158 ANATOMY OF THE WOOD RAT 



dinosus, the biceps femoris posticus, and a part of the semi- 

 membranosus posticus. It is thus well nigh impossible to 

 unravel the myological stimuli operating upon the bone. 

 The presence or absence of the semimembranosus anticus 

 is individually variable, and to this may be attributable a 

 part of the osteological differences encountered. Upon the 

 caudal border of the ascending ramus of the pubis originate 

 the ischiocavernosus, gracilis posticus, and the more ventral 

 portion of the origin of the semimembranosus posticus. 

 Differences in the pull of these muscles are undoubtedly 

 accountable for the fact that the caudal border of the ramus 

 may be straight, or the ventral portion definitely indented. 



Over most of the lateral face of the ischium originates the 

 quadratus femoris, and along the obturator border of both 

 this bone and the pubis originates the obturator externus. 

 Upon the inferior tuberosity of the ischium arises Poupart's 

 ligament, and closely adjoinmg, parts of the gracilis posticus, 

 rectus abdominis, and adductor brevis. The variation in its 

 shape is great, but this is individual. The only muscle 

 arising upon the medial face of the ischium is the obturator 

 internus from the extensive fossa of the same name. 



The pubis articulates caudad with the ischium at the 

 symphysis, and craniad near the acetabulum with the coty- 

 loid and ischium. It, with the ischium, encloses the obtur- 

 ator foramen. This bone shows but little variation save in 

 length, the latter item being influenced not directly by 

 muscular attachments but rather in accordance with differ- 

 ences in the ascending ramus and the position of the inferior 

 tuberosity of the ischium. 



From the pubis originate a portion of the rectus ab- 

 dominis, adductor longus, gracilis anticus, the anterior por- 

 tion of the adductor brevis, obturator externus, and the 

 adductor magnus. The latter arises chiefly from a point 

 ventrad the acetabulum. The pectineus has origin from 

 the pectineal process upon the ventral border also ventrad 



