MECHANISM OP THE INFERIOR EXTREMITIES. 241 



bones in front; and may be also increased, at pleasure, by the 

 extension of the legs. If, under such circumstances, the trunk 

 of the body be slightly advanced, its equilibrium is so easily main- 

 tained as to require but a very little muscular action to continue 

 it. The most exposed part of the base is backwards; and, if the 

 trunk be kept perfectly erect, there is some tendency of it to fall 

 in that direction. Hence, the utility of backs to seats, and the 

 fatigue from such as have not. 



SECT. VI. ON THE MECHANISM OF THE INFERIOR EXTREMITIES IN 



REGARD TO LOCOMOTION. 



1. Of the Motions of the Thigh. 



These, like the motions of the os humeri, upon the scapula? 

 consist in extension, flexion, abduction, adduction, rotation, and 

 circumduction; but, in consequence of being performed upon an 

 immoveable basis, the acetabulum, they are much less extensive. 

 In order that they may be understood well, it will be useful to 

 assume certain points of reference in the os innominatum and 

 femoris. These are the trochanter major, the pubes, and the 

 anterior superior spinous process of the ilium. In standing;, the 

 lower external part of the trochanter major, where it forms a 

 bulge on the side of the thigh bone, is on a horizontal line with 

 the upper part of the symphysis pubis. A triangle, described by 

 lines drawn from the anterior superior spinous process to the 

 symphysis pubis from the latter to the point mentioned of the 

 trochanter, and from the latter to the anterior superior spinous 

 process, will be nearly a rectangle, of which the base is above, 

 and the shortest side behind. 



The flexion of the os femoris is that motion in which its lower 

 extremity is carried forwards. It is performed with great ease 

 and freedom, in consequence of the arrangement of the articu- 

 lar surfaces of the bones, and of the capsular ligament. The 

 head revolves freely in the acetabulum, the ligamentum teres 

 is put into a slight tension, and the end of the trochanter major 



VOL. I. 21 



