465 



lUae Muscles (extensors of knee in part). 



1. Tensor vaginjE femoris, . . . ^ 



2. Sartorius,. ........> 6,^ ozs. 



3 Rectus femoris, ) 



The resultant plane of the portion of the hiceps attached to the is- 

 chium, and of the two internal hamstring muscles, is at right angles to 

 the ilio-pectinseal plane ; and the resultant of the action of the tensor 

 vagina and of the sartorius coincides with the plane of the rectus, and 

 also is at right angles to the ilio-pectinseal plane. Considering that only 

 a portion of the hiceps acts on the hip, and that in the erect posture the 

 leverage of these muscles on the head of the femur is equal and opposite, 

 it is manifest that these two groups of muscles, as well as the pos- 

 terior and anterior groups, balance each other's action. This plane of 

 resultant moments may be called the ilio-ischial plane. It is at right 

 angles to the ilio-pectingeal plane, and intersects it along the line join- 

 ing the centre of the sacro-lurabar articulation with the centre of the 

 acetabulum — that is to say, the line joining the centre of motion of the 

 body on the pelvis with the centre of motion of the hip upon the thigh. 



In the erect posture, neither of these planes is vertical, and the di- 

 ameters of the acetabulum corresponding to them make angles of about 

 45° at each side of the vertical diameter. 



The ilio-ischial plane makes a section of the os innominatum, not 

 so strong as that made by the ilio-pectinseal plane ; and its curvature is 

 in the opposite direction, being slightly concave outwards, while the 

 curvature of the ilio-pectinaeal section is strongly convex outwards. 

 Trom this and other considerations, it follows that the ilio-ischial plane 

 has relation rather to the support of the weight of the body than to re- 

 sistance to forces acting from without. 



Fig. 2. 



The above figure represents the os innominatum of the right side, 

 drawn from a point of view situated on the line joining the sacro- 



