CHAP. VI.] THE PELVIS. 141 



triple curvature of the spine enables it to yield with less jerk than if 

 it were a straight spring, or one that could bend only in a single 

 direction. " It yields in the direction of its curves, and recoils, and 

 so forms the most perfect spring, admirably calculated to carry the 

 head without jar or injury of any kind." 



The pliancy of the spinal column favours its flexion in various 

 directions, in obedience to the action of the numerous muscles 

 which are inserted into the vertebral processes. Nothing is more 

 common than to see a misshapen and crooked spine produced 

 by the predominance of action given to certain sets of muscles, 

 through the habitual assumption of awkward attitudes : most of 

 the curved spines which occur in weakly females may be traced to 

 uncorrected bad habits as their origin.* 



The spine, gradually expanding at its lower part, rests upon the 

 base of the sacrum ; and the last lumbar vertebra is separated from 

 that bone by a fibre-cartilaginous disc. The sacrum forms a wedge 

 separating the pelvic bones, and is admirably adapted to transmit 

 the weight of the spine to them. 



Of the Pelvis. "The spinal column," to use the words of Mr. 

 Mayo, " rests on an elastic hoop, in the extreme circumference of 

 which on either side the deep cups are wrought which receive the 

 heads of either thigh-bone. But this elastic hoop is not disposed 

 vertically, but slants in such a manner, that, when we alight upon 

 our feet, the force of the arrested motion tells in great measure on 

 the extensor muscles of the hip." 



In the articulation of the sacrum with the ossa innominata we see 

 remarkable provision against its displacement backwards, by a force 

 acting from above downward, the direction in which the superin- 

 cumbent weight bears, or even by one acting from before back- 

 wards. This security is obtained not only by the strong ilio-sacral 

 ligaments, which tie the bones together behind, and the cartilage, 

 which intervenes between the ilium and sacrum, and adheres firmly 

 to both, but also by the double wedge-like shape of the sacrum 

 itself; for this bone is wider above than below, so that it can 

 thereby resist the downw r ard pressure ; and it has a greater width 

 before than behind, which enables it to oppose the pressure in front. 

 And Mr. Ward has shewn that the sacrum is also well secured 

 against displacement forwards, not only by the general compact- 

 ness which the sacro-iliac joints derive from their ligaments and 

 cartilages, but also by the cuneiform character which the bone 



* Dr. Arnott's remarks bearing upon this subject deserve a careful perusal. 

 See his Elements of Physics, vol. i. p. 223, et seqq. 



