6 Williams, Fisher, and Udall: The Spavin Group. 
manifestation. With such a view, it follows that no articular 
tissue occupies a first place in the history, but any one of the 
tissues concerned may be the first to become visibly affected. 
Bone AND ARTICULAR CaRTILAGE. The short bones, composed 
of cancellated tissue and having an abundance of red marrow, 
take a prominent, if not the chief part in the known pathologic 
changes, and, according to many recent investigators, is the 
tissue in which the first visible lesions of the disease appear. 
These lesions are first discoverable chiefly in the subchondral 
layer, and may be seen by the naked eye in -early stages as 
variable sized, dark reddish-blue spots through the compara- 
tively transparent articulat cartilage, as is shown at I, 2, 4; 5, 
6, 7 in Fig. I and at B in Figs. II and JIT. These areas are 
irregularly circular in form and tend to occur simultaneously at 
corresponding points on the ends of the two bones concurring 
in the articulation. 
‘Histologically examined, the Haversian canals widen greatly, 
assuming sac-- or ampulla-like enlargements designated How- 
echip’s lacunee. The bone cells decrease in size, their prolonga- 
tions disappear and their outline becomes more regular. The 
canaliculi vanish in the severely affected parts. The bone matrix 
liquefies, forming cavities of considerable size, and, by exten- 
sion toward the articular cartilage, finally reaches it, and the 
granulation tissue interrupting the nutrition to the superposed 
cartilage, it, too, becomes involved. The disease of the cartilage 
generally begins in the deepest layers, bordering the subchon- 
dral excavations in the osseous tissue, the superficial cartilagi- 
nous layers remaining for a time intact. The disease processes 
within the cartilage are analogous to those taking place in the 
bone. The cartilage cells multiply, enlarge, lose their normal 
character, and assume the appearance of giant cells or osteo- 
clasts. The chondral matrix softens, becomes fibrillated and 
destroyed. When the superficial layer of cartilage is finally 
dissolved or thoroughly undermined by the erosive process tak- 
ing place in the subchondral bone; it breaks and falls into the 
excavation beneath, forming deep pit-like cavities, opening upon 
the articular surface by a mouth usually much smaller in diam- 
eter than the excavation beneath, producing the macroscopic 
‘appearances shown in Figs. II IV. 
These pits appear to be always the seat of active inflam- 
mation, are occupied in part by tissue débris which has become 
