344 HUMAN ANATOMY. 



greater lightness of the female skeleton shows particularly in this part of the pelvis. 

 It is owing to the greater divergence of the rami that the front of the obturator fora- 

 men is straighter in the female, making it more triangular and less oval than in the 

 male. The spines of the ischia are farther apart in woman. According to Verneau/ 

 those in man are rarely more than 10.7 centimetres (4^ inches) apart, and often 

 less than 9 centimetres (3% inches) ; while in woman they are often more than 10.7 

 centimetres apart, and never less than 9 centimetres. He states also that in man 

 the spines of the ischia are sometimes internal to the posterior inferior spines of the 

 ilia, but that they are always external to them in woman. The sacro-sciatic notch 

 is usually wider and less deep in the female. There is much irregularity in regard 

 to the false pelvis. The anterior superior spines of the ilia are farther apart in 

 woman. It does not follow that the same is true of the most lateral points of the 

 crests of the ilia. In powerful male bodies they are farther apart than in female 

 ones. The vertical depth of the false as well as of the true pelvis is greater in 

 the male. As has been stated elsewhere, the male sacrum is the more regularly 

 curved. 



Development. — The pelvis of the foetus and infant is strikingly small, and 

 continues relatively so for some years. The peculiarity of its shape is largely due 

 to the sacrum. Even at birth there is but a very rudimentary promontory, and the 

 sacrum is straight or nearly so. Consequently the pelvis is funnel-shaped, being 

 largest above. The height is greater in proportion to the breadth than later. It 

 has been shown by Fehling ' and Thompson ^ that the sex of the pelvis may be 

 recognized by the usual signs as early as the fourth month of foetal life. In the 

 foetus the transverse diameter of the inlet exceeds the conjugate, especially in the 

 female. The average subpubic angle of the foetus is 50° in males and about 68° in 

 females. In the latter the ischial spines are farther apart and the sacro-sciatic 

 notches wider. Although after birth the promontory becomes stronger, it has a 

 tendency to be double partly above and partly below the first sacral. This is cor- 

 rected at a very indefinite time in early childhood. Of the details of the changes by 

 which the great difference between the sexes is brought about we know very little. 

 Waldeyer* states that the external measurements of the female pelvis surpass those 

 of the male from the eleventh to the fifteenth year, but particularly from the four- 

 teenth to the sixteenth. The growth of the male pelvis is more regular. 



Mechanics of the Pelvis. — The mechanical function of the human pelvis, 

 apart from protecting the viscera, is chiefly to support the spine, whether sitting or 

 standing. The interruptions of the bony girdle at the symphysis and the sacro-iliac 

 joints add to the strength of the structure and break shocks. There is, however, a 

 real motion at the sacro-iliac joints which, slight under ordinary circumstances, is of 

 importance in childbirth. The weight of the body transmitted through the spine 

 m'^y theoretically be said to tend to force the sacrum down between the innominate 

 bones and also to carry the promontory downward and forward into the pelvis, the 

 sacrum rotating on a transverse axis passing through the second piece at the sacral 

 canal. Motion in the former direction does not occur, but in the latter it may to 

 a slight degree.^ With the body lying on the back, if the legs are strongly flexed 

 and pressed against the abdomen, the pelvis rotates on the sacrum, the symphysis 

 rises, and the antero-posterior diameter of the inlet is lessened ; if the legs be strongly 

 extended by being brought down over the edge of the table, this diameter is in- 

 creased, the difference between the extremes being one centimetre. At the end 

 of pregnancy these joints, as well as that of tha symphysis, are loosened so as to 

 admit of more motion, which is no doubt of real value. Assuming, as at first, the 

 pelvis to be the fixed part, the tendency to displacement of the sacrum in either of 

 these directions is resisted by the posterior sacro-iliac ligaments. The sacrum is not 

 really a keystone, for the anterior surface is broader than the posterior, except in 

 some few sections. 



^ Le bassin dans les sexes et dans les race, Paris, 1870. 



' Arch, fiir Gynakol., Bd. x., 1876. 



' Journal of Anatomy and Physiology, vol. xxxiii., 1899. 



* Das Becken, Bonn, 1899. 



' G. Klein : Zeitschrift fiir Geburtshiilfe und Gynakol., Bd. xxi., 1891. 



