ARRANGEMENT OF THE FASCLE OF THE PELVIS. h 



homologous muscles of the levator aui, and in most cases it 

 position is perineal rather than pelvic. This is due to the 

 extension of the ilio- and pubo-coccygei muscles upwards to a 

 higher level on the side wall of the pelvis. In ungulates, the 

 obturator fascia is not so uniform in thickness, being here and 

 there strengthened by longitudinal fibres, but no muscles take 

 origin from it. It is noteworthy that in these animals, in which 

 neither the ilio-coccygeus nor the pubo-coccygeus are apparently 

 represented, the fascia obturatoria exhibits a condition which 

 seems peculiar to the group. 



Apparently, the most important determining factor in modify- 

 ing the arrangement of the fasciae on the side wall of the pelvis 

 is the change in position of the origin of the ilio-coccygeus 

 muscle. At least one stage in the phylogeny of this muscle is 

 the descent of its origin on the lateral wall of the pelvic cavity 

 from the ilio-pectineal line, to the ischium as found in the rabbit 

 (Holl), or to the fascia obturatoria as in man. 



I have already given an account of the descent of this 

 muscle ^ in a young kangaroo. In the specimen described and 

 figured in PI. XIII. the ilio-coccygeus consisted of three parts, 

 viz., the upper, middle, and lower divisions. 



The upper division, dorsal in position, arose from the iiio-. 

 pectineal line just above the strongly marked ilio-pectineal 

 tubercle, and quite close to the sacrum. The middle and lower 

 divisions, ventral in position, were attached to the side wall of 

 the pelvis ; the former of these was situated immediately below 

 the obturator foramen, the latter a short distance further down. 

 The lower division could only be satisfactorily studied by 

 removing the middle layer, and it was then seen to have a 

 remarkable tendinous origin, one quarter to half an inch in length, 

 which appeared to come from a ' white line.' On close examina- 

 tion, however, it was clear that this was simply the attachment 

 of the tendon to the periosteum covering the bone. The line of 

 attachment coincided with one drawn horizontally backwards 

 from the lower margin of the symphysis pubis to the posterior 

 border of the ischium. 



Lower down, the three divisions fused together and formed a 

 rounded muscular bundle, which passed on to the under surface 

 1 The Myology of the Pelvic Floor, 1899, p. 76. 



