298 The Nerves and Muscles of the Leg 
nerve of the thigh and the sural nerve are both shifted over the dorsal 
(primary medial) surface of the thigh during development, but none 
the less distribute their cutaneous branches from a line which corre- 
sponds to some extent to the posterior margin of the lmb-bud. The 
line along which the anterior cutaneous nerves of the thigh reach the 
fascia likewise corresponds with the original anterior margin of the 
limb-bud. In Plate VII, Figs. 1 and 2, a schematic diagram is given 
to illustrate the mode of distribution of the cutaneous nerves of the 
adult limb. So far as possible each main nerve is represented approxi- 
mately as it occurs with the greatest frequency. 
This mode of distribution of the cutaneous nerves from the region 
of the margin of the limb-bud is probably due to a differentiation of 
function, the musculature of the limb-bud being differentiated on the 
medial and lateral surfaces and the margins serving for the primary 
development of the cutaneous areas. In sharks the cutaneous branches 
suppled to the dorsal and ventral surface of the fin extend upwards in 
numerous branches between the muscle bundles. In most higher forms 
a distribution of cutaneous nerves from the margins of the limb is well 
marked, although numerous exceptions occur. 
It is of interest to inquire whether or not anything may be found 
during embryonic development to account for the segmental distribu- 
tion of the nerves of the limb described on the basis of physiological 
and clinical evidence by Head, Sherrington, Bolk and a large number 
of other investigators. It is quite certain that no evident dermatomes 
associated with specific spinal nerves are to be found in the embryo. It 
seems probable that the cutaneous nerve fibres contained in a given 
spinal nerve find a path of least resistance toward the marginal area 
lying most directly opposite and that to any given area one or two 
nerves may thus serve to furnish the bulk of the fibres. In Figs. 2 and 
3 I have shown diagrammatically the approximate marginal areas to 
which each spinal nerve most directly contributes in the embryo. Sub- 
sequently these areas become extended by the growth of branches from 
the margins of the limb over the medial and lateral surfaces. 
Thé great variation in the distribution of the nerves supplied to 
different areas can be best accounted for, I think, by assuming that the 
nerves grow as plants grow: in part they are guided in their course by 
definite paths, as climbing plants may be guided by strings, but 
where definite paths are not offered great variation in the distribution 
of the cutaneous rami may be seen. Extensive development of one nerve 
tends to retard its neighbors, lack of development tends to excite them 
to more active growth. 
