344 The Nerves and Muscles of the Leg 
Plate IX, Fig. 1, the tendon passes lateral to the anlage of the lateral mal- 
leolus. In one of 20 mm. it passes behind this anlage (Plate IX, Fig. 2). 
Proximally in an embryo 20 mm. long (Plate IX, Fig. 2) the origin 
of the muscle extends to the lateral condyle of the tibia next to the 
attachment of the m. flexor digitorum longus. At this period the two 
heads characteristic of the adult muscle may be distinguished. 
A nerve may be seen entering the anlage of the peroneus longus in 
embryo CXLIV (Plate IX, Fig. 1) and in embryo XXII (Plate IX, 
Fig. 2) two nerves to the muscle may be seen. One of these enters the 
deep surface of the anterior head, the other passes distally into the pos- 
terior head. 
From the peroneal nerve as it passes beneath the muscle two branches 
usually arise in the adult. One of these passes to the central third of the 
anterior portion of the muscle, the other extends down across the middle 
third of the deeper muscle bundles which run obliquely from the fibula 
to the tendon of the muscle. The latter branch may arise from the n. 
peroneus superficialis and it may extend to supply the m. peroneus brevis. 
The M. peroneus brevis, (Plate IX, Figs. 1 and 2) arises proximally 
under cover of the peroneus longus and relatively higher up on the fibula 
than in the adult. It lies a httle more on the flexor side of the leg than 
the peroneus longus. When first developed the tendon of insertion of the 
muscle is closely associated distally with that of the m. peroneus longus. 
It hes somewhat near the m. extensor digitorum brevis, but, as mentioned 
above, I can find no such intimate union with this muscle as that which 
Schomburg describes as lasting till the third month of development. 
It is attached to the base of the fifth matacarpal in an embryo 20 mm. 
long (Plate IX, Fig. 2). 
In an embryo 14 mm. long (Plate IX, Fig. 1) a branch of the peroneal 
may be seen entering the muscle. In one 20 mm. long (Plate IX, Fig. 
2) this branch may be readily followed beneath the peroneus longus to 
the peroneus brevis which here occupies a more distal position than in 
the 14 mm. embryo. In the adult the nerve may arise either from the 
distal nerve to the peroneus longus or from the n. peroneus superficialis. 
The nerve enters the proximal margin of the muscle and extends distally 
about midway between the origin and insertion of the constituent fibre 
bundles. 
In amphibians the femoro-fibularis, which extends from the lateral epicon- 
dyle of the femur to the fibula probably represents the peroneal musculature 
of the higher vertebrates. In the reptiles two peroneal muscles are recog: 
