THE HORSE IN MOTION. 69 



forcing the body over it. As there is no loss of force in indirect 

 action, so there is none spent in adduction or abduction, or in sup- 

 porting weight ; that office is performed by the muscles of the limb 

 acting automatically, and the effect of its traction upon the shoulder 

 is to support it and prevent it from giving way while the limb is 

 playing its independent part in sustaining the superimposed weight 



of the body. 



There seems no room for a doubt that the conjoined action of the two 

 sets of muscles last described is the most powerful propelling force in 

 the whole locomotive organism of the horse. To make this apparatus 

 complete, there was necessary some force to return the limb to its posi- 

 tion forward when the act of propulsion was completed. This force is 

 found in two sets of muscles, the mastoido humeralis and the stiperficial 

 pectoral; the former has its fixed insertion at the mastoid process of the 

 temporal bone, or base of the skull, behind the ear, and to the first four 

 cervical vertebra.* It is shown in Plate 1 1 1., tn, m, m, passing downward 

 and backward along the whole length of the neck and over the point 

 of the shoulder, enveloping it, and is inserted at the humerus, about 

 half-way from its two extremities. (See Plate IV., /, where the muscle 

 has been cut away from its tendon of insertion. It is also severed at/, 

 leaving only its upper portion iti si/ic) It is six inches in width where 

 it envelopes the shoulder joint, and an inch in thickness, and gives 

 off, about thirteen inches above its insertion, a branch to be inserted 

 at the anterior border of the sternum, or breast-bone. This branch, 

 which could not be well shown in the drawing, is known to anatomists 

 as the cuticularis colli. There does not appear to be any occasion to 

 consider it a distinct muscle ; its fibres are interwoven with those of 

 the muscle under consideration; its function is to aid that muscle, 

 and fix it in its position over the shoulder joint. Though so thin, 

 the weight of the mastoido humeralis is not less than five pounds. 

 To give effect to this muscle, it is necessary that its base, the head, 

 should be fixed. This is effected by the complcxus, and its allies of 



* This relation is not well shown in the drawings, owing to displacement, caused by tlie 

 cord used in suspending the subject ; the artist drew the parts as he saw them, and the 

 inaccuracy was overlooked until too late to be corrected. 



