14 Williams, Fisher, and Udall: The Spavin Group. 
bone (Fig. XII), which may be due to mechanical manipulation of the 
tissue. The following changes were noted in the bone: the canaliculi of 
the lacune entirely atrophied in the region of the empty space (Z), the 
lacune roundish in form, the marrow cavities completely filled with cells 
and capillaries (Fig. XII, D1). The bone apparently in a condition of 
active hyperemia; obviously a case of osteo-porosis in the early stages. 
Case V.—A bay gelding draft horse twenty years old, bred and 
raised upon the University farm. The diet consisted of hay with five 
pounds of corn and oats mixed night and morning, and five pounds of 
oats at noon. The animal had always been vigorous and in excellent 
health. April seventeenth, nineteen hundred, he was presented to the 
clinic to be treated for lameness in the hind limbs, which had come on 
gradually, and at times was not noticeable. Diagnosed as gonitis. The 
following October the animal was returned to the clinic without improve- 
ment and owing to his age and severe lameness was left for disposal. 
Autopsy: — Pronounced changes were found on the proximal extremi- 
ties of both tibial bones; articular cartilage entirely absent from median 
side of left tibia, around this area the cartilage was very much thickened. 
Exostoses in the form of nodules were abundant around the articular sur- 
faces, some were firmly attached while others were held in place by 
fibrous tissue. The articular cartilage of the condyles of the femur 
presented a rough appearance; slight abrasions were found in some of 
the other joints. Histological examination, thin sections from the 
proximal extremity of the tibia presented bone and cartilage undergoing 
rapid and complete degeneration (Fig. XIII); the solid bone matrix: 
(trabecule) had lost nearly all of its characteristic structure; the lacune 
were.entirely absent over a great extent of the sections examined. Just 
beneath the abraded cartilage (Fig. XIII, A) the changes were very 
marked, only an occasional round space indicated the presence of a 
former lacune. The marrow cavities visibly enlarged and entirely filled 
with marrow cells, among which were many giant cells (Fig. XIII, D); 
‘Howschip’s lacune were abundant. 
Examination of this bone with a microscope that magnified fifty 
diameters revealed many places where the marrow cavities and trabecule 
had a very similar appearance; the cavities were so completely filled with 
cells and capillaries, and the bone had lost so many of its typical features 
that a higher magnification was necessary to determine the exact location 
of the boundary line. In certain of the marrow cavities reparative pro- 
cesses were beginning to take place. Marrow cells that had the appearance 
of osteoblasts in developing bone of young animals were arranged around 
the periphery of the cavity in contact with the bone trabecule. Blood 
vessels and connective tissue were present as in the reticular tissue of 
normal bone marrow, the amount of such connective tissue, was, however, 
abnormally small. The areas in which reparative processes were taking 
place were very small in comparison with the whole amount of diseased 
tissue; they were very suggestive, however, of the method by which 
such osteoporotic bone may become hard again, and finally pass to 
osteo-schlerosis. 
