358 The Nerves and Muscles of the Leg 
posterior surface of the muscle. Rarely a slender second branch enters 
the superior margin of the muscle (2 in 25 instances). 
The place of the popliteus is taken in the lower mammals, the amphibia 
and reptiles by an interosseous muscle, pronator tibie, which passes obliquely 
from the fibula to the tibia. A popliteus muscle corresponding essentially 
to that of man is found in nearly all mammals except the monotremes and 
marsupials. A popliteus in addition to a pronator tibie is likewise described 
for Myrmecobius (Leche). The popliteus is said to be absent in most chirop- 
tera (Leche). In the dog in addition to the popliteus there is a small fibulo- 
tibial muscle in the proximal part of the interosseous space. A similar muscle 
(the peroneo-tibialis, Gruber) has been found in a number of mammals and 
not infrequently as a variation in man (128 times out 860 instances, Gruber). 
It seems probable that the popliteus is an especially differentiated portion of 
the fibulo-tibial muscle of the lower vertebrates, and that its origin has ex- 
tended from the fibula to the lateral condyle of the femur. LHisler, g5, 
considers it homologous with the brachialis anterior of the arm. According 
to McMurrich the muscle in the mouse receives two nerve branches, one asso- 
ciated with that for the soleus from the “internal popliteal stem,” the other 
from the deep muscle nerve of the crus. The former is supplied to the more 
tibial oblique-fibered portion of the muscle, the latter to the more vertical 
fibular portion. From these facts McMurrich concludes that the popliteus is 
a compound muscle consisting of a portion derived from the “ plantaris super- 
ficialis’”’ and a portion which represents a part of the pronator tibie of the 
marsupials and the interosseous of the lower vertebrates. That it is there- 
fore similar to the pronator teres of the arm. While this may be true 
of the muscle in some of the mammals it does not seem to be true for the 
muscle as it is found in man. A double innervation is infrequent in man. 
During embryonic development the muscle appears to come from a single 
anlage which lies deeper than the tibial nerve. Gordon Taylor and Victor 
Bonney, o5, conclude that the popliteus is homologous with the deep por- 
tion of the pronator teres while the superficial portion of the pronator teres 
is homologous with the gastrocnemius. Occasionally in man a second head 
may arise medially from above the lateral condyle. This may possibly be 
equivalent to the superficial portion of the pronator teres. According to Le 
Double the m. popliteus biceps coincides frequently with the absence of the 
plantaris. 
b. Deep cruro-pedal group. M. flexor hallucis longus.—The anlage 
of this muscle is distinct from those of the other muscles of the calf in 
an embryo of 14 mm. (Plate IX, Figs. 3 and 4). Lateral to the anlage 
lies the caleaneus, the tendon of Achilles and the distal end of the soleus. 
On the tibial side it slightly overlaps the anlage of the tibialis posterior. 
Proximally it extends nearly to the head of the fibula. It lies beneath the 
n. plantaris lateralis which in this embryo separates high up from the 
n. plantaris medialis. Distally it terminates in an aponeurosis common 
