360 The Nerves and Muscles of the Leg 
tibialis (digitorum) and tibialis posterior are derived. McMurrich* bases 
this idea chiefly on the supposition that the flexor fibularis is supplied by the 
equivalent of the ramus superficialis medialis, while the flexor tibialis is sup- 
plied from the ramus profundus. In man, at least, the nerves passing to the 
two muscles are very frequently bound up for some distance in a common 
trunk. The flexor tibialis and tibialis posterior seem, however, to be more 
intimately connected during ontogeny than is either of these muscles with 
the flexor fibularis. In many mammals the tibialis posterior is absent 
(Leche). In these it may be undifferentiated from the flexor tibialis. On 
the other hand in several mammals the tibialis posterior is doubled, the 
deeper portion sending a tendon to various structures in the tarsus, or even 
to the base of the first phalanx of the big toe (Le Double, 97). The inti- 
mate relations between the tibial and fibular flexors are revealed by the 
fasciculi which so frequently have been found passing from one to the other 
as well as by their tendons (see Le Double, g7). 
The tibial and fibular flexors are inserted primarily into a deep plantar 
aponeurosis in which tendons are developed in accordance with varied func- 
tions of the foot and digits (Keith, 94). The arrangement of the tendons 
varies greatly in different forms. In many forms the flexor tibialis is rudi- 
mentary. In the chiroptera it is highly developed. For the variation of the 
tendons in the anthropoids and man see Le Double, 97. 
3. Nerve Supply of the Muscles of the Back of the Crus in the Adult. 
a. Relation of Muscle Branches to the Spinal Nerves. 
The difficulty of tracing these nerves back to their sources from the 
spinal nerves is so great that no statistical study of the subject has been 
attempted. It is evident, however, that the main bulk of the nerve fibres 
distributed to the gastrocnemius-soleus group has in general a somewhat 
more distal origin than those going to the deep muscles of the calf. The 
special dissections which I have made serve in the main to support the 
spinal nerve origins given in Quain’s Anatomy. These are as follows: 
popliteus, 4th and 5th lumbar, 1st sacral; soleus, 5th lumbar, 1st and 2d 
sacral; gastrocnemius, lst and 2d sacral; deep musculature of the calf, 
5th lumbar, 1st and 2d sacral. The nerve to the plantaris is given as 
arising from 4th and 5th lumbar and 1st sacral, but the 5th lumbar, 
1st and 2d sacral nerves seem to be the more probable sources of supply. 
* According to McMurrich the flexor tibialis and tibialis posterior of the 
mammals are represented in the reptiles (Lacertilia) and amphibia (uro- 
deles) by the plantaris profundus I (tibialis posterior of Gadow). The flexor 
fibularis is according to this author derived from a portion of the plantaris 
profundis III-II of reptiles (flexor longus digitorum, caput internum, Gadow) 
and the plantaris profundus II of the urodeles (femoro-fibule-metatarsales 
I-III, Hoffmann). 
