THE JOINTS. 35 



The next bone of importance belonging to the hock is the os 

 calcis, which corresponds to the human heeL In the horse it 

 does not bear any weight, but receives the insertions of some 

 important tendons, and thus acts as a considerable lever. 



The other bones of the hock are placed below the astragalus 

 in two rows, the lower of which rests on the metatarsus. These 

 comparatively small bones contribute very much in taking off 

 the jar, and are closely bound together by ligaments, by which all 

 motion is prevented. 



The bones below the hock being similar to those below the 

 knee, will need no further description. They are, however, 

 somewhat longer and slighter in their construction. — Ed. 



CHAP. VIL 



THE JOINTS. 



The chief peculiarity in the construction of joints in tlie horse 

 is that motion is almost entirely limited to one direction, and 

 every security is afforded against hiteral movement ; the result 

 of which is that we scarcely ever hear of dislocation in the 

 horse. Strength and speed are the chief objects required, and 

 accordingly we find that the joints admit of a great extent of 

 motion, but in one direction only. We have before observed 

 that the ends of bones forming a joint are tipped with cartilage, 

 which, being elastic, takes off the jar from the bones. This 

 cartilage is lined by a fine membrane, which secretes a iluid, 

 somewhat resembling the white of an egg, by which means, 

 however rapid or violent may be the motion of the animal, the 

 part is yet effectually secured from friction. This fluid is pre- 

 vented from escaping by a capsular ligament, which is strongly 

 attached to the neighbouring bones, and thus surrounds and en- 

 closes the joint, being however sufficiently loose as to admit the 

 requisite motion. The capsular ligament is also lined internally 

 by the synovial membrane, and is thus secured from injury. 



Many oi the joints of the horse are constructed on the prin- 

 ciple of the hinge motion, being admitted extensively at one 

 part and limited at another. 



The Shoulder joint is formed by the round head of the hu- 

 merus fitting into a socket in the lower part of the scapula. 

 This socket, however, is much shallower than in the human 

 subject, there being no lateral motion required. The joint has 

 no ligaments to protect it with the exception of the capsular ; 

 but this office is effectually performed by the tendons of the 



strong muscles of the shoulder. 



D 2 



