410 The Phylogeny of the Plantar Musculature 
In the ship to the second digit there is a similar transformation of the 
median portion into tendon and an insertion of the muscle fibers adjacent 
to this tendon into the metatarso-phalangeal fibro-cartilages, but there is ” 
only one slip passing to the head of the metatarsal, namely, that to the 
fibular side. The slip to the fifth digit behaves essentially lke that to 
the fourth or third, the only striking difference being the large size of the 
fibular metatarsal slip; but that to the first digit differs from the rest in 
that it fails to separate into subordinate slips, but inserts entirely into the 
metatarso-phalangeal fibro-eartilages. 
In addition to the portions of the flexor brevis superficialis described 
above, another portion is probably represented by the abductor quintt 
digiti, or, as it may be more accurately termed, the abductor ossis meta- 
tarsi V., which arises from the fibular border of the tarsus and inserts 
into the base of the fifth metatarsal, a sesamoid cartilage being developed 
at its insertion. 
The flexor brevis medius takes its origin from the aponeurotic layer 
which lies immediately dorsal to the plantares profundi. It appears as 
four distinct slips, one of which (Fig. 1, fom **) later divides, so that 
there is a slip for each digit. Toward the distal ends of the metatarsals 
the slips which pass to the third, fourth, and fifth digits divide into two 
portions, one of which (Fig. 2, fm), much smaller than the other, hes 
upon the plantar surface of the mediam slip of the corresponding flexor 
brevis profundus, while the other portion (fm’) rests upon the fibular 
slip of the same muscle. This latter portion inserts into the side of the 
head of its metatarsal, in more or less close association with the fibular 
slip of the flexor brevis profundus, and the smaller portion inserts into 
the metatarso-phalangeal fibro-cartilage. The slips to the second and 
first digits do not divide in this manner, but insert entirely into the 
articular fibro-cartilages. 
The flezor brevis profundus is composed of three slips for each digit, a 
median and two lateral (Figs. 1 and 2). The lateral slips arise from the 
tarsal bones, and, in the cases of the marginal digits, partly from the 
plantar aponeurosis. The median slip, on the other hand, arises from the 
plantar surface of its metatarsal, and in the central digits separates the 
lateral slips, which, up to the level of its appearance, form a single mass. 
The lateral slips insert into the heads of the metatarsals, the fibular slips 
of the four tibial digits being intimately associated with the intermeta- 
tarsals, and the same slips of the third, fourth and fifth digits with the 
fibular slips of the flexor brevis medius for those digits. The median slips, 
which are the metatarso-phalangei of Humphry, 72, extend further dis- 
