72 THE HORSE IN MOTION. 



lower insertions as one, overlaying the joint beneath the mastoido 

 humeralis, and acting on the head of the humerus as a direct extensor 

 of the humerus ; its weight is two pounds, and its length seventeen 

 inches. 



The shoulder joint is constructed on the same principle as that 

 of the hip, but the head of the humerus is broader and less convex, 

 and the cavity of the opposing articular surface of the scapula too 

 small to lodge it; but it is supplemented by cartilage and ligaments, 

 and held still more strongly in its position by the powerful tendons 

 which envelop it. The head of the humerus is held in its place 

 by the further assistance of the atmospheric pressure equal to one 

 hundred pounds. Though freedom of motion is not so great as in the 

 corresponding articulation in man, it is much greater than that of 

 the hip joint. 



The two muscles that especially guard the joint and prevent later 

 displacement are : the infraspinatus (i s), which is attached to the ex- 

 ternal surface of the scapula, and nearly fills the space below its spine. 

 It is inserted into the head of the humerus, at <?, directly opposite the 

 shoulder joint; the other is the subscapularis ( Plate XL, /), having 

 its attachment on the inner surface of the scapula, and occupying the 

 whole face of the bone below the insertion of the serratus, s, and 

 is inserted into the inner side of the head of the humerus, directl) 

 opposite to the insertion of the infraspinatus. These two muscles 

 are of the same power, each weighing two and a half pounds, and of 

 the same length. Acting simultaneously, they neither flex nor extend 

 the humerus, the abduction of the one cancelling the adduction of 

 the other, but they are powerful braces to the joint. 



There is another pair of muscles, whose functions cannot be under- 

 stood unless considered together. If the reader will refer to any one 

 of the silhouettes of the trotting horse, and watch the action of the 

 fore leg from the time that the foot leaves the ground until it takes a 

 new position in advance, he will perceive that all the joints are flexed 

 rapidly before the foot passes the perpendicular. The flexion at the 

 shoulder is performed by these two muscles. One is called, by Chauveau, 

 the long adductor ; teres minor, by Percivall ; and the scapula hume- 



