332 The Nerves and Muscles of the Leg 
c. Embryonic Development of the Short Head of the Biceps. 
In an embryo of 11 mm. (Plate 3, Fig. 2) the anlage of the short head 
of the biceps extends along the distal half of the fibular margin of the 
femur dorso-lateral to the peroneal nerve. Proximally it is continued to 
the anlage of the gluteus maximus. 
In an embryo of 14 mm. (Plate IJ, Fig. 3; Plate VIII, Fig. 4) it 
does not extend proximally quite to the femoral insertion of the gluteus 
maximus. Distally it is beginning to be attached to the tendon of the 
long head of the biceps. The nerve to the muscle which at the former 
stage was not evident may at this stage be seen entering the fibular margin 
of the muscle. 
G. D. Thane mentions an instance in which the nerve to the short head of 
the biceps arose in connection with the inferior gluteal nerve from the sacral 
plexus.” 
In an embryo 20 mm. long (Plate VIII, Fig. 5) both the femoral and 
distal attachments of the muscle are well marked. 
VI. THE MM. OBTURATOR INTERNUS, GEMELLI AND QUADRATUS 
FEMORIS. 
These constitute a distinct group of muscles which are differentiated 
on the ischial side of the anlage of the hip joint. Although closely as- 
sociated, the anlage of the obturator internus and gemelli seems to be 
from its earlier stages of differentiation distinct from that of the quad- 
ratus femoris. I do not find the anlages of these muscles fused at an 
early stage with the gluteal anlages as described by Grafenberg, 04. 
When they first appear (Plate III, Fig. 1) the anlage of the quadratus 
femoris has a somewhat more anterior position than that of the obturator 
internus. This may account for its nerve supply in the adult from a 
more proximal set of spinal nerves. 
a. Embryonic Development. 
Obturator internus and gemelli.mAn indistinct region of tissue differ- 
entiation near the ischium in Embryo CIX, length 11 mm. (Plate III, 
Fig. 1) I take to be the anlage of the obturator internus and the gemelli. 
To it a nerve is given from the sacral plexus. The anlage of these muscles 
is much more distinct in an embryo 14 mm. long (Plate VIII, Figs. 1 
and 4). Here it may be seen extending from the anlage of the great 
1% Quain’s Anatomy, 10th ed. 
