464 



Compounding the moments of these muscles as before, and using the 

 line ad as our origin of X, we obtain 



X = 4J + 2J cos 25° + 13 cos 49° + cos 64° 

 r= ^ sin 25° + 13 sin 49° + sin 64° 



/X2 + r2 = 19-89 ozs. 

 Y 



^ = tan (36° 470- 



The close agreement in magnitude between the resultant moment 

 of these muscles (19 '89) and that of the glutei (19'41) is very remark- 

 able ; and the difference of angle between them (11° 58') is not more 

 than might have been anticipated from unavoidable errors of observa- 

 tion. 



The resultant plane of the anterior muscles is shown in the figure 

 by the line x'y'. The bisector of the angle between the lines xy ando;'^' 

 is a tangent to the ischiadic notch, and coincides with the ilio-pectinseal 

 ridge. 



The diametral plane of tbe acetabulum just found, containing the 

 ilio-pectingeal ridge, and touching the ischiadic notch, possesses many re- 

 markable properties. 



1st. It passes through the centre of the anterior line of junction of 

 the fifth lumbar and first sacral vertebrae ; i. e. through the centre of 

 motion of the body on the pelvis. 



2nd. It gives, both as respects distribution of matter and geometrical 

 form, the section of the pelvis, which offers the maximum resistance to 

 forces acting from the outside. 



3rd. It is the plane of the resultant moment of the muscular forces 

 acting on the hip joint, both with respect to the posterior and anterior 

 muscles. 



This plane may be called the ilio-pectineeal plane, and is the plane of 

 maximum moments acting on the hip joint. 



Remaining Muscles of the Sip Joint. 

 In addition to the eleven muscles whose action has been already 

 considered, there are six others which act upon the hip joint. They all 

 act upon the joint so as to cause it to rotate upon the head of the femur 

 in a plane at right angles to that already found to be that of the resul- 

 tant moment of the posterior and anterior muscles. This plane passes 

 through the tuberosity of the ischium, and falls just inside the anterior 

 rim of the ilium. Three of the muscles in question act on one side, and 

 three on the other side of the centre of motion, and in the erect posture 

 their moments on the head of the femur are balanced. They may be 

 called the ischiac and iliac muscles, with reference to their action on the 

 hip. 



Ischiac Muscles (flexors of knee). 



1. Biceps femoris (part),_ . ... J 



2. Semi-tendinosus, ...... \l\ ozs. 



3. Semi-membranosus, .....) 



