THK IloRSi: IN MOTION. 55 



centre of motion is actually lowered several inches that its practical 

 length may be increased. For this purpose the system of levers is 

 iiM'd, which, by their flexion and extension, practically shorten and 

 lengthen the limb. The acutcness of the angles at which these bones 

 intersect each other is, therefore, an important element in the mechan- 

 ical action ; the angles to be acute require long levers, and long levers 

 -Mtate long and powerful muscles to "man" them. These quali- 

 ties must be bred. 



Flexibility of articular ligaments may be acquired by early training 

 and regular exercise, but the proportions of the body are inherited. 

 Length of muscular fibres and acute angles of the levers on which they 

 act, give sweep of limb, and strength depends upon the number of them, 

 and the effective power of both depends upon the will or courage ; but 

 all these qualities would be vain if the motion of the extremities were 

 not so co-ordinated that their functions should be performed without 

 interference one with another. 



When the speed of the horse is twenty-five miles an hour the rate 

 of the hind foot in passing that on the ground is twice that, or fifty 

 miles an hour. It is even greater than that, for the velocity of the foot 

 in its stride is an accelerated one during most of the distance, and may 

 be supposed to be most rapid midway. Now the movements of the 

 posterior extremity on its centre are controlled by voluntary muscles, 

 liable from various causes to be irregular, as they must necessarily be 

 from the ever-changing centre of gravity which it is designed to sup- 

 port. There would have been danger of one foot striking the other leg 

 in passing, an accident technically called interference, but another 

 danger still greater existed at the stifle from the blows that joint would 

 be liable to give the abdomen in its extreme and violent flexions. It 

 is the duty of the iliacus muscle to guard the abdomen from this vio- 

 lence, and when it performs its office well, it gives the " stifle action " 

 so much admired ; but while the upper end of the leg (tibia) is thrown 

 out in this action, the lower end is correspondingly thrown in, and the 

 foot would be still more so but for the unique construction of the hock 

 joint. .The interlocking grooves of this joint are not direct, as in other 

 hinge joints of the body, and as the corresponding joint in man is, but 



