Williams, Fisher, and Udall: The Spavin Group. 37 
navicular disease by so shoeing the animal as to relieve the 
painfully affected parts from compression. One of our most 
useful diagnostic agents in spavin, the spavin test, consists of 
exalted pain brought about by unusual compression. We prob- 
ably all recognize the evil influence of the low heel and long toe 
in determining navicular disease, and regard neglect in the 
changing of shoes and proper shortening of the toes as a highly 
important factor, and its significance. to us is found in the 
increased compression of the navicular bone by the vicious posi- 
tion of the foot. In the same way we regard unnatural dryness 
of the hoof as acting injuriously through the compression of 
the bony structures included within it. We meet with strong 
proof of the importance of pressure as a causative force, when, 
after netirotomy, the natural protection of the diseased parts 
through the agency of pain is destroyed, the consequent pres- 
sure resulting often in increased virulence of the rarefying osti- 
tis, leading to destruction of the integrity of the bones, ending 
in “breaking down.” Compression probably acts chiefly by 
disturbing the circulation and causing congestion, and accelerat- 
ing in this way the pathological changes occurring, especially 
necrosis and atrophy of the tissues. 
Strain appears to play an important part in localization. We- 
have noted, in several cases, especially in old-standing, quiescent 
spavins, a sudden eruption of lameness dating from becoming 
cast in the stall or from overexertion in drawing a heavy load. 
Perhaps this was due to a fracture or interruption between two 
more or less anchylosed bones. In other instances the exostoses 
have not been observed until shortly after a severe strain, but 
these cases with a definite history of strain constitute the excep- 
tion and not the rule. Probably the great influence of strain 
in localization is due to overtension of a degree which would 
be counted normal under proper conditions, and acquire impor- 
tance only because the tissues are weakened by disease. We 
‘might say it is the result of an attempt of a part to perform 
its normal function when unfitted by existing disease. 
The same may be said of concussion. In some cases we 
meet with concussion fractures, inducing sudden and extreme 
lameness as in fracture of the navicular bone where it has been 
weakened, as shown in Fig. I, or splitting of the first or second 
phalanx, injuries which may occur as the result of extreme 
violence, but most generally seen in animals suffering from 
some osseous disease. 
