364 The Nerves and Muscles of the Leg 
near the superficial surface of the muscle about midway between its origin 
and insertion and parallel with the tendon of the flexor digiti quinti 
longus. I have never seen the nerve for this muscle arise from the medial 
plantar nerve as described in the anatomy of Poirier and Charpy. 
In the adult this muscle is frequently reinforced by a fasciculus which 
may arise from either of the bones of the crus, from one of the deeper 
muscles of the crus, from the deep muscle fasciew, or from the calcaneus, 
Le Double, 97. The muscle may be inserted into any of the digital ten- 
dons, but most frequently into the 2d, 3d, and 4th; into that to the 5th 
toe less frequently ; into that to the great toe rarely. 
MeMurrich, 04, on phylogenetic grounds thinks that the quadratus plante 
is differentiated from the distal end of the same deep layer of crural muscles 
from which are derived the tibialis posterior and the flexor digitorum (tibial 
flexor). Schomburg, on the other hand, considers it more intimately related 
to the flexor fibularis, a point of view strengthened by the fusion which he 
found between the quadratus plante and the flexor hallucis longus in a young 
embryo. As mentioned above, I did not find this connection in the 14 mm. 
embryo. Nor does the nerve supply of the muscle indicate a close union be- 
tween it and the tibialis posterior or either the tibial or the fibular flexor. 
The quadratus plante ‘is clearly represented in the lacertilia where it is 
supplied by a branch of the ramus profundus.” (McMurrich, o4). 
In monotremes it arises from the calcaneus (Leche). In the majority of 
marsupials it is probable that it exists in a rudimentary condition (Mc- 
Murrich). In edentates it is absent in some forms, well marked in others. 
In some insectivora it is fused with the abductor metacarpi digiti minimi. In 
the higher mammals it is absent in some forms and well developed in others 
(i. e., dog and cat). In some apes it is fused with the flexor digitorum 
tibialis (Leche). 
M. abductor digiti quinti.—In the 14 mm. embryo (Plate IX, Fig. 4) 
the anlage of this muscle may be seen immediately distal to the tuber 
calcanei and lateral to the n. plantaris lateralis. In an embryo of a cor- 
responding age Schomburg, oo, pictures the muscle as extending to the 
4th metatarsal, but I have found no corresponding condition in the 
embryos I have studied. In embryo XXII, length 20 mm. (Plate IX, 
Fig. 6) the muscle extends to the base of the 5th metatarsal and has a 
more lateral position than in the 14 mm. embryo. At this stage a nerve 
may be seen extending into the medial margin of the muscle from the 
deep surface of the lateral plantar nerve. 
Jn the adult the muscle is developed medially so as partially to overlap 
the lateral head of the m. quadratus plante. It varies greatly in struct- 
ure. The main bulk of the fibre bundles usually extends somewhat 
obliquely from the caleaneus, the plantar fascia and the tendinous 
aponeurosis on the lateral side of the muscle near its origin to a tendon 
