Every one has observed that during sleep, or a fainting fit, the head inclines forward and falls upon the breast. In this case, it is in accordance with the laws of gravity; for the head, resting upon the vertebræ which support it at a point of its basis which is nearer the posterior than anterior part, cannot remain in an upright position in standing, except by an effort of the muscles at the back of the neck: it is the cessation of this effort that causes it to fall forward. The body also is unable to remain straight without fatigue. The vertebral column being placed behind, all the viscera or parts contained by the chest and belly are suspended in front of it, and would force it to bend forward unless strong muscular fibres held it back. A proof of this may be seen in pregnant and dropsical women, who are compelled, in consequence of the anterior part of the body being heavier than usual, to keep the vertebral column more fixed, and even thrown backward. The same observation may be made with regard to the pelvis, basin, or lowest part of the trunk, which by its conformation would bend upon the thighs, if not kept back by the great mass of muscular fibres that form the hips. In front of the thighs, again, are the muscles which, by keeping the kneepan in position, are the means of preventing the leg from bending. Lastly, the muscles forming the calves, by contracting, are the means of preventing the leg from bending upon the foot. Such is the general mechanism of the standing position. It is, therefore, as we observed, a concurrence of efforts: almost all the extending muscles are in a state of contraction all the time that this position is maintained, and the consequence is, a fatigue which cannot be endured for any great length of time. Hence we see persons in a standing position rest the weight of their body, first on one foot, then on another, for the purpose of procuring momentary ease to certain muscles. For this reason, also, standing still is more fatiguing than walking, in which the muscles are alternately contracted and extended.