416 The Phylogeny of the Plantar Musculature 
Of the slips which pass to the metatarso-phalangeal fibro-cartilages of 
the first, second, and third digits, one (fbp:) arises from the tibial sur- 
face and the other (fbp;) from the plantar surface of the next adjacent 
metacarpal on the fibular side. In the case of the fourth digit one slip 
(Fig. 4, fbp’:) arises from a strong ligament which extends distally 
from the cuboid; a second slip (fbp*;) arises partly from this same liga- 
ment, which, much reduced in size, accompanies it throughout its course, 
and partly from the tibial surface of the fifth metatarsal; while the 
third (fbp’:) takes its origin from the head of the fifth metatarsal in 
close association with the intermetatarsal ligament. All three slips 
insert into the metatarso-phalangeal fibro-cartilages of the fourth digit. 
The third slip from its position might readily be interpreted as a 
portion of the flexor brevis medius, stratum profundum. The fourth 
digit, however, has another slip which is plainly a part of that layer, 
and, furthermore, the deep branch of the lateral plantar nerve passes 
to its deep position between the slip under discussion and the flexor 
brevis medius, the slip, therefore, lying practically dorsal to the nerve 
layer and having the same relation to it as the other deep flexors. Its 
identification as one of these makes it seem probable that it really rep- 
resents one of a fifth pair, its fellow being a slip (Fig. 3, fbp’ ¢), which 
arises from a ligament extending between the talo-calcaneus and the 
base of the fifth metatarsal. It passes distally upon the fibular surface 
of the metatarsal, parallel to the lower part of the plantaris superficialis 
lateralis, from which it is separated by the lateral plantar nerve, and 
inserts into the fifth metatarsal near its distal extremity. 
In Iguana I find essentially the same arrangement of the flexores 
breves profundi, although there are slight differences in detail. The 
first and second digits each receive two slips, but the slip to the third 
. digit could not be divided into two portions. It arose, however, partly 
from the tibial surface and partly from the plantar surface of the 
fourth metatarsal and consequently agreed with the two slips found in 
Scincus. To the fourth digit three slips can be distinguished, of which 
that corresponding to fbp*; is much the most prominent and completely 
conceals fbp':, which is represented by a narrow and thin band of fibers, 
inseparable at its origin from fbp*;, although diverging from it distally. 
The third slip, which arises from the base of the fifth metatarsal, is quite 
small and after a short course unites with the fourth intermetatarsal. 
The single slip to the fifth digit is much stronger than in Scincus, cov- 
ering the whole plantar surface of the metatarsal and having upon it 
in the median line the tendon of the flexor plantaris profundus. 
