The Lower Extremity and Pelvis 



123 



Consequently (Fig. 101) the axis of the cavity, extending from inlet to outlet, is 

 curved with a backward convexity. 



The size of the cavity or passage of the true pelvis is naturally a matter of great importance to 

 the obstetrician, and it is necessary to know the average measurements of the different parts of the 

 cavity. For this purpose arbitrary points are taken for measurement, and different observers have 

 made use of different standards ; but there are certain diameters which are recommended by their 

 natural value, so so speak, and these are : (i) Antero-posterior or conjugate, in the middle line ; 

 (2) Transverse, at the widest spot : (3) Oblique (valuable in considering the mechanism of labour, 

 in which the foetal head is obliquely placed), between the sacro-iliac joint and the situation on the 

 opposite wall of the cavity corresponding with the level of the ilio-pectineal eminence. These 

 measurements on the upper aperture of the male pelvis, i.e., at the brim, give as an average 



(1) 4 inches, (2) 5 inches, (3) 4} inches. In the 

 female they are all increased to (i) 4$ inches, 



(2) 5J inches, (3) 5 inches. (The conjugate is 

 measured from the sacral promontory to the top 

 of the symphysis.) 



At the lower aperture or outlet the antero- 

 posterior diameter admits of increase through 

 mobility of the coccyx, but the averages in the 

 male may be given as (i) 3$ inches, (2) 3$ inches, 



(3) Scinches ; in the female as (i) 4^ to 5 inches, 



(2) 4$ inches, (3) 4^ inches. Speaking roughly, 

 the diameters in the female show an increase of 

 J inch or more at the upper aperture and i inch at 

 the lower aperture. 



The corresponding diameters half-way down 

 the cavity are also of interest. In the male they 

 average about (i) 4^ inches, (2) 4! inches, (3) 4$ 

 inches ; in the female (i) 5$ inches, (2) 5 inches, 



(3) 5$ inches - 



The least increase is in the transverse dia- 

 meter : possibly the less curved sacrum in the 

 female accounts for the greater increase in the 

 other measurements. 



If we now compare the measurements in the 

 female pelvis from above downwards we find that 

 the greatest diameter is transverse above, ob- 

 lique in the middle, and antero-posterior below : 



this change is correlated with and responsible for the rotation of the fcetal head that places its long 

 axis in the sagittal direction. 



FIG. 101. Diagram to show the angles made 

 with the horizontal line by the brim and 

 ischip-pubic ramus respectively when the 

 pelvis is in the standing position ; also to 

 show the direction of the axis of the cavity. 



Fig. 102 is an outline of male and female types drawn to the above measurements 

 and reduced to half size. It exhibits also certain sexual features of the Ilium, for 

 the male bone is seen to have a wider iliac fossa with an anterior superior spine some- 

 what incurved : the female bone has a smaller fossa, but more open, and the anterior 

 spines are slightly wider apart, though the crest is not so prominent. The outlines 

 are placed so that the pubes and anterior superior spines fall together in both which 

 is not quite a natural coincidence, for the bony spine is further forward in the female 

 and it can be seen that the pelvis, looked at from above, has its sexual characters, 

 though these are not nearly so marked as they are when it is seen from below. 



The general characters of female bones are usually well exemplified in the pelvis. 



Looked at from the side (Fig. 99) the greater and lesser sciatic notches on the 

 innominate bones are seen to be converted into foramina by the great and small sacro- 

 sciatic ligaments : the former of these bands can be traced as a falciform edge along 

 the inner side of the tuber ischii. 



