664 MAMMALIAN ANATOMY 



free from muscular attachment, although covered by the obturator 

 interims, which has its origin partly anterior and partly posterior to 

 the obturator foramen. 



The posterior (dorsal) border (Fig. 520) runs downward, back- 

 ward, and outward. Its upper third is gently arcuate, sharp, and 

 presents at the lower end the blunt, inconspicuous spine which gives 

 attachment on the outer side to the gemellus superior, and on the 

 inner side to the levator ani muscle. Below the spine the border is 

 slightly emarginate and rolled to the outer side, producing a smooth 

 surface, over which the internal obturator glides in order to join the 

 external obturator and the gemelli muscles passing to the femur. 



The inner (ventral) border faces inward, downward, and forward. 

 It is emarginate and continuous with the border of the ramus of the 

 ischium and the border of the pubes, and forms part of the edge of 

 the obturator foramen. 



The external border (Fig. 520) faces downward, outward, and 

 forward, and is directed from the acetabulum downward, backward, 

 and slightly outward. It is often obscure above, but is more distinct 

 at its lower end, which joins the tuberosity. It forms the external 

 margin of the area of origin of the gemellus inferior muscle. 



The ramus, or ascending ramus, of the ischium is a thin, hook- 

 shaped, bony plate which is given off from the inner side of the lower 

 part of the body of the bone at right angles to its long axis. From 

 its origin it becomes gradually narrower, passes inward and forward, 

 and then curves upward and forward to join the descending ramus of 

 the pubes. It thus forms the bony arch which encloses the lower 

 posterior third of the obturator foramen. It presents an external 

 (ventral) and an internal (dorsal) surface and a ventral and a dorsal 

 border. 



The external surface of the ramus of the ischium (Fig. 518) is the 

 continuation inward of the lower part of the external surface of the 

 body of the bone. Its emarginate upper edge is the dorsal border 

 bounding the obturator foramen. It is separated from the pubes by 

 an imaginary transverse line at the lower third of the foramen. Below, 

 it is limited by the arcuate ventral border, which is formed behind by 

 the union of the two lips of the tuberosity, then turns upward at the 

 inner end, and joins the corresponding part of the border of the 

 ramus of the other ischium to form the lower part of the median 



