54 THE HORSE IN MOTION. 



of cellular or fibrous tissue than others, and will have less power, other 

 things being equal. For example, the gluteus and vastus are coarse 

 muscles capable of resisting external force, and therefore popularly 

 believed to be strong; but it is in a meaning corresponding to tough- 

 ness, and that quality depends upon the amount of interstitial cellular 

 tissue they contain, which tissue has no contractile property, and can- 

 not originate motion; while the psoas and iliacus, having but little 

 such cellular or fibrous tissue, have little power to resist external force, 

 but have a larger contractile power as measured by the areas of their 

 sections. 



Muscles do not often have their force concentrated at both extremi- 

 ties, but it is distributed over the face of their levers at different dis- 

 tances and at different angles, as in penniform muscles, and nearly all 

 others in a greater or less degree, and at different angles at each change 

 in the position of the levers. Though we recognize the same general 

 mechanical principles, we cannot apply the same mathematical rules 

 usual in mechanics ; add to these elements of uncertainty the com- 

 position of forces often in the same muscle, and we see how for- 

 midable are the difficulties in the way of reducing animal mechanics 

 to an exact science. 



But while we cannot accurately determine the forces in detail, we 

 can in the aggregate. We see all these different and often antago- 

 nistic forces united in their action around a common centre of motion, 

 as the hip joint, to effect one result. There are certain general princi- 

 ples, however, that we can deduce from the facts before us. In order 

 that the foot shall reach the ground as far in advance as possible, to 

 support the centre of gravity as early as may be, and as long as pos- 

 sible, and that it may use its propulsive force later, it is necessary 

 that it should be possessed of sufficient length ; but it is bearing a 

 burden whose weight we will suppose to be a thousand pounds, and 

 going at the rate of twenty miles an hour, and the momentum is the 

 product of that weight multiplied by the velocity. This is a respon- 

 sibility that could not be borne on stilts. The difficulty is overcome 

 by so constructing the whole limb that it shall be extensible, thus 

 having all the advantage of length without its disadvantage; and the 



