REVIEW—RIGOT’S ARTICULATIONS OF THE HORSE. 491 
action, when we compare them with the inferior surface of a 
diseased navicular bone. In the one we have an alteration of 
structure, and sometimes a loss of substance, which is never to 
be met with, except in cases where it has been preceded by such 
a considerable degree of inflammation that lameness of the most 
painful nature is the almost invariable consequence; while in the 
other, we have simply a removal of matter, evidently for the pur¬ 
pose of forming a reservoir, by the existence of which a large 
supply of lubricating fluid is continually kept in the vicinity of 
those parts of the articulatory surfaces most liable to be afl’ected 
by friction. They are useful in another way,—they allow a free 
passage of the synovia from the anterior to the posterior parts 
of the articular cavity: this is of great consequence when there, 
is an accumulation of this fluid. 
The synovial cavity formed by the articulation of the tibia with 
the astragalus, may be divided into tw'o parts; one anterior to the 
tibia, the other posterior to it. Their capacities are not at all 
times the same. They are altered by the flexion and extension 
of the limb. When the cavity in front is rendered smaller by a 
motion of the joint, the one posterior is at the same moment ren¬ 
dered larger, and by exactly the same movement. Therefore, 
such a communication between the two parts of the articular ca¬ 
vity as will admit of a ready passage of the fluid contained there¬ 
in, is, in cases of hydrops articuli, a desideratum of no little 
importance. Even in the hock-joint not affected with this dis¬ 
ease, were it not for the little space between the middle articula¬ 
tory ridge of the tibia and the grove between the condyles of 
the astragalus, along with some other communications betu'een 
the anterior and posterior parts of the articular cavity, the pres¬ 
sure of the fluid contained in one of them, when its capacity is 
being diminished by either the flexion or extension of the joint, 
would distend or even rupture the synovial membrane, and thus 
produce bog-spavin, or thorough-pin, or, perhaps, both. 
From the fact of these excavations being more frequently 
found, and to a greater extent, in the old than in the young 
animal, it would appear that they are constructed for the purpose 
of acting as synovial reservoirs, in order to assist in counteracting 
the bad efl'ects likely to result from long-continued and inordinate 
exertion. 
Let us, but for a moment, reflect on the great degree of pres¬ 
sure, and the intensity of the friction resulting therefrom, wliich 
must exist between the tibia and astragalus of the hunter carry¬ 
ing a welter weight across a close and heavy country, or the gi¬ 
gantic dray-horse drawing a load of a couple ol tons over the 
rough stones, and up the steep hills of oui capital. As a matter 
