106 



SKELETON. 



small and shallow, that the bladder, even in the undistended 

 state, cannot be accommodated by it, but is contained princi- 

 pally by the abdomen. Its transverse diameter is much short- 

 er than the others. The superior strait faces much more 

 forwards than in the adult. 



SECT. VII. ON THE MECHANISM OF THE PELVIS. 



The pelvis has an important part in the several actions of 

 standing and of locomotion; besides its usefulness in giving a 

 support to the viscera of the abdomen, and in having attached 

 to, and contained within it, the organs of generation. 



In standing, the pelvis is impelled by two opposing forces, in 

 consequence of the attachment of the vertebral column at its 

 hind part, and of the ossa femorum at its anterior lateral parts. 

 The weight of the head and of the upper parts of the body, 

 falling upon the sacrum, acts upon a lever, which is represent- 

 ed by the distance between the acetabula and the sacro-iliac 

 junction, and has a tendency to depress the posterior part of 

 the pelvis, by rotating it upon the heads of the thigh bones. 

 This movement is obviated by the iliacus internus, psoas mag- 

 nus, and some other muscles, which keep the front of the pelvis 

 from rising up. It is also prevented by the principal weight 

 of the trunk being in front of the spine, and therefore inclining 

 forwards, so that the centre of gravity, in the erect position, 

 gives a continual tendency to fall forwards instead of back- 

 wards. 



The wedge-like shape of the sacrum is highly favourable to 

 the erect position: from having it's base upwards, whenever 

 the weight of the trunk is thrown upon it, it is driven down 

 between the ossa innominata, and has the tightness of its arti- 

 cular connexion, therefore, much increased by the position 

 which it is intended to sustain. In illustration of the. usefulness 

 of the triangular or wedge-like shape of the sacrum, it may be 

 observed, that it is much less so in animals which are intended 

 to go upon all fours than in the human subject. 



The articulation of the several bones of the pelvis with each 

 other, is so close as not to admit of any motion between them, 

 with the exception of the os coccygis, and of the relaxation 



