STRUCTURE OF THE HOCK-JOINT. 
*243 
are. that the pulley-like heads of the astragalus and tibia fit 
deeply into each other, and are strongly confined in theL 
position by extremely powerful ligaments, which are so con¬ 
trived that they admit of the necessary freedom of the 
hinge-like motion of the joint, but completely prevent that 
lateral or side motion to which the joint is exposed in rapid 
movements, or passing over uneven surfaces. It will be 
seen, by inspecting the figure referred to, that the weight 
of the hind quarters is principally thrown upon the tibia, b, 
and that it rests almost entirely on the astragalus, a; but 
it will be observed that this weight does not press perpen¬ 
dicularly, but in an oblique direction, so that much of the 
concussion which would otherwise take place is avoided, by 
the springy action this slanting pressure produces among 
the several bones which compose the joint. This will be 
more readily understood by a reference to the skeleton, 
plate ii. fig. 21 As we have already shown, this joint con¬ 
sists of six bones, all of which are covered with elastic 
cartilage, and each admitting of a certain degree of motion, 
which diminishes concussion by the weight, pressure, and 
action being diffused among them all, and thus the concus¬ 
sion is neutralized and rendered harmless. Besides the car¬ 
tilaginous covering, each of these bones has a membranous 
covering, which secretes the synovia or oily fluid, which we 
have already mentioned at page 111 and elsewhere. In 
fact these bones may be considered as so many distinct 
joints, all separated from each other and protected from 
injury, yet united by different ligaments, binding them so 
firmly together as to prevent the possibility of dislocation, 
yet permitting sufficient motion for the important office 
they have.to fill. Beautiful and wonderfully powerful as 
this joint is, it is sometimes injured, too frequently, we 
lament to say, by the brutal cruelty of those to whose care 
