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 

 serve all the purposes as a part 

 of the living organism." 1 



Bone. The elements of bone 

 are lamella and small cor- 

 puscles ; the latter are possibly 

 merely spaces between the for- 

 mer, in which is deposited the 

 earthy substance. The lamellae 

 have for their basis a cartila- 

 ginous substance combined 

 with earthy matter, or salts. 

 These salts are chemically com- 

 bined with the organic basis. Acids dissolve only the 

 earthy salts, and leave the organic basis of the same 

 form as the bone itself. The lamellop. 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. 



Fig. 345. A transverse section of the 

 human clavicle, or collar-bone, mag- 

 nified 95 diameters ; which ex- 

 hibits the Haversian canals, the 

 concentric laminae, and the con- 

 centric arrangement of bone-cell* 

 around them. Some of the Ha- 

 versian canals are white, others 

 black ; the latter are filled with 

 a deposit of opaque matter, used 

 in the grinding and polishing the 

 section. When viewed under a 

 lower power, they appear to be 

 only a series of small black dots, 

 as shown in fig. 346. 



(1) Profesaor Simonds, on the "Structure and Development of Teeth o* 



