466 



lumbar articulation with the centre of the acetabulum, and therefore 

 shows the traces of the ilio-pectinseal and ilio -ischial planes as two right 

 lines intersecting at an angle of 90°. 



The Fig. 3 shows the section of the os innominatum made by the 

 ilio-pectinseal plane, in which, as I have shown, the resultant couples of 

 the principal muscles acting on the hip joint are situated. The cancel- 

 lated portion of the bone is shaded, and the dense part is left white. 



It would require a separate paper to show how admirably adapted 

 this form of section is either to resist a shock acting in the direction of 

 the arrow, which the bone receives in j umping down from a height on 

 one foot, or to counteract the strain produced by the muscles acting from 

 the periphery of the bone upon the femur. 



Fig. 3. 



In Fig. 4 I have shown the section of the os innominatum made by 

 the ilio-ischial plane, at right angles to the ilio-pectinseal plane. 



This section of the bone is rarely called upon 

 to resist any strain in a transverse direction ; and 

 when the cavity of the acetabulum is completely 

 filled by the head of the femur, its strength to 

 resist vertical pressure, as in sitting, is very 

 great. 



Some interesting deductions may be made from 

 the weights of the muscles, classified into groups 

 suggested by the preceding analysis. 



The total weight of the muscles of the hip 

 and knee joints, named h, c, d, e, is found to be 

 73*50 oz. ; of this amount 21 '75 oz. are included 

 in the three glutm; 21 oz. in the group of eight 

 muscles antagonistic to the glutei ; 23*5 oz. in the 

 extensors of the knee (including the tensor vaginm, 

 which aids the quadriceps extensor) ; and 7*25 oz. 

 are included in the flexors of the knee joint. 



Expressed in percentages of the hip and knee tuber isci-iii 

 joint muscles, these groups have the following Fig. 4. 



values : — 



