152 



DISEASES OF THE BONES AND ARTICULATIONS. 



!FiG. 19. — Inferior extremity of tibia 

 in a healthy condition, a, Laminal layer. 

 h. Cancellated tissue. 



In the first form, the Haversian canals and the cancellated 

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



an ivoiy-lilvc 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 being 

 enlarged, with thinning of its 

 osseous layers, and the forma- 

 tion of communications between 

 its various interspaces. Whilst 

 this is going on, the outer sur- 

 face of the bone may become 

 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-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- 

 position going on upon 

 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. 



Fig. 20. — Interior extremity of tibia in a 

 state of ulceration, a, Thickened laminal layer. 

 h. Carious spot, c, Cancellated tissue opened 

 up. 



