424 The Phylogeny of the Plantar Musculature 
nings, or, as lumbricales, but they cannot properly be regarded as belong- 
ing to that group of muscles. 
The flexor brevis medius stratum superficiale. This layer is represented 
in the mammals in part by the lumbricales. In the opossum these are 
four in number (Fig. 6, fbms), arising from the tendon of the flexor 
fibularis just as it divides into the tendons for the four lateral digits. 
Three of the muscles consequently arise in the angle between the four 
diverging tendons and the fourth from the tibial side of the tendon to 
the index. The muscles pass to the tibial side of the base of the proxi- 
mal phalanx of the second, third, fourth, and fifth digits. In the mouse 
there are also four lumbricals arising in the angles formed by the split- 
Fic. 6. Transverse section through the foot of the opossum. abV’ =ab- 
ductor quinti digiti; fom, —flexor brevis medius str. profundum; fom, = 
flexor brevis medius str. superficiale; fop—flexor brevis profundus; fbs,= 
flexor brevis superficialis str. profundum; fbs;— flexor brevis superficialis 
str. superficiale; fid—tendon of flexor longus digitorum; flm—=tendon of 
flexor longus hallucis; 7p =lateral plantar nerve; mp — medial plantar nerve; 
Ss = metatarso-phalangeal sesamoid cartilage of hallux; J-V = metatarsals. 
ting of the long flexor tendon and passing to the same digits as in the 
opossum, while in the cat there are only three muscles in the group, that 
for the second digit being lacking. 
In the opossum and mouse the muscle to the second digit is supphed 
by a branch of the medial plantar nerve, while the other three are sup- 
plied by the lateral plantar. In the cat all the muscles of the set are 
supplied by the lateral plantar, the muscle to the second digit being, as 
stated, wanting. 
In the Jacertilia a large muscle belonging to the superficial layer of 
the flexor brevis medius passes to the hallux, and in the opossum what I 
believe to be the same muscle (Fig. 6, fbm’s) occurs. It arises in close 
association with the hallucal portion of the flexor brevis superficialis, 
