STRUCTURE OF THE HOCK-JOIJJT. 243 



are, that the pullej-like heads of the astragalus and tibia fit 

 deeply into each other, and are strongly confined in theij 

 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, h, 

 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 difiused 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 oil}^ 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 



