Charles R. Bardeen 319 
lateral and medial plantar nerves are separated from one another. 
Nerves are given to the various muscle anlages. In an embryo 20 mm. 
long (Plate V, Fig. 1; Plate VIII, Figs. 2 and 3; Plate IX, Figs. 5 and 
6) the muscles of the foot are beginning to be differentiated, the medial 
and lateral plantar nerves on the back of the leg have become fused and 
the branches to the various muscles somewhat resemble those of the 
adult. 
b. Adult Conditions.. 
1. Separate Origin of the Peroneal and Tibial Nerves. 
During early embryonic development, as mentioned above, the per- 
oneal and tibial nerves arise separately from the plexus. In about 10% 
of instances studied at the Johns Hopkins University this condition was 
found present in the adult, the two nerves being separated by a portion 
of the piriformis muscle or more rarely by the whole muscle (see Bar- 
deen and Elting, o1). Eisler, 92, found the condition in 18.1%. of 123 
plexuses and Paterson, 94, in 13% of 23 plexuses. The nerves arise 
separately from “normal,” proximal or distal types of plexuses with 
about equal frequency. 
2. Frequency of Variation in Origin of the Peroneal and Tibial Nerves. 
In Tables XVIII and XIX are shown the frequency of various 
modes in origin of the tibial and peroneal nerves from the sacral plexus, 
and the types of plexus with which these various modes of origin were 
associated. No detailed explanation of these tables seems requisite. 
Tabulation of the relation of the various types of origin in relation to 
race, sex, and side of the body has revealed no facts of special interest, 
and hence tables covering these points are here omitted. 
3. Relations of the Branches Springing from the Peroneal and Tibial 
Nerves to the Nerve Roots. 
It is not often possible to trace back with certainty to their origin 
from the plexus the various branches springing from the peroneal and 
tibial nerves. It can be done only under special conditions and cannot 
be well carried out by students in the dissecting room. For this reason 
no attempt has been made to collect statistical data on this subject. The 
following diagram based on special dissections, indicates roughly the 
regions occupied by the chief nerve branches in the peroneal and tibial 
trunks, and their relations to the spinal roots of these nerves. Although 
the peroneal and tibial nerves are usually bound up on the back of the 
