CHAP. VI.] LOWER EXTREMITY. 145 



junction of the neck with the shaft throws the thigh-bone outwards 

 to a certain distance,, and leaves abundant room for the play of the 

 adductor muscles : this could only be attained otherwise by greatly 

 enlarging the pelvis in the transverse direction. And again, in the 

 movements of the femur, much is gained ; for rotation can be per- 

 formed with little muscular effort by reason of the favourable lever- 

 age afforded by the neck of the bone and the trochanter major. 

 The increased power thus given to the rotator muscles has consider- 

 able effect in walking, which is on that account performed more 

 lightly, and without any circumduction of the limb. 



It is owing to this disposition of the neck of the thigh-bone, that, 

 when its lower extremity advances in the vertical plane, its head and 

 neck turn on a horizontal axis ; in other words, that the angular 

 motion of its shaft is converted into a rotatory movement at the hip- 

 joint. And from such an arrangement this great advantage results, 

 that in the various motions or states of the joints, as extreme or par- 

 tial flexion, extension, &c., the same, or very nearly the same, extent 

 of the articulating surfaces is exposed to pressure : for, as Mr. Ward 

 expresses it, the rotation of a hemispherical head within a socket of 

 the same form involves no diminution of the extent of the contiguous 

 articulating surfaces ; but the angular motion of a joint of this kind 

 throws part of the ball out of the socket, and leaves part of the 

 socket without bearings to rest upon, so that the weight, instead of 

 being distributed equally over the whole surface of the head, is con- 

 centrated upon that portion which remains within the cavity.* 



Moreover, the joint gains as regards the extent of flexion by this 

 conversion of angular into rotatory motion, for an angular motion in 

 the acetabulum would be readily checked by the edge of the cavity 

 coming in contact with the neck of the bone ; " whereas rotation 

 meets no such check in the conformation of the joint itself, but 

 may be continued indefinitely, until opposed by the tension of liga- 

 ments, or some other adventitious obstacle/' 



In the structure of the extremities of the femur, we find evidence 

 of much beautiful mechanical contrivance, in the disposition of the 

 compact tissue, and of the rods or fibres of the cancellous texture, 

 having an obvious reference to the direction of the weight to be sus- 

 tained as well as to the dispersion of concussions. The neck of the 

 thigh-bone, having to bear considerable superincumbent weight, is 

 strengthened on its inferior surface by an arch of compact tissue, 

 gradually increasing in thickness as it proceeds from above down- 



* Ward, loc. cit. 

 VOL. i. i. 



