428 The Phylogeny of the Plantar Musculature 
partly over the fourth space and partly over the tibial border of the fifth 
metatarsal. The two more tibial bundles, which probably represent but a 
single muscle, take their origin from the sheath of the peroneus longus 
tendon, while the fibular one seems rather to come from the base of the 
fifth metatarsal. Additional fibers are added to the fibular muscle from 
an origin on the tibial surface of the fifth metatarsal. These fibers are 
easily recognizable from their decidedly oblique direction, and it seems 
probable that they represent a distinct muscle, namely, the intermetatar- 
salis IV. Eventually the two more tibial bundles insert into the tibial 
metatarso-phalangeal sesamoid, while the fibular muscle, together with 
the intermetatarsalis 1V terminates on the fibular sesamoid. : 
Over the plantar surface of the fifth metatarsal two muscles, which 
arise from the sheath of the peroneus longus tendon, occur. They are 
separated by a distinct tendinous partition, and, as they are traced distally, 
separate to be inserted into the two sesamoid cartilages of the metatarso- 
phalangeal joint. 
Finally, I find beneath the hallucal slip of the flexor brevis medius str. 
superficiale and between that muscle and the conjoined flexor brevis pro- 
fundus I and intermetatarsalis I a small bundle of muscle fibers, which 
may possibly represent an additional slip of the flexor brevis profundus 
layer. It is so rudimentary, however, that it is impossible to assign to it 
an origin or an insertion; it is distinctly separate from the combined in- 
termetatarsal and flexor profundus I and its fibers have a direction almost 
at right angles to those of the flexor brevis medius str. superficiale above 
it. No equivalent of the muscle occurs either in the mouse or the cat, in 
both of which the hallux is considerably reduced as compared with that 
of the opossum. 
In the mouse the arrangement of the flexor brevis profundus is very 
similar to that of the opossum, except that the various slips are much 
more intimately fused and consequently much more difficult to recognize. 
Over the base of the first metatarsal there is a mass of muscular tissue 
in which no indications of a composite nature €ould be detected. It passes 
to the tibial metatarso-phalangeal sesamoid of the second digit and seems 
therefore to correspond to the combined intermetatarsal and flexor pro- 
fundus I of the opossum. Over the second metatarsal are two muscle 
slips separated only by a tendinous partition. They both insert into the 
fibular metatarso-phalangeal sesamoid of the digit, the arrangement 
differing in this respect from that of the opossum. ‘The muscles of the 
third digit are arranged as in the opossum, though much more exten- 
sively fused. The mass which they form curves dorsally around each 
surface of the metatarsal and the lateral parts of the crescent so formed 
