Chap, xvi ii.] THE P ELY is. 373 



be seen that its thickest and strongest parts are such 

 as are situate in the line of these. " When very con- 

 siderable strength is requisite in an arch, it is 

 continued into a ring so as to form a counter-arch, or 

 what is called a tie is made to connect together the 

 ends of the .arch, and thus to prevent them from 

 starting outwards. By these means a portion of the 

 superincumbent weight is conveyed to the centre of 

 the counter-arch, and borne in what is called the sine 

 of the arch. The body and horizontal raaai of the 

 pubes form the tie or counter-arch of the femoro-sacral, 

 and the united rami of the pubes and ischium the tie 

 of the ischio-sacral arch. Thus the ties of both arches 

 are united in front at the symphysis pubis, which, like 

 the sacrum or keystone, is common to both arches. 

 .... This explains how it is that so much strain 

 is made upon the symphysis when any increased weight 

 has to be supported by the pelvis, as in pregnancy ; 

 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 spinae, 

 glutens 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 becomes 

 greatly narrowed in its antero-posterior diameter. 

 In severe cases the anterior arch may practically 



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

 able account of the mechanism of the pelvis will be found. 



