1 44 LOCOMOTION. [CHAP. vi. 



again rolls upon the sternum, and the scapula rolls upon the con- 

 vexity of the ribs. On the other hand, the pelvis, as regards the 

 viscera, is intended merely for their support ; and if, during labour, 

 a temporary enlargement of its lower aperture is requisite, the flexi- 

 bility of the joints of the os coccygis in the female skeleton, with 

 the temporary yielding of the ligaments, affords a sufficient pro- 

 vision for this object ; the inferior extremities again require to be 

 articulated to a solid, unyielding platform, upon which they may 

 poise the incumbent weight of the trunk and head. The bones of 

 the pelvis are for these reasons few, weighty, massive, and knit 

 together immovably. Thus accurately do the points, in which 

 a resemblance is wanting between the chest and pelvis, preserve the 

 analogy between these parts/' 



Of the Extremities. In no part of the skeleton is the adaptation 

 of anatomical disposition and structure to function more strikingly 

 obvious than in the bones of the extremities. 



The lower extremities form powerful pillars of support for the 

 trunk in the erect posture. The great strength of the femur and 

 tibia, which form the principal portion of each pillar, fit them admi- 

 rably for this office ; and it is interesting to notice that dense osseous 

 tissue in each of these bones is most abundant in those situations 

 where the greatest strain of the pressure from above is felt. This 

 may be seen in a transverse section of either, when a dense spine 

 of bone is found corresponding to the concave surface of each; this 

 is most distinct in the femur, where the dense bone alluded to con- 

 stitutes the linea aspera. 



The femur is curved forward, and this incurvation gives elasticity 

 to the bone, and aids in distributing the force of concussions. Its 

 shaft is inclined downwards and inwards, so that the opposite femora 

 approach each other inferiorly at the knee-joints, while they are 

 separated by a considerable interval above. And this interval is 

 increased by the head and neck of each femur being united to the 

 shaft at an obtuse angle: this angle is one of about 135 degrees in 

 the male ; it is somewhat smaller in the female. 



It is evident that the femur must suffer in point of strength from 

 this mode of junction of its neck and shaft, for a bone without any 

 bend in its axis must necessarily be more capable of resisting down- 

 ward compression, than one consisting of two pieces united at an 

 angle. But we observe here, as in other parts of the mechanism of 

 the human body, that the disadvantage in one respect is more than 

 counterbalanced by certain advantages which this peculiar arrange- 

 ment offers, and which could be attained in no other way ; for t 



