Williams, Fisher, and Udall: The Spavin Group. 25 
of strain or clinical proof of its frequent occurrence, we are com- 
pelled to conclude that strain bears but a secondary part in the 
etiology of this group of affections. 
Concussion. Concussion is so closely related to strain, it is 
the sudden impact of one body against another in a manner 
which tends to crush the weaker of the opposing bodies, rather 
than to divide it by fracture or rupture, that we might define it 
as the same force acting in a different direction. The effect of — 
concussion depends largely upon the resiliency of the surface of 
contact and the facility by which it may glide away from the 
concussing force. The articular ends of the long bones and the 
entire short bones are cancellated and elastic and the angular 
direction at which the bones of the extremities meet in a horse, — 
as already described, admit of their ready recoil in obedience to 
concussion. 
The disease does not most frequently attack those bones most 
subject to concussion, nor at the points where concussion is 
apparently the greatest. We observe the navicular bone swing- 
ing in the hammock-like tendon more frequently seriously at- 
tacked than the adjoining pedal bone, which certainly receives by 
far the greater concussion; but more interesting is the fact 
that the plantar surface of the navicular bone is universally the 
portion most obviously affected, whereas if concussion caused it, 
the chief pathologic changes should be found on the dorsal sur- 
face which, so far as we know, always escapes. In ringbone con- 
cussion plays no very evident part. In horses with exceedingly 
oblique pasterns, the disease is probably as common as in those 
with upright pasterns and certainly more recalcitrant to all at- 
tempts at cure, yet the obliquity of the pastern obviates concus- 
sion and supplants it with strain on the tendons. 
In spavin it is difficult to see how there can be greater con- 
cussion between the lesser bones of the tarsus and the metatarsus 
than between the astragalus and tibia, yet the former constantly 
suffers and the latter only very rarely. 
Some have suggested that the anchylosis of spavin is a pro- 
cess of evolution tending ultimately to efface the latter joints, but 
the tendency is seen only in diseased animals which, if the con- 
, tention were true, would tend to show that it is a pathologic 
evolution. 
Gonitis can ‘scarcely be regarded as the result of concussion 
as the parts usually affected are those most widely propped apart 
3 
