408 The Phylogeny of the Plantar Musculature 
the flexor brevis superficialis (fbs). Dorsal to this is a layer, consisting 
at this level of four distinct portions, which is the flexor brevis medius 
(fbm) ; resting directly upon the metatarsals is the third layer, showing 
indications of division into a number of subordinate portions, and form- 
ing the flexor brevis profundus (fbp) ; and, finally, extending between the 
adjacent surfaces of contiguous metatarsals, are the representatives of the 
fourth layer, the intermetatarsales (vm). 
The plantar aponeurosis and flexor brevis superficialis. The plantar 
aponeurosis is the direct continuation of the strong aponeurosis which 
covers the muscles of the crus, and over the metatarsals it divides into 
five slips, which pass to the various digits; the slips to the hallux and 
Fig. 1. Transverse section through the foot of Amblystoma. fbm = flexor 
brevis medius; fobp—flexor brevis profundus; fbs=— flexor brevis super- 
ficialis; im Zintermetatarsales; 7p—lJateral plantar nerve; mp —medial 
plantar nerve; pa—plantar aponeurosis; J/-V —=metatarsal bones. 
minimus had already separated at the level of the section shown in Fig. 1. 
More proximally, over the tarsals, the aponeurosis receives upon its dorsal 
surface the insertion of the majority of fibers of the plantares profundi 
of the crus, these muscles acting on the phalanges through the aponeuro- 
sis. In tracing a series of sections from the crus downwards into the foot 
one finds the plantares gradually diminishing in size as their fibers insert 
into the aponeurosis, until they are represented only by a few slips which 
are prolonged further distally than the main masses of the muscles. But 
just as one begins to expect these slips to completely disappear, they begin 
to enlarge and more distally form the continuous sheet of muscle which 
is represented in Fig. 1 as the flexor brevis superficialis, this muscle, ac- 
cordingly, appearing to be the direct continuation of the plantares pro- 
