146 



PELVIS. 



the double sacral wedge had a small diameter 

 directed forwards as well as downwards, in 

 which case it would be prevented by the ilia 

 from moving in that direction at all. 



Thus the pelvic supports of the trunk are 

 a peculiar and admirable combination of the 

 arch and the suspension bridge. Under heavy 

 weights, the preparatory tension of the pelvic 

 muscles, as the psoae, pyriformes, and great 

 glutei, will, by more closely approximating the 

 sacrum and ilia, produce the conditions of the 

 arch. But in sudden shocks, the strain will 

 fall more immediately upon the ligamentous 

 suspensory structures, as the sacro-iliac, sacro- 

 sciatic, and ilio-lumbar ligaments, more calcu- 

 lated, by their resiliency, to break their force 

 gradually, and finally overcome them. 



The thick, strong, and elastic fibro-cartilagin- 

 ous pads inserted between the opposing os- 

 seous surfaces of the sacro-iliac and pubic 

 symphyses may be mentioned also, as con- 

 tributing to deaden and break the force of 

 shocks passing through the pelvic arches ; and 

 these being generally, as we have seen, arranged 

 in two layers allowing of limited sliding motion 

 between them, are better calculated to resist 

 sudden shocks passing obliquely than single 

 discs, such as the vertebral, which are chiefly 

 disposed to resist pressure passing directly. 



The posterior projection of the iliac tubero- 

 sities protects the sacrum, deeply situated be- 

 tween them, from direct force tending to 

 produce anterior dislocation, while their in- 

 ternal direction prevents that bone from slip- 

 ping backward between them. 



The great breadth and size of the sacrum 

 in the human pelvis it being proportionally 

 larger than that of any other animal indi- 

 cate its importance as the basis of support to 

 the spine, and the crown of the pelvic arch ; 

 and, in connection with the admirable mecha- 

 nical and architectural arrangements just de- 

 scribed, present a wide contrast to the pelvic 

 structures of animals ; and prove the erect 

 position to have been designed for the habi- 

 tual expression of the dignity of man. 



The thigh of man, when standing, forms one 

 line with the trunk, and makes an obtuse angle 

 with the posterior arm of the pelvic lever ; but 

 in quadrupeds it is directed much more for- 

 wards, and forms an acute angle with both the 

 ilia and the spine. In quadrupeds, the thighs are 

 much closer together and more pressed upon 

 the flanks, and, even when they rest on their 

 haunches, they naturally support themselves 

 on their fore legs. This is even seen to a 

 great extent in the apes and monkeys, so 

 difficult is it for them to maintain the centre 

 of gravity in an erect posture. The extensors 

 and flexors of the thigh on the pelvis are also 

 more developed in man, in order to sustain 

 with more firmness the erect posture. Hence 

 the greater breadth of the hip and buttock, 

 and the bulk of the thigh. The breadth of 

 the pelvis also gives a greater leverage to 

 these powerful muscles. 



In walking, the human pelvis is thrown 

 alternately, on each side, upwards, forwards, 

 and sideways, as the leg on that side is lifted ; 



the trunk keeping its centre of gravity over 

 the bearing leg by swaying regularly to that 

 side, the pelvic hoop being at the same time 

 drawn over the supporting leg by the powerful 

 abducting muscles, the glutei. 



On account of the greater width of the 

 pelvis and trochanters in the female, the 

 centre of gravity oscillates through a greater 

 space, and takes longer time to pass over from 

 one leg to the other, and hence the greater 

 amount of undulation in their gait, especially 

 when running. 



Mechanism of the human pelvis in regard to 

 parturition. As a containing cavity, when 

 completed by its muscular and fascial struc- 

 tures, the pelvis offers a basin-shaped struc- 

 ture with a somewhat triangular superior 

 aperture, the sides of which are formed by the 

 psoae muscles, and the base by the pubes ; and 

 with a moveable floor, formed by fasciae and 

 the levator ani muscle, and perforated by the 

 rectum and generative organs. Its walls are 

 interrupted laterally by the sacro-sciatic and 

 obturator foramina, which are filled by soft 

 and yielding muscular and ligamentous struc- 

 tures, and give way considerably to pressure 

 from within, enlarging the pelvic diameters 

 opposite to them. They afford, in common 

 with the superior and inferior openings, the 

 outlets for the nerves and vessels passing from 

 the lumbar and sacral plexuses and iliac 

 trunks to the inferior extremities and peri- 

 neum. The inferior outlet also transmits the 

 external communications of the pelvic viscera. 

 These are, the bladder supported by the 

 pubis ; the rectum, supported by the sacrum 

 and coccyx ; and the internal organs of gene- 

 ration placed between them. 



In the female these internal organs are more 

 bulky than in the male, and consist of the 

 uterus and its ovarian and vaginal appendages. 

 That there is a relation between the greater 

 size and the functions of these organs, and the 

 greater extent of the female diameters, is 

 evident from the consideration of their simul- 

 taneous development at the period of puberty, 

 and corresponding increase afterwards. 



This relative development of the pelvis 

 seems to extend not only to sex, but to the 

 varieties of mankind, either as an irrespective 

 consequence of primitive formative type, or in 

 regard to the adaptability of the foetal head to 

 the pelvis in the processes of parturition. In 

 either case this adaptation is strikingly illus- 

 trated by the different pelvic forms prevailing 

 in different races of men, which will be found, 

 when considering that branch of our subject, 

 to be markedly assimilated to the form of the 

 skull. 



The pelvic bone, which is of the greatest 

 importance in an obstetric point of view, 

 entering as it does into the formation of 

 both the pelvic brim, cavity, and inferior 

 outlet, is the pubis; and deformities of this 

 bone produce the greatest obstacles to par- 

 turition. The sub-pubic arch is a peculiarity 

 of the human species, it being only imper- 

 fectly developed in the lower animals ; and it 

 has an important bearing upon human par- 



