336 SURGICAL APPLIED ANATOMY. [Chap. xvm. 



why there is such powerlessness, with inability to 

 stand or sit, in cases in which this joint is weakened 

 or diseased ; and why the anterior portion of the 

 pelvis yields under the weight of the body, and 

 becomes deformed in rickets and mollities ossium." * 

 The pelvic deformity in rickets, it may be here 

 observed, varies greatly according to the age at which 

 the disease sets in, and the usual attitude of the child 

 when it becomes affected. The deformity sometimes 

 produced in very young infants has been ascribed 

 to muscular contraction (ilio-psoas, erector spinse, 

 gluteus medius, etc.). In the rickety pelvis, par 

 excellence, the two acetabula approach one another, 

 the anterior part of the pelvis yields, so that the 

 symphysis is pushed forward, and the cavity be- 

 comes greatly narrowed in its antero - posterior 

 diameter. In severe cases the anterior arch 

 may practically collapse, and the horizontal rami 

 of the pubes be for some little way parallel to one 

 another. 



In the erect attitude the pelvis is so inclined that 

 the plane of the brim of the true pelvis forms with the 

 horizon an angle of from 60 to 65 ; the base of the 

 sacrum is about 3f inches above the upper border of 

 the symphysis, while the tip of the coccyx is a little 

 higher than its lower border. The centre of gravity 

 of the whole body (adult) is at a spot just above the 

 sacro-lumbar angle, and exactly over the mid-point of 

 a line drawn between the heads of the femora. The 

 obliquity of the pelvis materially assists in breaking 

 shocks, and in distributing forces applied from below 

 throughout the pelvic ring. In modifying the effect 

 of shocks, also, it is aided by the arches of the pelvis, 

 and by the buffer-like discs of cartilage at the 

 symphysis and sacro-iliac joints. 



* Henry Morris on "The Joints, "p. 116, where a most valuable 

 account of the mechanism of the pelvis will be found. 



