PELVIS. 



141 



direct distance from the centre of the cotyloid 

 support (/#. 87. a' b', a' c').* This gives the 

 anterior muscles of the thigh greater power in 

 resisting the downward force of the trunk at 

 the sacral extremity of the lever. 



But by the addition of the iliac and sacral 

 projections posteriorly, the cotylo-sacral arm 

 is increased in length by 2^ inches, as will be 

 observed on reference to the figure (c', d'), 

 a disposition which evidently increases the 

 power of the glutei muscles in maintaining 

 the habitual erect position, and resisting any 

 tendency to fall forwards, by extending the 

 femora on the pelvis. 



Again, if we follow the lateral curve of the 

 sacrum at the brim of the pelvis, we shall 

 find that it projects forward in the promontory 

 of the sacrum, immediately under the point 

 supporting the spine, so that its profile, taken 

 with that of the ilium, as seen in the figure 

 (^/), presents a curve with the concavity for- 

 ward, in the manner of a C spring. It is 

 worthy of remark, that, in the erect position, 

 a plumb line, dropped opposite to the centres 

 of the bodies of the axis and last lumbar ver- 

 tebra, passes across the centre of the sacral 

 promontory, and directly between the centres 

 of the cotyloid cavities, as was proved by the 

 experiments of Weber. Such a plumb line 

 marks exactly the line of the transverse 

 vertical plane of the spinal column before 

 mentioned, which, when continued down- 

 wards, passes through the sacral extremity of 

 the pelvic lever, and also through the cotyloid 

 fulcrum, dividing equally and vertically the 

 heads of the thigh bones, and crossing the 

 ischio-pubic rami about their suture ( a'). 

 Thus, the oblique C-like curve of the cotylo- 

 sacral arch, or posterior bent arm of the lever, 

 meets this plane at its two extremities, di- 

 recting its concavity towards it like the arc 

 of a circle to its chord ; and contributes, by 

 its elastic reaction, to break the force of 

 shocks operating through the spine and fe- 

 mora. In deformed pelves, we generally find 

 that the sinking of the sacrum, the crown of 

 the pelvic arch, under the weight of the trunk, 

 produces an increased curvature of the iliac 

 bones forwards by the yielding of the C spring, 

 and thus still further encroaches upon the 

 dimensions of the pelvic brim. 



The above considerations will illustrate the 

 fallacy of the deduction of Cruveilhier in re- 

 spect to the statics of the pelvis. This cele- 

 brated anatomist remarks, that " the arti- 

 culation of the vertebral column with the 

 pelvis is situated at the back part of that 

 cavity, while the articulation of the femurs is 

 anterior and lateral. The distance between 

 them increases the space in which the centre 

 of gravity can oscillate, without being carried 

 so far forward as to pass beyond a perpen- 

 dicular, from the cotyloid cavity to the base 



* The line c c', in the diagram passes through the 

 centre of the sacro-iliac joint, at which point, a line 

 drawn from the cotyloid fulcrum of the bent level- 

 falls perpendicularly upon the line of action of the 

 weight, transmitted obliquoly in the snor.il axis, 

 and this consequently, the effective point of power 

 of the bent arm f. e. 



of support at the feet" (p.514-. vol. j. Anut. 

 Dcscrip.). Now these experiments of Weber 

 prove that the centre of gravity is directly 

 ever the cotyloid support, and cannot oscil- 

 late between these two articulations. The 

 only oscillation of the line of gravity which 

 can take place without falling is along the 

 length of the basis of support the feet. 



Soon after, in reference to the sitting po- 

 sition, he says, " the tuberosities of the ischiu 

 being a little anterior to the cotyloid cavities, 

 and near the front of the pelvis, the centre 

 of gravity tends to pass behind the base of 

 support; and the body easily falls backward 

 in that position." Now, the tuberosities of 

 the ischia, in the erect posture, are consider- 

 ably behind the line of gravity, or transverse 

 vertical plane, which crosses at or near the 

 ischio-pubic ramal suture ; and though, in 

 the sitting posture, they are brought a little 

 nearer the line of gravity, yet a much more 

 satisfactory reason of the trunk more easily 

 falling backwards than forwards, is because of 

 the support of the hams in front, and the eleva- 

 tion of the cccyx behind above the plane of 

 support. 



Again, by the projection of the sacrum and 

 tuberosities of the ilia behind the sacro-iliac 

 joints, another lever, less powerful than the 

 foregoing, is formed, having also the cotylo- 

 femoral supports for its fulcrum, and "the 

 spinal column for its weight, the anterior arm 

 of this lever being the cotylo- sacral arch 

 (fig. 87. a', c'), and the posterior, the over- 

 hanging tuberosities of the ilia and projecting 

 sacrum ( c', d']. Measuring from the centre 

 of the sacro-iliac articulation, the anterior 

 arm is J inch in direct length, and the 

 posterior about 2 inches and a half. The 

 power in this lever resides in the power- 

 ful muscles which pass from the sacral and 

 iliac bones posterior to the sacro-iliac joint, 

 to the osseous spinal projections and append- 

 ages above viz. the longissimus dorsi and 

 sacro-lumbalis, and its action is shown in the 

 increase of the pelvic inclination on the change 

 from the sitting to the standing position ; the 

 principal movement taking place in thesacro- 

 luinbar joint. It acts to the most advantage 

 when the centre of gravity of the trunk, from 

 which it is derived, is thrown in advance of 

 the cotyloid fulcrum ( '), so that the lever, 

 though apparently one of the second order 

 (i. e. in which p and w are on the same side 

 of the fulcrum), is in reality one of the first 

 order, (in which the fulcrum may be between 

 them, and supports both the power, p, and 

 the weight, w. Hence, in the drooping of the 

 trunk forwards in old age, the action of 

 these muscles contributes, to produce the in- 

 creased obliquity of the pelvic lever in the 

 manner before described. This may be made 

 more evident by inspection of the diagram 

 0%. 86. u), which is taken from a small brass 

 model made to illustrate this point. It will be 

 seen that the pressure on the cotyloid fulcrum, 

 F, could not be w p, as in the second 

 order of levers, but must necessarily be 

 w + p, and therefore in the first order of 

 levers. P, in this case, may be represented 



