaths 

 the dentinal sheaths of Neumann. The matrix intervening between the 

 tubuli is clear and homogeneous, and in softened specimens gives evidence 

 of being deposited in lamellae, concentric with the pulp-cavity (Sharpey). 

 Near the outer surface of the dentine, and also forming certain arched con- 

 tour lines at the crown of the tooth, is a peculiar modification of the matrix, 

 which appears to be broken up into spaces bounded by globular masses per- 

 forated with dentinal tubuli the so-called iuterglobular layer. The enamel 

 (Fig. 28) is composed of solid hexagonal prisms of about the ^ J^-th of an 

 inch in diameter, arranged vertically to the deutiual surface on which they 

 rest, and firmly adherent to one another. The course of these prisms is 

 generally wavy, and their surfaces, in section, are marked by transverse 

 strise. Enamel is the hardest of all the tissues of the body. The crusta 

 petrosa, or cementum, corresponds in all essential particulars with bone, pos- 



