THE HORSE IN MOTION. 67 



the pair act unitedly, and to bend the neck laterally when each 

 muscle acts separately. 



We have shown how the anterior extremity is used as a supporter 

 to the trunk, or crutch, and how it is itself supported in its position 

 when not so acting. The mechanical principles involved are very 

 simple. The method in which mechanical power is applied to the 

 same limb as a lever in locomotion will be found to be no less so, and 

 if the contrivance does not display as great ingenuity as some parts 

 of the locomotive organs, it is because there was no occasion for such 

 display : it has the merit, at least, of being very primitive. 



While there is no bony connection between the anterior extremity 

 of the horse and its trunk, therefore no fixed point of resistance 

 and reaction, as in the posterior extremities, the centre of motion is 

 attained equally well, and it is difficult to conceive how it could serve 

 its different relations to the trunk any better. The centre of motion 

 in the anterior extremity is in the scapula, as high as a bony base could 

 be reached. This, if not anatomically so, is mechanically a joint, and 

 corresponds to the hip joint of the posterior extremity, the shoulder to 

 the stifle, and the elbow to the hock. In this view, there is no reversed 

 order in the joints, as has been stated, but the same mechanical relation. 

 The freedom of motion at its centre in the limb is less than in the 

 corresponding joint in the posterior extremity, but there is all that 

 is required ; it is placed considerably higher than in the latter, in 

 order that more motion should not be required ; and the restriction 

 at that point is compensated for by the superior flexibility of the lower 

 joints. The total result is that the stride of one limb is just equal to 

 that of the other. 



The limb, acting as a lever of the third order, having its centre of 

 motion as high as possible, should have the power to move it applied as 

 low down as possible, within the periphery of the body ; but the farther 

 from the fulcrum, or centre of motion, the power is applied, the greater 

 will be the space moved over, and, consequently, the longer must be 

 the fibres of the muscle.* This requisite is furnished by the great 

 dorsal (Plate III., d, d), which has for its base the spines of the last 



* See page 31. 



