PELVIS. 



sacral arch, but it resists their displacement 

 inwards, which would result from the pressure 

 of the femora in the direction of the necks of 

 these bones. The effect of this pressure, when 

 the pnbes yield to it, is shown in the de- 

 formity which has been termed the rostrated 

 pelvis, resulting from the crushing of these 

 bones together. The cot3'lo-pubic arch also 

 receives, in its concavity, part of the weight 

 of the abdominal viscera, though, from the 

 attachment of these to the spine, their chief 

 weight is concentrated upon the common 

 centre of pelvic arches, the sacrum. The 

 ilia are also generally supposed to support 

 the intestines in a great measure ; but this 

 support, on account of their great obliquity in 

 the erect position, cannot be so important as 

 is commonly imagined, except, as in the case 

 of the ccecum and rectum, through peritoneal 

 attachments. The human pelvis, when thus 

 taken in conjunction with the thorax, forms 

 the base of a cone, the apex of which is 

 the neck, a disposition for supporting the 

 contained viscera which the erect position 

 demands, and which contrasts strongly with 

 the structure of quadrupeds. 



Again, the cotylo-sacral and pubic arches 

 on each side, united at their extremities in 

 the acetabula, form two lateral arches, on the 

 centres of which rest the thigh bones. Against 

 the lateral pressure exercised by the thigh 

 bones, these two arches, connected, at their 

 anterior and posterior extremities, by the 

 symphysis pubis and sacrum, form, as Mayo 

 observes, an elastic hoop. The ischia also 

 contribute to this resistance against lateral 

 pressure, and form, with the two other por- 

 tions of the innominate bones, a sort of arched 

 tripod, on the apex of which the femur is 

 supported. 



In addition to the buttresses already de- 

 scribed, there is, placed vertically above the 

 cotyloid cavity, a thick rib of bone, which 

 transmits to the arched crest of the ilium, 

 and through it and the sacro-iliac joint to 

 the sacrum, a portion of the direct vertical 

 pressure from the heads of the thigh bones. 

 This thickened portion of the iliac wing has 

 been mentioned in the general description 

 of the bone as impinging on the iliac crest 

 in the middle of its anterior curve. The 

 division of the pressure thus produced, no 

 doubt calls into action much more com- 

 pletely the elastic resistance of the pelvis, 

 in sudden increase of weight. Thus in 

 the sacro-iliac joint meet three buttresses 

 or thickened lines of pressure, of which the 

 direct cotylo-sacral is the central and principal 

 one, the ischio-sacral the lowest and next in 

 strength, and the superior or indirect cotylo- 

 sacral the weakest. 



But, besides merely supporting quiescent 

 superincumbent weight, the pelvic arches are 

 required to resist and break the force of 

 shocks and concussions meeting with the 

 inertia of the trunk, and passing from the 

 lower extremities of the body to the vital and 

 delicate cranial and thoracic structures. 

 These dynamic requirements are met by pe- 



culiar modifications of the simple arch, com- 

 bining with it, by an admirable adaptation, 

 the qualities of an elastic spring. 



First, the cotylo-sacral arch, on which 

 the greatest number and force of shocks falls, 

 is not placed vertically, but obEquely upwards 

 and backwards, while the cotylo- pubic arch, 

 being united to it at its extremities, and con- 

 tinued in the same plane over the femoral 

 supports, forms the anterior arm of a bent 

 lever of the first order, of which the cotylo- 

 sacral arch is the posterior curved arm, the 

 spinal column the weight, and the heads of the 

 femurs the fulcrum (see Jigs. 87. and 86. u). 



Fig. 87. 



Drawing of a section of the pelvis in the coti/lo- 

 sacral arch, removing the left iliac wing, a, a', line 

 of fulcrum falling in the transverse vertical plane 

 of trunk; c, e', line of weight passing through centre 

 of sacro-iliac joint ; b,V, line of power or pubic pro- 

 jection ; d, d', line of sacral projection ; e,f, cotylo- 

 sacral curve ; a', If, pubic arm of lever ; a', c', co- 

 tylo-sacral arm ; a', d', length of gluteal arm ; c', d\ 

 posterior spinal arm ; g, posterior iliac projec- 

 tion. 



The anterior or pubic arm of this lever 

 giving insertion to the powerful extensor 

 muscles of the thigh, which represent the 

 power, is thrown upwards by the operation 

 of downward force on the crown of the 

 cotylo-sacral arch, calling these muscles into 

 contractile reaction, which overcomes gradu- 

 ally the force of any shock operating at the 

 posterior extremity of the pelvic lever over 

 the fulcrum of the thigh bones. In well- 

 formed male pelves, the pubic arm of this 

 lever is increased in power by being longer 

 than the cotylo-sacral by or f of an inch, 

 the one being 2 inches, the other 1| inch, in 



