THE HIP-JOINT. 



373 



The retinacula ^ probably strengthen the union of the head and neck before the 

 union of the epiphyses. 



Movements. — As a ball-and-socket joint, the hip permits motion on an indefi- 

 nite number of axes. If the ball were on the end of a straight rod, we could assume 

 that flexion and extension occur on a transverse axis and adduction and abduction 

 on an antero-posterior one, but the inclination of the shaft of the femur and that of 

 the neck in two directions complicates the problem, so that accurate analysis of the 



Fig. 389. 



Crest of ilium 



Symphysis pubis 



Head of femur 



Tuberosity of 

 ischium 



Obturator membrane 



The inner wall of the hip-joint socket has been cut away, exposing the head and round ligament without disturbing 



the capsule. 



movements is practically impossible. Rotation is motion on a vertical axis which 

 is generally assumed to pass through the head and the intercondylar notch. This 

 must, of course, vary with the shape of the bone. Although the angular motions in 

 the four conventional planes are far from simple, they may be assumed to be so 

 for practical purposes. Flexion is stopped in life by the contact of the thigh and 

 the trunk before the limits of the motion are reached. Extension is limited by the 



^ Fawcett : Journal of Anatomy and Physiology, vol. xxx., 1896. 



