BONE. 



713 



tion of the cavity is gradually effected by an increased 

 formation of dentine ; and this is not supplanted by an 

 abnormal or diseased growth, as would be the case were 

 the pulp to become ossified, but as the pulp diminishes, 

 so is the supply of nutriment to the tooth lessened, and at 

 length entirely cut off from the interior. "To provide for 

 the vitality of the tooth under 

 these circumstances, the crusta 

 increases in quantity on the 

 fang, at the expense of the per- 

 fectly-formed dentine, which is 

 lying in immediate contact with 

 its inner surface. Through the 

 medium of the canals in the 

 crusta, which open on its 

 borders, the tooth now draws 

 its nourishment from the blood- 

 vessels of the socket ; and thus 

 it continues, long after the obli- 

 teration of its pulp cavity, to ii^y 

 serve all the purposes as a part f^ 

 of the living organism." ^ '■ -'y^^' \ /i^^^:'':k:^::b^!.j:^--:^'- 



Bone. The elements of bone rig- 'Ho.—A transverse section of the 



liuman da vide, or collar-hone, mag- 

 nitied 95 diameters ; which ex- 

 liibits the Haversian canals, the 

 concentric laininie, and the con- 

 centric arrangement of bone-cells 

 around them. Some of the Ha- 

 versian canals are white, others 

 black ; the latter are tilled with 

 a deposit of opaque matter, used 

 in tlie grinding and polishing the 

 secition. When viewed under a 

 lower power, they appear to be 

 only a series of small black dots, 

 as shown in fig. 346. 



Acids dissolve only the 



are lamellae and small cor- 

 puscles ; the latter are possibly 

 merely spaces between the for- 

 mer, in which is deposited the 

 earthy substance. The lamellaa 

 have for their basis a cartila- 

 ginous substance combined 

 with earthy matter, or salts. 

 These salts are chemically com- 

 bined w^ith the organic basis. 



earthy salts, and leave the organic basis of the same 

 form as the bone itself. The lamelhp. are homoge- 

 neous throughout, like the intercellular substance of car- 

 tilage, but chemically different, being resolved by boiling 

 in water into colla, whereas cartilage is resolved into 

 chondrine. 



(1) Professor Simouds, on the 

 .\nimals." 



Structure and Devcloonieut of Teeth of 



