196 



PELVIS. 



necessary that the line of gravity of the whole 

 body viz. that from the lower centre should 



Fig. 122. 



A. Diagram of the lines and centres of gravity of 

 the trunk, a b ; and of the whole body, c d. 



b. Outline of the lines of pressure in the pelvis and 

 legs in the ovate rickety distortion, during the 

 standing posture. a b lines of direction of, the 

 pressure of the heads of the femurs in the ace- 

 tabula, upwards and outwards. 



c. Outline of the pelvis and legs in the angular 

 rostrated pelvis of the adult, resulting from molli- 

 ties ossium. ab, a c, direction of pressure in the 

 acetabula when the legs are not deformed, 

 upwards and inwards. 



D. A similar outline in the angular rickety pelvis of 

 the child, when the legs are bent outwards, di- 

 rection of pressure inwards. 



E. Outline of abnormal antero-posterior curves of 

 the spine, pelvis, and legs, a b, direction of the 

 pressure in the acetabuia backwards increased by 

 the forward curve of the femora, c d, line of 

 traction of the "psoas muscles increasing the de- 

 formity of the pelvis. 



fall anywhere between the extent of longitude 

 of the feet (c d), 



If the trunk, however, bend forward, its 

 centre and line of gravity is advanced beyond 

 that of the whole body, and a share of the sup- 

 port of the trunk, equivalent to the degree of 

 distance of these two lines (b, d), falls upon the 

 muscles and ligaments of the posterior part of 

 the spine, and a corresponding strain upon their 

 attachments to the sacrum and posterior part 

 of the pelvis. This instance may be taken as 

 an example of many others, in which the me- 

 chanical position of the line of gravity in- 

 fluences muscular action, the effects of both 

 falling upon the lines of pressure and support 

 in the pelvis. 



For the preservation of the sitting position, 

 it is only necessary that the line of gravity of 

 the trunk (a b) should fall within the extent 

 of the basis of support, which is, from more 

 or less of the whole extent of the hams in 

 front to the ischial tuberosities behind. Hence 

 the greater facility with which a person sitting 

 down is pushed backward than forward. 



The line of pressure, on the other hand, 

 passing down the centre of the bodies of the 

 vertebrae will, in the well-made subject, when 



standing on both legs or sitting, divide at 

 the sacral promontory, into two equal parts, 

 each of which traverses, first the lateral sacral 

 masses to the sacro-iliac joint ; from this 

 point, in the upright position, it passes along 

 the cotylo-sacral rib to the heads of the 

 femurs, describing in its course the C-like 

 curve. In the sitting posture, it passes down 

 the ischio-sacral buttress to the tuberosities. 

 It will be borne in mind that each of these 

 standing and sitting arches has its tie, which 

 prevents it equally from starting outwards or 

 pressing inwards at the extremities ; that for 

 the cotylo-sacral arch being the united supe- 

 rior pubic rami, and that for the ischio-sacral 

 arch, the united ischio-pubic rami. The co- 

 tylo-sacral arch and its pubic tie, united at 

 the acetabula, and placed in the same plane, 

 form in man, as we have seen, a lateral arch 

 made up of the two halves, which supports 

 on its culm the inward pressure of the head 

 of the femur. The cotylo-sacral portion also 

 sustains, in addition, its upward and backward 

 pressure. 



The first effect of the softening of the os- 

 seous supports in this line of pressure is to 

 increase the natural curves which occur in it. 

 Thus we see an increase in the dorsal, lumbar, 

 and sacral curves, in the cotylo-sacral, fe- 

 moral, and tibial (fig. 122. E). The next effect 

 is to produce lateral curves, which present 

 generally their concavities towards the line of 

 gravity, and are always associated with com- 

 pensating curves, so as to keep the line of 

 gravity within them, about which they pro- 

 duce a wavy line, as is seen in the deformed 

 spine. When this is not the case, the 

 support of the weight falls more upon the 

 tension of the muscles, and ligaments, and 

 parts of bone on the convex side. In the 

 pelvis, and, to some degree, in the bones of 

 the legs, however, these results are modified 

 by the lateral duplication and division of the 

 lines of pressure ; and in the pelvic skeleton 

 this effect is still further increased by the cir- 

 cular union of the lateral structures, and by 

 the pressure or traction of the bones of the 

 legs, conjointly or individually. 



The alteration of the position of the centre 

 of gravity of the trunk, by deformity of the 

 spine low down, will also have its effect upon 

 the pelvis, by necessitating a constrained and 

 unnatural position to prevent the body falling. 

 Deformities confined to the upper part of the 

 spine are seldom accompanied by deformed 

 pelvis, owing, probably, to the little effect 

 they have in altering the centre of gravity. 

 In addition to these general changes from 

 mechanical pressure, there is, in this softened 

 state of the bones, the powerful effect of 

 combined muscular action. 



The influence of continued posture on these 

 changes will be found to be the origin of most 

 of the differences of form we have seen in 

 pelvic distortions. Let us consider the effects 

 of mechanical position and muscular action in 

 the recumbent, sitting, and standing posi- 

 tions respectively, on the softened pelvis 

 In lying upon Ike back, the softened pelvis 



