426 The Phylogeny of the Plantar Musculature 
In the mouse the layer is represented by only three muscles, that to the 
fourth digit being wanting. They arise from the strong fibrous sheath 
which invests the peroneus longus and insert into the fibular side of the 
bases of the proximal phalanges of the first and second digits and into 
the tibial side of the corresponding phalanx of the minimus. In the cat 
the reduction in number-is carried one step farther, in that the muscles 
pass to only two digits, namely the index and minimus (Fig. 7, fbm,'’), 
but the latter digit receives two slips, one of which is inserted into the 
tibial side of the base of the proximal phalanx, while the other inserts 
into the tibial side of the metatarsal near its head and forms what has 
been termed the opponens minimi digiti. In both forms all the muscles 
are supplied from the lateral plantar nerve. 
Fic. 7. Transverse section through the foot of the cat. abV —abductor 
quinti digiti; fbm,—flexor brevis medius str. profundum; fbm, = flexor 
brevis medius str. superficiale; fbp— flexor brevis profundus; /fbs,=— flexor 
brevis superficialis str. profundum; fbs— flexor brevis superficialis, str. super- 
ficiale; im —intermetatarsalis; JJ-V = metatarsal bones. 
The flexores breves profundi and the intermetatarsales are so intimately 
associated in all the forms studied that they may be considered together. 
Compared with the lacertilia it is noticeable that the intermetatarsales 
are represented by muscles, instead of ligaments, and that there is a very 
different arrangement of the flexores profundi with reference to the vari- 
ous digits. Instead of a general inclination of the slips from the fibular 
to the tibial side one finds that in the mammala they are almost directly 
longitudinal in their course and that certain of them are so fused with 
the intermetatarsales as to be distinguishable from them only by inter- 
vening fibrous bands, in some cases by a more or less pronounced differ- 
ence in the direction of their fibers and by their more plantar position. 
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