ANIMAL LOCOMOTION. 41 



owing to the fact that the great backward movement of these 

 limbs on the trunk is made possible by the fixation of the bones 

 to the pelvis, and through this structure to the vertebral column. 

 Not only is this the case, but the hind limbs alone possess the 

 power of propelling the body, so as to throw upon the fore limbs 

 the labor of accoraraodatiiig themselves to the rate of work of 

 their more powerful associates. When an animal is moving at a 

 high rate of speed, as in the gallop, the synchiral action of the 

 hind limbs projects the body with such force as to compel the fore 

 limbs to act simply as props, which successively carry the body 

 forward until one of the hind limbs is again in position to give 

 the body a second impetus. In proof of this assertion it is only 

 necessary to observe that the greatest height attained by the 

 trunk is that secured by the rump when both hind feet are off 

 the groimd. The statement generally made that the horse leaves 

 the ground by one of the fore feet creates the impression that he 

 gains the springing force from this foot, all the previous move- 

 ments being in preparation for such a spring. In place of this 

 statement another is here substituted, — viz., that the horse springs 

 from that hind foot which last leaves the ground, and is " off" 

 from all feet when he simply relinquishes the support afforded by 

 the last prop, — that is to say, the last fore foot. (See pp. 53, 62.) 

 If the fore and hind limbs were based on the same plan, the 

 motion of an animal would be either a series of springs — the two 

 feet pushing against the ground at the same moment — or a series 

 of steps, the two feet moving alternately. While closely resem- 

 bling one another, the two limbs are not on the same plan. If 

 any motion takes place in the vertebral column at the time that 

 the fore limb is moving, it is noticed that it occurs in the region 

 of the neck. The scapula has a slight motion downward and 

 backAvard. The trunkal motion for the hind limbs occurs in the 

 region of the lumbar vertebrae, while the pelvic bones are fixed. 

 The limit of the forward motion of the liind limbs is dependent 

 upon the flexibility of the lumbar vertebrae. The limit of the 

 similar motion of the fore limb is determined by the action of the 

 muscles alone. The forward motion of the fore limbs is essentially 

 the same in all animals, but the forward movement of the hind 

 limbs is variable, because the lumbar vertebrae differ in degrees of 

 flexibility. In clawed forms there is more lumbar flexibility than 



