THE HORSE IN MOTION. 7 1 



limb, rendering tense the tissues connecting it with the trunk, and by 

 so doing extending the Hmb to enable it to take the ground farther in 

 advance, and leads us to infer how great importance was attached by 

 the Master Mechanic to utilizing every available means to enable the 

 fore foot to reach the ground as far in advance as possible, that no 

 time might be lost in giving support to the centre of gravity. 



We have thus far considered the anterior extremity as a passive 

 tool taken as a unit; it remains to study it as an active automatic 

 machine. It is difficult to trace any analogy between the mechan- 

 ism of the anterior and posterior extremities thus far; but in the 

 system of levers, by the closing of which the limb is shortened, and 

 in the opening of which it is lengthened, we recognize the same 

 mechanical combinations that are employed for the same purpose in 

 the posterior extremity. 



In Plate IV. the external view of the shoulder and arm is given 

 showine its relation to tiie trunk and that of the muscles to each other. 

 The pearly-colored upper border of the scapula, ;/, n, is seen with 

 the levator anguli scapulae still attached. This border, which is car- 

 tilaginous, is not seen in the prepared skeleton, but a rough margin 

 to the bone indicates its former connection. No muscle of the ante- 

 rior extremity, as an automatic machine, is attached to this cartilagi- 

 nous border. It has not sufficient firmness to resist force from below, 

 but its tenacity is sufficient to withstand great traction, and its flexi- 

 bility is such as to prevent any danger of fracture by force so applied. 

 The spine of the scapula may be traced from the cartilage downward, 

 near the middle of it, to which the trapezius and branch of the yellow 

 cord were attached. This spine divides unequally the scapula; in front 

 of it is the superspinatus muscle, s s, whose terminal tendons pass 

 over the head of the humerus, or shoulder, one to be inserted into the 

 external tubercle on the outside, and one third of the distance from 

 the point of the shoulder to the elbow. This is the most consider- 

 able division, and acts to extend the humerus on the scapula and 

 rotate it outward. The other tendon is inserted near the internal 

 tuberosity; it unites with its fellow in extending the humerus. Con- 

 sidering this muscle mechanically, it would be proper to regard its 



