Charles R. Bardeen 359 
to it and the m. flexor digitorum longus. The nerve enters the proximal 
extremity of the muscle anlage. 
In an embryo 20 mm. long (Plate IX, Figs. 5 and 6) the muscle occu- 
pies a relatively somewhat more proximal position and is somewhat more 
under cover of the soleus. It is attached to the blastema of the shaft 
of the fibula and distally is inserted into the deep surface of the plantar 
aponeurosis. The nerve runs along and enters the tibial margin of the 
muscle. 
In the adult the nerve or nerves to the muscle run along its tibial 
margin or deep surface and send twigs into its substance. 
M. flexor digitorum lonyus—This is differentiated from an anlage 
medial to that of the m. flexor hallucis longus. In the 14 mm. embryo 
(Plate IX, Fig. 3) it lies beneath the n. plantaris medialis which gives 
two branches to the upper extremity of the anlage. The muscle extends 
into a somewhat irregular plantar aponeurosis of which mention has 
been made in connection with the m. flexor hallucis longus. The tendons 
are partially differentiated. ‘The anlage of the muscle nearly covers that 
of the m. tibialis posterior. Schomburg found in the leg he reconstructed 
that the tibial side of the muscle had not reached the tibia. In embryo 
CXLIV this is also true. The tibial attachment has begun to take place 
in embryo XXII, length 20 mm. (Plate 1X, Fig. 5). In this embryo 
also the pedal aponeurosis has become still further differentiated into 
tendons, but it is not yet possible to distinguish clearly the tendons be- 
longing to the fibular flexor (flexor hallucis longus) from those belong- 
ing to the tibial flexor (flexor digitorum longus). ‘Two nerves enter the 
muscle on its superficial surface. One of these extends to the fibular 
side of the muscle, the other to the tibial side. A similar arrangement is 
usually found in the adult. 
M. tibialis posterior—This muscle is formed from the deeper layer of 
the tibial portion of the flexor anlage near the lateral portion of the lower 
half of the tibia (Plate IX, Fig. 4). Its tendon is differentiated early 
and may be followed to the anlage of the navicular. In subsequent de- 
velopment, as pointed out by Schomburg, it develops in a proximal and 
lateral direction (Plate IX, Fig. 6). Its nerve enters near the tibial 
border of the anlage. In the adult the nerve enters the posterior surface 
of the muscle in its proximal third and gives off one or two branches for 
the tibial fasciculus. The main trunk descends across the centers of the 
fasciculi arising from the fibula. 
Comparative anatomy of the deep plantar muscles of the crus.—Hisler, 
95, and MecMurrich, 04, consider that the flexor fibularis (hallucis) is 
derived from a layer primarily superficial to the layer from which the flexor 
