452 



MOTION. 



those of the posterior; this tends to throw 

 the centre of gravity a little in advance of the 

 middle of the quadrilateral figure described by 

 the four legs of the quadruped on the plane 

 of position. 



Although the bones of the posterior extre- 

 mities are inclined to each other at a greater 

 obliquity than those of the anterior, they have 

 the calcaneum projecting considerably beyond 

 the axis of the calcaneo-tibial articulation, so 

 that, acting with the advantage of a powerful le- 

 ver, the wasteof muscular force is diminished, and 

 the disadvantage arising from their obliquity is 

 partially compensated. The time and order in 

 which the legs of quadrupeds succeed each 

 other in motion determine the paces deno- 

 minated the walk, the trot, the gallop, the 

 amble, and the leap, or bound. 



In order to illustrate many of the general 

 principles on which the progression of qua- 

 drupeds is effected, we shall select the horse as 

 an example. In standing, the animal rests its 

 trunk on the legs which form the four columns 

 of support. Let us suppose the legs to be 

 placed on the ground at A B E D (fig. 245) : 

 if these points be joined, they form a rectangular 

 parallelogram. 



When the animal walks slowly with his 

 right side in advance, the left hind leg moves 

 first ; the right fore leg second ; the right hind 

 leg third ; and the left fore leg fourth. During 

 these four successive motions, the centre of 

 gravity is propelled forwards over a space equal 

 to the length of one step. Let us now investi- 

 gate what takes place during these successive 

 motions. The hind foot having been pre- 

 viously extended, and having urged the centre 

 of gravity forward, is first moved ; it is then 

 flexed, lifted from the ground, and advanced 

 from E to e. Whilst the leg E is in the act of 

 advancing to e, the trunk is supported on 

 three legs, A B D, thus having the base 

 of support transferred from the plane of a rec- 

 tangular parallellogram to that of a right-angled 

 triangle. 



In this movement, the centre of gravity G 

 must fall within the plane A B D, which is 

 effected by an oblique lateral movement of the 

 trunk towards B D. The foot E having taken 

 a new position at e, the second foot B is set 

 in motion, raised, and advanced to t>, and the 

 base of support becomes transferred from the 

 rectangular triangle, A B D, to the oblique- 

 angled triangle, A e D : by an oblique lateral 

 motion of the trunk, the centre of gravity is 

 propelled towards A , within the new base of 

 support, leaving the leg free to move without 

 danger of the horse falling. The leg B having 

 advanced and taken a new position at 6, the 

 leg D is next raised and advanced to d, 

 during which the base of support is transferred 

 from the plane A e D, to that of A b e, 

 within which the centre of gravity is propelled ; 

 lastly, the leg A is advanced to a, and the 

 base of support is transferred from the plane 

 A b e to that of b e d. When the leg A 

 has reached the point a, the base of support 

 becomes a new parallelogram abed, equal 

 in dimensions to that of A B E D. In walk- 



Fig. 245. 



ing, the four legs move in the order above- 

 mentioned successively. The time occupied in 

 performing the series of movements to com- 

 plete a step varies. In horses of large dimen- 

 sions, one foot moves the length of a step every 

 second, and, therefore, each leg swings one quar- 

 ter, and rests on the ground three-quarters of 

 a second. In walking at a more rapid pace 

 each leg moves in rather a less period, and the 

 interval between the setting down of one leg 

 and the rising of the next vanishes. 



The trot. In the trot the legs move in 

 pairs diagonally. If the legs A D (fig. 246) 

 be first raised and advanced, then B E will 

 generally be raised the instant that A D reach 

 the ground ; on the other hand, when the legs 

 B E are raised before those of A D reach the 

 ground, the trot approximates to the gallop of 

 two beats, the four legs being at the same time, 

 for a minute interval, above the plane of mo- 

 tion. The bases of support in the trot are the 

 lines A D and B E alternately. The same 



