

THE HORSE IN MOTION. 31 



are limited. The muscles are subject to fatigue, and are unable to 

 respond indefinitely with equal force to the will. 



Muscular fibre has other properties to be considered in relation to 

 motion. Its contractility is limited to one fourth* or one third t of the 

 length of the fibre, and with a power proportioned to the area of the 

 transverse section of the muscle. It will be found that the relation of 

 length to thickness is as action to power. 



Deep-seated muscles are often attached to the bones upon which 

 they act directly ; but as there is insufficient space on the surface of the 

 bones for all that depend upon them, the extremities of the muscles are 

 often changed into tendon, a substance altogether different in its me- 

 chanical properties, being compact, very flexible, and incapable of elon- 

 gation, in order that it may not give away the contraction effected by 

 the muscular tissue. By means of this tendinous tissue the power of 

 the muscle is transmitted when necessary to a considerable distance, or 

 its direction may be changed by the tendon passing through a sheath 

 or groove, as a pulley, over an angle. In a humanly contrived machine 

 it has been found necessary, when the direction of the action of the 

 power requires to be changed, to use a friction roller or pulley ; but 

 nature has done better, and contrived a way to avoid friction and wear 

 that human ingenuity cannot hope to rival. By these means the power 

 generated in the heavy muscles is exerted at the extremities of the 

 limbs where all needless weight requires such great expenditure of 

 power to give it the needful velocity. The power which is conserved 

 in the body as momentum would be lost in the extremities, for the 

 motion of the limbs is arrested at every stride. $ The attachment 

 of these tendons to the bones and the periosteum enveloping them is 

 so great that detachment by natural means is not mentioned in 

 works on farriery as among the possible accidents to which the horse 

 is liable. 



* Bishop. 



t Bowman, Cyc. Anat. and Phys. 



J If a weight of 25 Ibs., sustained by the hand of an arm extended horizontally, requires the 

 expenditure of an energy equal to 2oq times that weight, or 5,225 Ibs., what amount of muscular 

 force is expended by the muscles of one of the extremities of a horse to move a 4-ounce shoe on 

 his foot when he is trotting at the rate of a mile in 2 min. 20 sec.? 



