122 



Anatomy of Skeleton 



aperature. The sacrum is fixed between the innominate bones by strong posterior 



sacro-iliac ligaments (see p. 125). It is also attached to these bones (Fig. 99) along the 



length of its borders by the great sacxo-sciatic 

 ligaments passing to the tuberosities of the 

 ischia, and by the lesser ligaments deep t 

 these and fastened by their apices to the ischial 

 spines : these ligaments also reach the coccyx. 

 Now examine the outlet of the pelvis 

 (Fig. 100). Its bony boundaries antero-late- 

 rally are provided by the ischial tuberosities 

 and ischio-pubic rami, and by meeting in front 

 at the lower part of the symphysis they form 

 the pubic or subpubic arch. The coccyx limits 

 the outlet behind, and between this and the 

 tuberosity of the ischium stretches the great 

 sciatic ligament on each side : in the recent 

 state a ligamentous transverse band below 

 the symphysis, called the subpubic ligament, 

 rounds off the angle made by the meeting of 

 the bones of each side. 



It is in the width of the pubic arch that 

 the most striking difference is seen between 

 the male and female pelvis (Fig. 100) : in the 



latter the lines of the rami form a right angle with each other, whereas the male arch 



is only about 70 degrees to 75 degrees or even less. 



It is apparent at a glance that the posterior wall of the true pelvis is higher than 



the anterior : the vertical depth of the back wall is between 5 and 6 inches, and that 



FIG. 99. Right innominate in situ, with 

 sacrum and ligaments. 



FIG. 100. Outlet of pelvis seen from below, with the pubic outlines in male and female. 



of the front wall about. 2 inches. The plane of the pelvic inlet forms an angle of about 

 60 degrees with the horizon, whereas that of the outlet is inclined at an angle of about 

 15 degrees, open in the opposite direction. 



