Charles R. Bardeen Bae 
trochanter across and then upwards for a short distance on the pelvic 
surface of the ischium toward the obturator foramen. No distinction 
can at this time be made between the obturator internus and the two 
gemelli. From the sacral plexus a nerve branch may be seen extending 
across the outer surface of the muscle. Beneath the muscle another nerve 
may be traced to the anlage of the quadratus femoris. 
In an embryo 20 mm. long (Plate VIII, Fig. 5) the obturator internus 
has extended well over the obturator foramen and in its growth into the 
pelvis has carried its nerve in the same direction. The gemelli cannot 
yet be clearly distinguished from the obturator internus. A good descrip- 
tion of the architecture of these muscles in the adult and of the distribu- 
tion of nerves to them is given in the J’raité @anatomie humain of Poirier 
and Charpy. In the adult the chief variations in structure are those of 
a greater or less independence of the gemelli and a greater or less extent 
of the pelvic attachments of the obturator internus. 
Quadratus femoris ——This is differentiated comparatively early in a re- 
gion lying between the anlage of the great trochanter and that of the 
tuber ischii (Plate III, Fig. 1; Plate VIII, Fig. 4). It soon forms at- 
tachments which correspond well with those of the adult muscle (Plate 
VIII, Fig. 5). In the embryo, as in the adult, the nerve enters the deep 
surface of the muscle near the junction of the middle and ischial thirds. 
In the adult the muscle is frequently fused either with the inferior 
gemellus or with the adductor minimus. Its nerve of supply may extend 
into the adductor minimus. 
The quadratus femoris, gemelli, and obturator internus muscles of mam- 
mals are apparently related to the ischio-femoral musculature of urodeles 
and the pubi-ischio-femoralis posterior (Gadow) of reptiles. Among the 
mammals the obturator internus is said not to be found in the monotremes 
(W. Leche) but it occurs in most, although not all, of the higher forms. The 
degree of isolation of the gemelli and the mode of attachment of the obturator 
internus vary considerably in different mammals. The quadratus femoris 
seems to be a fairly constant muscle in the mammalian series. In a con- 
siderable number of mammals, however, it is innervated by the obturator 
nerve instead of by a special branch from the sacral plexus (see W. Leche). 
I do not know of an instance of this kind being reported as a variation in 
man. The innervation of the adductor minimus portion of the adductor 
magnus by the nerve to the quadratus femoris is, however, frequent and 
rarely this nerve may send a branch to the M. obturator externus. The 
adductor minimus is normally supplied chiefly by the obturator nerve. In 
Talpa the quadratus femoris and obturator externus are fused and the com- 
bined muscle is supplied both from the obturator nerve and from the sacral 
plexus (W. Leche). 
