THE PELYIC ARCH. 



137 



united to the sacrum, and another, the pubis, joins its fellow 

 from the opposite side, forming the crown of the arch, whilst 

 the ischium is wedged in between them ; these three bones form 

 the ossa innominatum of the human anatomist. 



Figs. 127 and 128 represent these haunch bones, (i) is the 

 ilium (n), the ischium, and (in) the pubis. The broad iliac 

 bones form the brim of the pelvis (fig. 125), they afford sup- 

 port to the viscera of the abdomen, and give attachment by both 

 their surfaces to the large and powerful muscles by which 

 the thigh is moved, and the trunk retained erect upon the 

 lower extremities. The brim of the pelvis (, , a, a) differs in 

 the two sexes. In the male (fig. 126), the greatest diameter is 

 in the antero-posterior ; in the female (fig. 125), in the trans- 

 verse direction. A comparative view of the outlet (b, b, b, b) 

 (figs. 129 and 130) in a male and female pelvis, shews this 

 opening to be of a diamond form, having the angles before, 

 behind, and on the sides. In the male (fig. 130), the outlet is 



Fig. 129. Outlet. Fig. 130^ 



Female. 



Male. 



small; in the female (fig. 129), it is large. The greatest 

 diameter is from the sacrum to the pubis in the female, in con- 

 sequence of the sacrum being less curved than in the male. The 

 space comprised between the brim and the outlet is called the 

 true pelvis, in which the pelvic viscera are lodged. On each 

 side of the pubic arch a large oval hole (obturator foramen), 

 is formed by the ischium and pubis. It Fig. 131. 



gives passage to blood vessels and nerves, 

 and is partly closed by a ligament. On 

 each side of the obturator hole, but some- 

 what behind that opening, is the cup- 

 shaped cavity for receiving the head of the 

 thigh bone (acetabulum) (fig. 131, e), 

 formed by the junction of the ilium (i), 



