228 THE ERECT POSITION IN MAN. 



c. The vertical direction of the axis of the iliac bone is 

 due to the lumbar and sacral curvature, and to the pro- 

 montory. 



6.— The Thigh. 



a. The human thigh-bone is relatively longer and more 

 massive than in any animal form. 



b. In no animal thigh-bone is the neck so elongated and 

 so oblique as in the human. 



c. In no animal thigh-bone are the areas for the attach- 

 ment of its proper muscles so limited in extent, but at the 

 same time so precisely expressed as in the human femur. 

 The linea aspera is only met with in its proper form in man, 

 and is to be regarded as an area, not as a mere line. The con- 

 centration of muscular attachments on it is a necessary pro- 

 vision for freedom of movement at the hip joint. 



d. The human femur is the only thigh-bone which can be 

 extended beyond the line of axis of the vertebral column. 



e. The hip-joint can only be fully extended in man, and 

 in man only does it become immovable in its extended posi- 

 tion. All animals can fully flex the hip. 



/. The human hip-joint is the only hip-joint in which the 

 extensor area is fully developed. Extension may take place 

 from 140° to 160°, according to the mass of muscle which is 

 interposed. This complete movement of extension at the 

 human hip-joint is not due to any laxity of structure, but to 

 the presence of a posterior or extensor area on the head of the 

 femur. In apes and quadrupeds this extensor area is much 

 more feeble than in man ; in birds it is scarcely to be seen. 



g. The proper muscles of the thigh are in man principally 

 devoted to the balance of the trunk on the hip-joint. The 

 corresponding muscles in the animal are chiefly devoted to 

 the propulsion of the body. 



h. The extension of the human femur beyond the line of 



