BONE. 



713 



tion of the cavity is gradually effected by an increased 

 formation of dentine ; and this is not supplanted by ar 

 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 foi 

 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." * 



Bone. The elements Of bone Fig- 345. A transverse section of the 



i -11 I -i -i human clavicle, or collar-bone, mag- 



are Iamelia3 and Small COr- nified 95 diameters ; which ex- 

 puscles j the latter are possibly hibits * h . e Haversian canals, the 

 * - ' ... J-IP concentric lammse, and the con- 



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. 



earthy salts, and leave the organic basis of the same 

 form as the bone itself. The lamellae 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. 



centric arrangement of bone-cella 

 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 tha 

 section. 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 



(1) Professor Simonds, on the ''Structure and Development of Teeth of 

 Animals." 



