335 



considerable, and which is estimated by multiplying the single motion 

 by the number of vertebrae. 



The centre of the motions, by which the spine is extended or bends for- 

 ward or backward, is not situated in the articulation of the oblique pro- 

 cesses, as it is maintained by Winslow, in the Memoirs of the Academy 

 of Sciences for the yeur 1730, nor in the invertebral substance. The exten- 

 sion and flexion of the vertebrae are not performed in two centres of mo- 

 tion, the one in the intervertebral substance, the other in the articulation 

 of the articulating processes, as was imagined by Cheselden and Barthez, 

 but on an axis crossing the bone between its body and its great aperture. 

 The anterior part of the bone and its spinous process perform, around 

 this imaginary axis, motions forming part of a circle, and whirh though 

 limited, are not the less marked ; and in these motions, the articulating 

 surfaces separated by the intervertebral substance are brought into close 

 contact, and this substance is compressed, while the oblique processes 

 move on one another, and tend to part from one another: this is what 

 happens in bending the trunk, while, in straightening it, the anterior sur- 

 faces are removed from each other, the posterior surfaces approach, come 

 closer and closer together, and finally touch throughout the whole of 

 their extent, when the extension of the trunk is carried as far as the spi- 

 nous processes will allow. 



The use of the ridge of projections which arise from the posterior part 

 of the vertebrae, is to limit the bending of the trunk backwards, and to 

 enable the muscles which straighten it, to act with a more powerful lever. 

 When, from the habit of an habitually erect posture, these processes have 

 been prevented from growing in their natural direction, the trunk may be 

 bent backward to such a degree, that the body forms, in that direction, 

 an arc of a circle. It is thus, that they train, from the earliest infancy, 

 the tumblers who astonish us by the prodigious suppleness of their loins, 

 in bending backward so as to change the natural direction of their spinal 

 processes. 



It was of consequence, that the motions of the vertebral column should 

 take place, at once, in a great number of articulations, as the curvatures 

 are thus less sharp, and thus the organization of the spinal marrow, 

 which is very delicate, is not injured. Thefibro-cartalaginous substances 

 which connect together the bodies of the vertebrae, between which they 

 lie, possess a remarkable degree of elasticity, like all bodies of the same 

 kind, and support, in a favourable manner, the weight of the body. When 

 the pressure which they experience is long continued, they somewhat 

 yield, and diminish in thickness, and this effect taking place, at the same 

 time, in all the intervertebral substance, our stature is sensibly lowered. The 

 body is, on that account always shorter in the evening than in the morn- 

 ing, and this difference maybe considerable, as is mentioned by Buffon to 

 have been the case in several instances. The son of one of his most 

 zealous coadjutors (M. Gueneua de Montbeillard, to whom is due the 

 greatest part of the natural history of birds,) a young man of tali stature, 

 five foot nine inches when he had reached his complete growth, once lost 

 an inch and a half, after spending a whole night at a ball. This differ- 

 ence in the stature depends, likewise, on the condensation of the cellular 

 adipose tissue at the heel, which forms, along the whole of the sole of 

 the foot, a pretty thick layer. 



The thigh bone is longer in man than in quadrupeds, and this relative 



