TISSUES OF THE BODY. 265 



have to discuss this point at greater length in a future section, when con- 

 sidering the termination of the nerves of the pulp. 



In passing, it will be convenient to touch here on the nature of the 

 cement, or crusta petrosa, of the teeth. This commences at the 

 termination of the enamel as a thin layer clothing the root (figs. 250 

 and 254), increasing in thickness below until it attains its greatest thick- 

 ness at the point of the latter. It is, however, nothing but simple 

 osseous tissue (fig. 252, a), and, like this tissue, generally greatly 

 inferior in hardness to dentine and still more so to enamel. It is not 

 always sharply defined against the ivory of the tooth. Its ground- 

 substance is sometimes homogeneous and sometimes streaked : when very 

 thick it may also appear faintly laminated, but it rarely comes to the 

 formation of Haoersian canals. No bone-cells at all are found in the 

 cement around the neck of the tooth, and they only become numerous 

 towards the point of the root. Their size and shape, and the number of 

 their ramifications, which is often considerable, are more liable to varia- 

 tion than those of ordinary bony tissue. Some of these ramifications 

 are united with the canaliculi of the tooth which have penetrated as far 

 as the cement ; others form anastomoses with adjacent cells (fig. 252, 

 in the middle of a). 



These lacunse must not be confounded with clefts which are frequently 

 to be met with in the cement of old teeth in the form of irregular, branch- 

 ing interstices. 



152. 



Dentine, whose specific gravity is 2 '080 according to G. Krause, con- 

 tains, notwithstanding its hardness, several per cent, of water : some 

 analyses give 10 per cent. It consists, like bone, of a glutin-yielding 

 substratum, rendered hard by a considerable excess of calcium and also 

 magnesium salts. 



The organic substratum, determining the form of the structure, is 

 collagenic matter without any admixture of chondrin. An interesting 

 observation has been made in regard to the walls of the canaliculi, 

 namely, that though they may be isolated by means of the stronger 

 acids and alkalies, they remain for a time undissolved in a Papwts 

 digester, in which the ground-substance is transformed into glutin 

 (Hoppe). showing that these canals are not formed of glutin-yielding 

 matters. We have thus a similar condition of things as in the 

 lacunse of bone and their ramifications. The dentine globules also are 

 not convertible into glutin,' and their substance offers even a more 

 determined resistance to the action of acids than the other portions of 

 the tissue. 



The bony earths of dental tissue consist of a considerable proportion 

 of phosphate of calcium, with a smaller quantity of carbonate, and also 

 taking a more subordinate place fluoride of calcium and phosphate of 

 magnesium. , The carbonate of calcium appears to be subject here to more 

 variation in amount than in bone. Fluoride of calcium was originally 

 determined by Berzelius, and Bibra made the interesting discovery that 

 the dentine of many mammals is comparatively very rich in phosphate 

 of magnesium. 



Beside these, many other salts and mineral constituents are met with 

 in the teeth, and also a small proportion of fat. 



The bony earths, taken quantitatively, amount in human dentine from 



