738 THE BONES AND JOINTS. 



yellow zone, from 2 to 4 mm. in thickness, which forms a loose, readily 

 separated connection between the cartilage and the diaphysis. The 

 white, friable zone consists mainly of irregular rows of degenerated and 

 distorted cartilage cells lying in a calcified basement substance, of irreg- 

 ular masses of atypical bone tissue, and of blood-vessels surrounded by 

 variously shaped cells. The soft zone consists of more or less vascular 

 tissue with homogeneous basement susbtance, and round and spindle 

 shaped cells. This soft zone is not sharply outlined against the adjoin- 

 ing new-formed spongy bone, which, instead of consisting of the normal 

 marrow spaces with bony lamellae beween them, is largely composed of 

 granulation tissue. 



Different phases of this faulty development may be seen in different 

 bones in the same individual. According to Wegner the lesion is usually 

 most advanced in the lower end of the femur, then in the lower ends of 

 the leg bones and of the forearm, then in the upper ends of the tibia, 

 femur, and fibula. 



Not infrequently there is fatty degeneration of the marrow cells and 

 blood-vessels, giving the marrow a reddish-yellow color. These altera- 

 tions of the bones may occur, not only in children who have gummata in 

 other parts of the body, but also in those in whom other evidences of 

 syphilitic infection are absent. So uniform is their occurrence that their 

 presence alone suffices for the establishment of a diagnosis. 



NECROSIS. 



Necrosis is the death of a larger or smaller portion of bone. This 

 may be induced by conditions which deprive the bone of its proper vas- 

 cular supply from the periosteum and medulla. It may be associated 

 with suppurative periostitis, osteomyelitis, and osteitis, traumatic sepa- 

 ration of the periosteum, ulcers of neighboring soft parts, eniboli, the 

 action of phosphorus vapor, and exhaustive infectious diseases. Necrosis 

 is a pure form of gangrene, differing from gangrene of soft parts in that 

 the dead bone has at first, and may retain for a long time, the general 

 outward characters of the normal bone ; while in dead soft parts rapid ab- 

 sorption may occur, or should bacteria of various forms be present, or gain 

 access to the dead tissue, putrefaction, with complex changes, may ensue. 



When a portion of bone has died inflammation occurs at the dividing- 

 line between the dead and living bone. This inflammation has the char- 

 acters of a rarefying osteitis (see above), and finally separates the dead 

 from the living bone. The dead bone, or sequestrum, may remain smooth 

 and unaltered (Fig. 488), or it may be eroded by the influence of sur- 

 rounding pus or granulation tissue or osteoclasts. In this way it is pos- 

 sible for the sequestrum, if it be small, to be entirely absorbed. More 

 frequently there is a production of new bone around the sequestrum, 

 either beneath the periosteum or in the substance of the bone, and this 

 becomes lined with granulation tissue, from which pus may continue to 

 be formed, bathing the sequestrum. 



