152 



DISEASES OF THE BONES AND 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 

 sawn across ; at the same time 

 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 beini^ 

 enlarged, Avith thinning of its 

 osseous layers, and the forma- 

 tion of communications between 

 ^ ,nTr. . ., ^xM . its various interspaces. Whilst 



r IG. 19. — inferior extremity of tibia , . . . 



in a healthy condition, a, Laminai layer. thlS IS gOing On, the OUtcr SUr- 

 h, CanceUated tissue. f^^.^ q£ ^^^^ j^^^^g ^^y. \)qqqj^q 



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

 by absorption ; 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-hke ulcer. 

 Again, it is generally found 

 that when this loss of sub- 

 stance goes on w^ithin the 

 bone, and upon its articular 

 surface, as in ostitis caused 

 by open joint, that there is 

 an active process of de- 



FlG. 20. — Interior extremity of tibia in a 

 state of ulceration, a, Thickened laminai layer. pOSltlOU gomg On upOU 

 h, Carious spot, c, Cancellated tissue opened its periosteal SUrface ; and 



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 of the in- 

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

 " fractured spavin," fractured navicular bone, &c. 



