THE ARM 26) 



olade. At tne uppe* part it has a large round head, received into the shallow cavity 

 of the shoulder-blade ; or, as Mr. Percivall has graphically described it, ' at is thi 

 segment af a globe, smooth and polished, evidently for the purpose of playing like a 

 spheiical 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 for- 

 ward and backward, but we can also, to a considerable extent, make it perform a sor* 

 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 protuberances 

 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 add- 

 ing 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 s, p. 259, the first proceeding from the upper part of this bone to the 

 elbow, and the 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. 



THE ARM. 



The arm extending from the elbow to the knee (see K and L, p. 68, and also cut, 

 p. 259), consists, in the young horse, of two distinct bones. The long and front 

 bone, called the radius, is nearly straight, receiving into its upper end the lower 

 heads of the humerus ; and the lower end corresponding with the upper layer of the 

 bones of the knee. The short and hinder bone is called the ulna. It has a very long 

 and powerful projection, received between the heads of the humerus, and called the 

 elbow ; it then stretches down, narrowing by degrees (see L, p. 68, and the cut, p. 

 259) to below the middle of the front bone, where it terminates in a point. The two 

 bones are united together by cartilage and ligament; but these are by degrees 

 absorbed and changed to bone, and before the horse becomes old the whole of the arm 

 consists of one bone only. 



It will be perceived that, from the slanting direction of the humerus, the weight of 

 the horse, and the violence of the concussion, will be shared between the radius and 

 the ulna, and therefore less liable to injure either. The circumstance, also, of so 

 much weight and jar being communicated to them, will account for the extensive and 

 peculiarly strong union between these bones in the young horse ; the speedy inflam- 

 mation of the uniting substance and absorption of it, and the substitution of bone, and 

 complete bony union between the radius and ulna, in the old horse. The immense 

 muscles that are attached to the point of the elbow likewise render it necessary that 

 the union between these bones should be very strong. 



The arm is a most important part of the horse, as will be seen when we describe 

 the muscles that belong to it. The muscles q, r, and s, proceeding from the shouldei- 

 blade and the humerus, and inserted into the elbow, have been already spoken of. 

 Fhey are the grand agents in extending the arm; and in proportion to the powei 

 which they exert, will be the quickness and the length of the stride. The strength 

 of the horse, so far as his fore-limbs are concerned, principally resides here. Thee 



* Veterinarian, vol. xv. p. 307. 



