333 



THE ARM. 



The arm extending from the elhow to the knee (see K and L, p. 108 and also 

 cut, p. 330), consists, in the young horse, of two distinct hones. 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. 108, and the cut, p. 330) 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 circum- 

 stance, 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 inflammation of the uniting substance and absorp- 

 tion of it, and the substitution of hone, 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 de- 

 scribe the muscles that belong to it. The muscles q, r, and *, proceeding from 

 the shoulder-blade and the humerus, and inserted into the elbow, have been 

 already spoken of. 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 there will naturally be a large and muscular 

 arm, and such a formation of the limb, and particularly of the elbow, as will 

 enable these muscles to act with most advantage. 



The principle of the lever (referred to at p. 328) is here beautifully applic- 

 able. 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 motion, as it is in the construction of this limb, so will be the 

 greater degree of energy requisite to be exerted. Supposing that the weight, 

 taking the knee to be the centre of it, is eighteen inches from the elbow-joint — 

 that the limb weighs 60lbs., and that the elbow projects two inches 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. If in another horse the point of the elbow 

 projects three inches from the joint, the weight of the leg remaining the same, 

 only six times the force, or 360 lbs., will be required, making a difference in, 

 or saving of, muscular action, equal to 180 lbs. in each extension of the arm 

 If a few pounds in the weight of the rider tell so much for or against the 

 horse in a long race, this saving of power must make an almost incalculable dif- 

 ference ; and, therefore, judges of the horse rightly attach so much importance to 

 the depth of the elbow, or the projection of the point of the elbow from the joint. 



When describing the proper obliquity of the shoulder, it was stated that the 

 power was exerted with most advantage in a line perpendicular to the arm of 

 the lever, and that the slightest deviation from that line was manifestly disad- 

 vantageous. If the reader will examine the cut, he will perceive that muscles 



