460 



joint, is also proportional to its weight. — Q. E. D. Hence it follows 

 that — 



Corollary 2. — The weights of the muscles surrounding the joint 

 may be regarded as moments of the forces, and may therefore be com- 

 pounded by the law of composition of moments or couples. 



The action of the muscles that move the thigh upon the hip is 

 usually referred by anatomists to three classes of motion : — 



a. Eotation outwards or inwards. 



h. Flexion or extension. 



c. Abduction or adduction. 



If we imagine three rectangular co-ordinates drawn at the centre of 

 the acetabulum in the following manner : — 



a. Yertical axis, 



h. Horizontal lateral axis, 



c. Horizontal antero-posteral axis ; 

 it is easy to see that rotation round these axes corresponds with the 

 three recognised classes of motions; and as every motion, however com- 

 plex, of the thigh upon the hip, must be a rotation round some diameter 

 of the sphere of which the acetabulum forms a portion, it is evident that 

 every such motion may be interpreted correctly in the usual way, by 

 the aid of the composition of rotations. 



Such a method of interpretation, although exact, is not simple, as 

 the axes of co-ordinates are not chosen with reference to the forces 

 and directions of the muscles themselves, but with reference to direc- 

 tions, vertical and horizontal, arbitrarily assumed beforehand. 



In the following note I shall endeavour to establish the existence of 

 three axes of co-ordinates, to which the motions of the hip joint may be 

 referred, and which possess not only greater simplicity than other sys- 

 tems of axes, but also other properties of great interest and importance. 



The centre of the acetabulum is the centre of motion of the thigh 

 upon the hip ; and the centre of motion of the body upon the pelvis is 

 situated in the junction of the fifth lumbar vertebra with the sacram. 

 If these two centres of motion be joined, we have a geometrical line to 

 which the motions of the hip joint ought to be referred. In the erect 

 posture in man, this line is the axis of the neck of the femur, and is 

 essentially an oblique line, making acute angles with all the three axes 

 of anatomical writers. 



The anatomical and mechanical problem which I propose to solve is 

 the following : — 



To find the simplest planes passing through the centres of motion 

 of the body on the pelvis, and of the hip on the thigh, to which the 

 forces of the muscles of the hip joint can be referred." 



I shall commence by recording the observations made upon a human 

 subject, which was a female, aged 40, weight 82 Ibs.,^ and height 65^ 

 inches. I selected a female subject, in consequence of my first compa- 

 rative dissections having been made on a female Cercopithecns. 



The weights of the body, viscera, and muscles of this subject were 

 found to be as follow : — 



