STANDING POSTURE : EQUILIBRIUM. 489 



shock produced by the sudden descent of the body upon the 

 ground. 



Of the Attitudes of the body, and the various kinds of Locomotion. 



650. A small number of Vertebrated animals, Serpents, 

 for instance, bear habitually on the whole length of their 

 bodies, which rest entirely on the ground ; and their only 

 movements are effected by undulations of the spinal column. 

 But the rest are supported upon their extremities ; and we 

 give the name of standing to that position in which the 

 animal rests supported by its limbs upon the ground or on 

 any firm horizontal basis. In maintaining this position, the 

 extensor muscles, by which the joints are straightened, must be 

 in continual action, since the limbs would otherwise bend 

 beneath the weight of the body. Now as the sense of 

 fatigue, in any set of muscles, depends in great degree upon 

 the length of time during which they have been in continuous 

 action, the maintenance of the standing posture for a long 

 period is, in most animals, more fatiguing than walking ; 

 since in the latter exercise the action of the flexors alternates 

 with that of the extensors. 



651. But this condition is not the only one essential to 

 steadiness in the standing posture ; for in order that the 

 body may rest firmly upon the members, it must be in equi- 

 librium. It has been shown (MEGHAN. PHILOS. Chap, iv.) 

 that equilibrium exists, or in other words, that a body 

 remains at rest in its position, not only when it bears upon 

 the whole of a broad sur- 

 face, but also when it is 



so placed that the tenden- 

 cies of its different parts 

 to descend or gravitate 

 towards the earth counter- 

 balance each other. This 

 is the case when its centre 

 of gravity is supported, 

 that is, when a line drawn 



perpendicularly from that centre falls within the base. In 

 order, then, that an animal may rest in equilibrium on its legs, 

 it is necessary that the vertical line from its centre of 

 gravity (or line of direction) should fall within the space 



