Chap, xviii.] THE PELVIS. 335 



serves to transmit the weight of the body both in the 

 standing and sitting postures. The transmission is 

 effected through two arches, one available for the 

 erect position, the other for the posture when sitting. 

 The sacrum which supports the spinal column is the 

 centre or keystone of both these arches. When stand- 

 ing, the arch is represented by the sacrum, the sacro- 

 iliac synchondroses, the acetabula, and the masses of 

 bone extending between the two last-named points. If 

 all other parts of the pelvis were to be cut away but 

 these, the portions left would still be able to support 

 the weight of the body, and would represent in its 

 simplicity the arch through which that weight is 

 transmitted. When sitting the arch is represented by 

 the sacrum, the sacro-iliac synchondroses, the tubera 

 ischii, and the strong masses of bone that extend 

 between the two last-named parts. Morris terms 

 these two arches the femoro-sacral and the ischio- 

 sacral. On examining the innominate bone it will 

 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 rami 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 ; 



