332 THE HUMERUS, OR LOWER BONE OF THE SHOULDER. 



part of the arm ; it is also attached to the inferior part of the body of the 

 humerus, and to the fascia covering the arm, and reaching a considerable 

 way down the arm. The use of this muscle is obvious and important. It 

 binds the arm to the side of the horse ; it keeps the legs straight before the 

 horse when he is at speed, that the weight of the body may be received on them 

 in a direction most easy and safe to the horse and to the rider, and most advan- 

 tageous for the full play of all the muscles concerned in progression. Con 

 sidering the unevenness of surface over which a horse often passes, and the rapid 

 turnings which are sometimes necessary, these muscles have enough to do ; and 

 when the animal is pushed beyond his strength, and these muscles are 

 wearied, and the fore-legs spread out, and the horse is " all abroad," the con- 

 fused and unpleasant manner of going, and the sudden falling-off in speed, are 

 well known to every rider. Mr. Percivall very properly observes, that this 

 muscle has probably more to do in enabling the arm to support weight than to 

 give it motion. 



THE HUMERUS, OR LOWER BONE OF THE SHOULDER. 

 Forming a joint with the shoulder-blade at the point of the shoulder is the 

 humerus. It is a short strong bone, slanting backward in an opposite direction 

 to the shoulder-blade. At the upper part it has a large round head, received 

 into the shallow cavity of the shoulder-blade, or, as Mr. Percivall has graphically 

 described it, " it is the segment of a globe, smooth and polished, evidently for 

 the purpose of playing like a spherical hinge within the cup-like concavity 

 occupying the place of the apex of the scapula. There are no two bones in the 

 skeleton whose articular connexion is of a nature to admit more varied and 

 extensive motion than exists between the scapula and the humerus. If we 

 attempt to lift a horse's fore-leg, we cannot merely bring it forward and carry 

 it backward, but we can also to a considerable extent make it perform a sort of 

 rotatory motion, in consequence of the mobility existing in this joint between 

 the socket of the scapula and the head of the humerus*." It has several protu- 

 berances for the insertion of muscles, and is terminated below by two condyles 

 or heads, which in front receive the principal bone of the arm between them as 

 in a groove, thus adding to the security and strength of the joint, and limiting 

 the action of this joint and of the limb below to mere bending and extension, 

 without any side motion. Farther behind, these heads receive the elbow deep 

 between them, in order to give more extensive action to the arm. In a well- 

 formed horse this bone can scarcely be too short, in order that the fore-legs may 

 be as forward as possible, for reasons already stated, and because, when the lower 

 bone of the shoulder is long, the shoulder must be too upright. Dislocation 

 can scarcely occur in either of the attachments of the bone, and fracture of it 

 is almost impossible. The lower bone of the shoulder and the shoulder-blade 

 are by horsemen confounded together, and included under the appellation of 

 the shoulder, and in compliance with general usage we have described them as 

 combining to form the shoulder. 



Among the muscles arising from the humerus are two short and very strong 

 ones, seen at r and *, page 330, the first proceeding from the upper part of this 

 bone to the elbow, and tho second from the internal part, and likewise going to 

 the elbow, and both of them being powerful agents in extending the leg. 



In front, at y, is one of the muscles of the humerus, the external one employed 

 in bending the arm, arising from the inner and back part of the neck and body 

 of the humerus, turning obliquely round that bone, and inserted into the inner 

 and upper part of the bone of the arm. 



* Veterinarian, vol. xv. p. 307. 



