Williams, Fisher, and Udall : The Spavin Group. 7 
- necrotic and fallen into the cavity, or the red marrow may 
proliferate, and a mass consisting of connective tissue, vessels, 
and lymphoid marrow cells completely occupy the cavity if the 
superficial layer of cartilage remains intact. The amount of 
necrotic tissue débris is unusually abundant in Figs. II and III 
at C. 
If the superficial cartilaginous layer has broken down com- 
pletely and the excavation has acquired an opening into the 
articular cavity, the granulation or marrow tissue may grow 
up into the joint cavity, and, meeting there with prolongations 
from corresponding erosions in the opposing bone, the two may 
fuse, producing fibrous anchylosis which constitutes in a measure 
a reparative process. 
The reparation of the affected parts is largely a reversal of 
the processes just outlined. The bone cells, lymphoid marrow 
cells or osteoblasts resume their normal properties, the osteo- 
clasts largely disappearing, new osseous lamella form about the 
Haversian canals, new lacune and canaliculi appear, and instead 
of rarefying ostitis, we observe an ostitis condensans, which 
generally more than replaces the prior bony lamella, producing 
an osteo-sclerosis as shown in Fig. IV, in which the former 
porotic bone has become exceedingly dense, almost like ivory. 
‘This represents the recovery of the osseous lesions in the most 
complete form we are able to recognize. 
If the marrow cavities which we have described push their 
way near to or upon the surface of the joint and ossification of 
these prolongations take place, they cause hard, rough, pointed 
elevations or calciffed points, readily felt in some, cases upon 
the joint surface, and capable of causing friction erosions on 
the opposing articular surfaces. 
Following the fibrous anchylosis, the prolongations tend to 
become ossified, producing an osseous anchylosis of varying 
degrees of completeness. Rarely the entire joint surface be- 
comes denuded of articular cartilage and the anchylosis may 
then become complete, but generally there are islets of articular 
cartilage of varying sizes, which leave the anchylosis incomplete, 
and according to incompleteness, insecure, exposing it to inter- 
ruption or fracture resulting in a renewal of pain and lameness. 
In other cases the prolongations of red marrow with their 
vessels and connective tissue fibers fail to fuse with those from 
the opposite joint surface, but push against them and into any 
depressions or cavities in the opposing surface, the prolongations 
