. Animal physiology. Physiology, Comparative. 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 up
. Animal physiology. Physiology, Comparative. 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 extensoi^ 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 j)osture 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 ce7itre ^^^^ of gravity is supported,— ^ ^.^ ^^^ d 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 i
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