152 



UISEASES OF THE fiONES AND jV ARTICULATIONS. 



In the "first form, the Haversian canals and the cancellated 

 spaces are blocked up by earthy matters, which give to the bone 



an ivory-like appearance when 

 ^v ^^^K sawn across ; at the same time 



\ ^ i.j^lLV the bone is heavier and denser. 



In the latter, or rarefaction of 

 bone, there is a diminution of 

 its density and weight, owing 

 to its tissues being expanded, 

 and to its canals and cells beincj 

 enlarged, with thinning of its 

 osseous layers, and the forma- 

 tion of communications between 



„ , . . . , ., . its various interspaces. Wliilst 



Fig. 19. — Inferior extremity of tibia ,, . . . , 



an a healthy condition, a, Laininal layer. thjS IS going on, the outer Sur- 

 J, Cancellated tissue. ^ idiCe of the bone may become 



thickened by a deposition' of new bone, or it may be removed 

 by absorptionj or both conditions may be co-existent upon the 



same surface of the bone 

 (see Fig. 20). Of this we 

 have other examples in 

 navicular disease, where 

 small nodules of bony mat- 

 ter are often found conti- 

 guous to a pit-like ulcer. 

 Again, it is generally found 

 that when this loss of sub- 

 stance goes on within the 

 bone, and upon its articular 

 surface, as in ostitis caused 

 by open joint, that there is 

 an active process of de- 



PlO. 20. — Interior extremity of tibia in a . . 



state of ulceration, a, Thickened laminallayer. position going OU UpOU 

 b. Carious spot, c, Cancellated tissue opened j^^ periosteal surface ; and 

 "P' this may be looked upon 



as a process of repair contemporary with that of destruction. 

 During the early stages of the process of rarefaction, the bony 

 texture is found softened, and has a porous appearance ; but 

 later on the effect is to render the whole substance o'. the In- 

 flamed part more brittle and liable to fracture, as in the cose of 

 "fractured spavin," fractured navioyilar bone, &o. 



C—-S 



