ANATOMO-PATHOLOGIC STUDY OF RINGBONE AND SPAVIN. 
993 
which becomes ulcerated over irregular areas and desquamates, 
as is frequently seen in cases of osteoporosis, constituting an 
osteo-arthritis. This inflammatory zone in the bone now be¬ 
comes the seat of a condensing ostitis (osteosclerosis). The soft 
vascular, embryonic material now becomes organized, infiltrated 
with calcareous matter and converted into condensed or com¬ 
pact bone. The articular cartilage of the opposing bone at a 
point opposite to articular ulceration of the bone first affected 
likewise commences to ulcerate and leads to an osteo-arthritis 
at that point. The two articular ulcerations being contiguous 
will finally form adhesions and become coossified. This centre 
of ossific union may be localized, or in aggravated cases may in¬ 
volve the entire articular surface and form a complete ankylosis. 
There is first a rarefying, then a condensing ostitis and, finally, 
ossific union of the two bones at those points where the process 
is completed. The articular ulceration usually commences 
towards the margin of the articular surface. 
The histologic alterations are the absorption of the bony tis¬ 
sue and the bone cells, enlargement and saccular dilatations (la¬ 
cunae of Howship) of the Haversian canals which are filled with 
embryonic cells and congested blood vessels—a picture which is 
repeated in the generalized bony lesions of osteoporosis and 
called osteitism or o^titic diathesis, which so often in these cases 
predisposes to fracture. 
2 . Periarticular .—The exostosis develops upon the peri¬ 
phery of the bone. It is an osteo-periostitis. It commences as 
a rarefying and then passes into a condensing ostitis of the super¬ 
ficial or subperiosteal layers of the compact tissue of the bone, 
which is communicated to the periosteum and provokes a peri¬ 
ostitis. The result is a bone tumor of variable dimensions. The 
exostoses of the ends of two contiguous bones may become co¬ 
ossified, forming a peripheral or false ankylosis, while the articu¬ 
lar surfaces themselves may retain their normal state. 
The structure of an exostosis differs slightly from normal 
bone : It is more porous, the Haversian canals are larger, sac¬ 
cular and filled with embryonic tissue and blood vessels ; the 
