7 8 



THE HORSE IN MOTION. 



while the impulse is transmitted to the humerus at the angle of the 

 elbow, too rapid flexion at that joint being prevented by the force of 

 the triceps at the point of the elbow and the contraction of the great 

 pectoral, which, acting on the shoulder, prevents the sudden flexion of 

 that joint, at the same time that it forces the body over the limb ; in 

 which action it holds an analogous relation to the vastus of the pos- 

 terior extremity, the application of mechanical power being utterly dis- 

 similar, but the result in locomotion the same. The great dorsal aids in 

 this office, though less efficiently. The superspinatus, acting from the 

 scapula upon the upper end of the humerus over the shoulder joint, per- 

 forms the same function for that joint, preventing its flexion too rapidly. 

 The traction of the triceps upon the scapula is so great that it would 

 be torn away from its position but for the counter action of the trachelo 

 subscapularis, which transmits it to the cervical vertebra, as already ex- 

 plained. In this order the angles at the shoulder and elbow close while 

 the fetlock joint is bent until the pastern is horizontal with the ground. 

 In this action the limb is practically shortened, until from the position 

 of the hypothenuse it becomes the perpendicular of a right-angled 

 triangle, and during this change of position it has given uniform 

 support to the centre of gravity without deviation of the direct line of 

 its motion. During this time the levator anguli scapulas has been 

 contributing its force by acting on the short end of the lever, drawing 

 it forward and adjusting the axis of the limb to its changing require- 

 ments. The passing of the body over the limb in a position perpen- 

 dicular to the ground enables the limb in its character as an automaton 

 to exert a propelling force as well as a sustaining one. It is necessary, 

 however, that the support should be constant, as before; the angles must 

 open as gradually as they had closed, and the fetlock joint must be as 

 gradually straightened. Some changes take place in the action of the 

 forces. The superspinatus, that had been yielding to allow of flexion, 

 now contracts with greater force, and its labor is rendered easy by the 

 continued traction of the two great propellers acting from the thorax. 

 The branches of the triceps acting from the scapula relax altogether ; 

 that from the humerus by its continued contraction extends the fore- 

 arm upon the humerus. This order continues to the close of the first 



