PELVIS. 



139 



8'7 millimetres above the sacro-lumbar joint, 

 or just above the pelvic arch. All weight on 

 the arch, such as that of the trunk, is sup- 

 ported most easily when its line of gravity 

 falls through this part, and coincides with that 

 of the whole body in the transverse vertical 

 plane, and the trunk will be found to be 

 thrown into such a position, when supporting 

 heavy weights, as will tend to produce this 

 effect. And, according to the researches of 

 Rokitanski, when the sacro-vertebral angle 

 and pelvic inclination is increased, as by hip 

 disease, there is always a compensatory curve 

 of the spine backward, to keep the centre of 

 gravity above this point. 



In artificial constructions for the purpose 

 of transmitting a weight downwards from 

 a central to two lateral points, a segment of 

 a circle, or arch is most commonly made 

 use of, and generally consists of two lateral 

 curved portions, composed of separate parts 

 or voussoirs, with an interposed substance 

 between them called a keystone or crown- 

 piece, of a wedge shape, and placed with the 

 broad end uppermost. This wedge shape 

 exactly corresponds to the interval which 

 would be left between the lateral curved 

 pieces, having their ends cut square. Any 

 force operating on the keystone from above 

 tends only to drive the broader part of the 

 wedge further between the lateral pieces, 

 at the same time pressing them nearer to each 

 other, and so to increase the firmness of the 

 arch, so long as the extremities are firmly 

 fixed in the ground and prevented from start- 

 ing outwards, which is generally accomplished 

 by means of abutments. 



Constructed upon this plan, the pelvis pre- 

 sents two lateral curved thickened buttresses 

 or columns, passing from the cotyloid to the 

 sacro-iliac articulations, and two others pass- 

 ing on a plane posterior to these from the 

 tuberosities of the ischia along their de- 

 scending rami, and through the ilia to the 

 same sacro-iliac articulations; and interposed 

 between each of these corresponding lateral 

 pieces is the common keystone of both the 

 arches thus formed, the wedge-shaped sacrum. 



The sacrum thus forms the common cul- 

 minating point of two arches, viz., the cotylo- 

 sacral or standing arch, and the ischio-sncral 

 or sitting arch. And the planes of these two 

 arches are so directed as to coincide or be- 

 come applied to each other at the top of 

 the great sciatic notch, as may be seen by 

 reference to the diagram (fg. 86. A, a a'). 

 In this comparison, however, it must be borne 

 in mind, that the extreme tenacity and strength 

 of the material used, bone, obviates the ne- 

 cessity of the use of many pieces in the 

 lateral portions, such as the "voussoirs" of 

 stone arches, and is more analogous to 

 the iron materials sometimes used for this 

 purpose. And it is in these lines of pressure 

 that we find the bulk and strength of the 

 osseous structure of the pelvis most displayed. 



The span of the cotylo-sacral arch being 

 greater, and more elliptical in the female 

 than in the male, where it is almost circular, 



renders them less able to support heavy weights; 

 and on account of the greater distance to 



Fig. 86. 



A, diagram of the pelvic arches: b, a, b>, cotylo- 

 sacral arch ; c, a, c', ischio-sacral arch ; b, d, I/, 

 cotylo-pubic tie ; c, d, c', ischio-pubic tie. 



is, diagram of pelvic levers : c, a, line of gravity ; 

 F, b, pubic lever; F, w, cotylo-sacral lever; 

 w, r, posterior spinal or iliac lever ; p, c, direction 

 of spinal power; w, e, direction of spinal weight; 

 F, cotyloid fulcrum ; d, femoral support. 



which it separates the femurs, contributes to 

 produce the waddling gait in running which 

 is characteristic of this method of female pro- 

 gression. 



Instead of abutments to prevent the extre- 

 mities of the arch starting outwards, we often 

 see, in artificial constructions, a connecting 

 link or tie extending between these extremi- 

 ties to hold them together, or the circle of 

 which the arch is a segment is completed 

 below, as in tunneling. 



Such a tie and completion of the circle 

 we have in the horizontal rami of the pubic 

 bones, for the cotylo-sacral arch, and in the 

 united ischio-pubic rami, for the ischio-sacral 

 arch ; and they are connected in front, at 

 the pubic symphysis, exactly as these two 

 arches themselves are behind at the sacro-iliac 

 joints. 



By the vertical ischio-pubic arch thus formed, 

 that portion of the pressure which has a tend- 

 ency to push forward and upward the extre- 

 mities of the ischio-sacral arch, is supported 

 and thrown upon the cotylo-sacral arch, the 

 whole weight of the trunk, in a sitting posture, 

 being thus divided between them The ischio- 

 pubic rami are the parts of the pelvis most 

 liable to fracture, according to Cruveilhier, 

 from the application of force acting on the 

 ischia. 



The cotylo-pubic arch not only resists the 

 starting outward of the ends of the cotylo- 



