118 LECTURE III. 



the top and bottom of a perpendicular pil- 

 lar, but in the back it projects posteriorly, 

 and describes a segment of a laro-e circle. 

 In consequence of this form, the back part 

 of the chest, the blade bones of the shoul- 

 der, and the weighty muscles of the back, 

 project behind the centre of gravity, and 

 become equiponderant to the weight in 

 front; and, we know, that in an upright 

 posture, the whole weight of the upper 

 part of the body is so perfectly balanced on 

 the base of the vertebral column, as to have 

 an equal propensity to preponderate in 

 every direction. 



Though the motion taking place between 

 the individual vertebrae is small, and such 

 as can produce no alteration in the form 

 of the vertebral canal injurious to the me- 

 dulla spinalis, yet the effect of the conjoin- 

 ed motion of all the vertebras is consider- 

 able ; and appears still greater when ob- 

 served in the motions of the head, which 

 it supports. We can incline the head far 

 forwards, backwards, or to either side, and 

 nearly in an equal degree. 



