336 THE HORSE. 



and back part of the neck and body of the lower bone of the shoulder, 

 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. 63, 

 and also cut, p. 230), 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 lower bones of the shoulder ; 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 lower bone 

 of the shoulder, and called the elbow ; it then stretches down, narrowing 

 by degrees (see L, p. 63, and the cut, p. 230) 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 lower bone of 

 the shoulder, 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; and the circumstance 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, and the speedy 

 inflammation of the uniting substance and absorption of it, and substitution 

 of bone, and complete bony union between the radius and ulna in the old 

 horse. The immense muscles which 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 the most important part of the horse, as will be seen when 

 we describe the muscles which belong to it. We have spoken of those at 

 q, r, and s, proceeding from the shoulder-blade and the lower bone of the 

 shoulder, and inserted into the elbow. They are the grand agents in 

 extending the arm ; and in proportion to the power 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. Then we 

 look for a large and muscular arm, and we look likewise for such a for- 

 mation of the limb, and particularly of the elbow, as will enable these 

 muscles to act with most advantage. 



The principle of the lever, to which we have referred (p. 231), is here 

 beautifully applicable. The elbow-joint is the centre of motion ; the whole 

 of the lower part of the leg is the weight to be raised ; and the power by 

 which it is to be raised in one act of progression, the extending of the 

 limb, is the muscles inserted into the elbow. In proportion as the weight 

 is more distant than the power from the centre of mption, as it is in the 

 construction of this limb, so will be the greater degree of energy requisite 

 to be exerted. We will suppose that the weight, taking the knee to be the 

 centre of it, is eighteen inches from the elbow-joint, that the limb weighs 

 60 lbs., and that the elbow projects two inciies from the joint ; then an 

 energy equal to nine times the weight, or .540 lbs., will be needed to move 

 and extend the limb, because the weight is nine times farther from the 

 centre of motion than the power is. We will suppose that in another horse 

 the point of the elbow projects three inches from the joint, the weight of 

 the leg remaining the same. Three arc one-sixth of eighteen ; and only si.t 



