BONE. 



573 



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- 

 jectly-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." l 

 Bone. 



The elements of bone 



Fig. 315. A transverse section of the 



are lamellae and small cor- 

 puscles j the latter are possibly SSnaVersian canals, the concentric 

 merely Spaces between the for- lami nse, and the concentric arrange- 

 . i i j , i ,1 ment of bone-cells around them. 

 mer, in WniCh IS deposited the Some of the Haversian canals are 



earthy substance. The lamellae ^ it - e ' ( ? t - hers - black ; the latter are 

 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 lamellaa are homogeneous 

 throughout, like the intercellular substance of cartilage, 

 but chemically it is different, being resolved by boiling 

 in water into colla, whereas cartilage is resolved into 

 chondrine. 



uman clavicle, or collar-bone, mag- 

 nified 95 diameters ; which exhibits 



filled with a deposit of opaque mat- 

 ter, used in the grindingand polish- 

 ing the section. When viewed 

 under a lower power, they appear 

 to be only a series of small black 

 dots, as shown in fig. 316. 



Acids dissolve only the 



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

 AHimals." 



