312 The Nerves and Muscles of the Leg 
The cutaneous branch of the obturator nerve is not clearly distinguish- 
able in the embryos studied. In embryo XXII the articular branch of 
the nerve to the adductor magnus muscle is well marked. 
2. Individual Muscles. 
Gracilis—The anlage of this muscle becomes distinctly differentiated 
at an early stage (Plate II, Fig. 3; Plate VIII, Figs. 1,2, anda). It 
first appears in a region which corresponds with that in which the nerve 
enters the deep surface of the muscle in the adult, near the junction of 
the proximal with the middle thirds. From this region the muscle ex- 
tends toward its pubic and tibial attachments. 
In urodeles and reptiles (Saurians) the place of the gracilis is taken by 
the pubi-ischio-tibialis, which is innervated by the tibial portion of the 
sciatic nerve in the former and by the sciatic and obturator nerves in the 
latter (Gadow, 82). In all the mammals it is innervated by the obturator 
nerve. In several mammals the origin takes place from the abdominal wall 
anterior to the pubis. The insertion is usually in considerable part into the 
crural fascia and in some forms (edentates) extends to the foot (W. Leche). 
In many mammals the gracilis near its insertion is fused with the sartorius. 
Origin by two heads and fusion near the insertion with the sartorius have 
been found as variations in man. On the whole, however, the muscle is 
singularly independent. 
Adductor brevis. Plate II, Fig. 3; Plate VIII, Figs. 1, 2, and b. 
This muscle is differentiated at first in somewhat close association with 
the obturator externus muscle and with the obturator portion of the ad- 
ductor magnus. From its anlage processes of attachment are sent toward 
the pubis and femur (Plate VIII, Fig. 2). In the adult the muscle is 
usually innervated by a nerve which enters its middle third near the 
proximal border. 
Adductor longus. Plate II, Fig. 3; Plate VIII, Figs. 1 and 2. This 
is differentiated from a muscle mass at first not perfectly distinct from 
that of the adductor brevis and in a region corresponding with that where 
the nerves enter the muscle. From here the muscle extends to its at- 
tachments. In the adult the nerve usually enters the deep surface of the 
muscle in several branches about midway between its tendons of origin 
and insertion. 
Obturator externus. Plate II, Fig. 3; Plate VIII, Figs. 1 and 2. 
This muscle is differentiated from dense tissue lying beneath the obtu- 
rator nerve in the obturator foramen and close to the embryonic hip joint. 
From here it extends to its attachment to the femur. The relations 
of the obturator externus to the pectineus muscle have been described 
