Charles Russell Bardeen 235 
lumbo-sacral nerves give rise to the main branches destined for the 
limb-plexuses, and the thoracic nerves give rise to dorsal and ventral 
divisions and to branches for the sympathetic system. Frequently the 
primary branches extending outward from the main nerve-trunk of a 
given cerebral or spinal nerve become combined with certain branches 
derived from another nerve, and thus give rise to a common peripheral 
trunk containing fibres derived from two or more sources. Thus in 
man, a branch from the tenth joins the trunk of the ninth cranial 
nerve and the fibre-bundles of the eleventh cranial nerve, themsélves 
derived from a series of anlages, become to a greater or less extent 
bound up with the main nerve-trunk derived from the anlage of the 
tenth eranial nerve. In the region of the limbs, by the processes of 
anastomosis and of plexus-formation, fibre-bundles immediately con- 
nected with the large areas of the spinal cord become united into a few 
nerve-trunks which can grow forward with a minimum expenditure of 
energy towards a region of ultimate distribution. 
The forces which direct the early embryonic nerves toward the anlages 
for which they are destined are at present shrouded in mystery. There 
is much, however, which indicates that to a certain extent the nerves 
take a path of least resistance offered by the surrounding tissues and that 
they are guided to a considerable extent by the more fixed structures 
lying in the line of their general growth. The nature of these paths of 
least resistance can hest be studied, however, in connection with the de- 
velopment of specific nerves. ‘Thus the intercostal nerves are, to a 
considerable extent, guided by the costal processes of the embryonic 
vertebral column and by the myotomes. There is certainly the possi- 
bility, however, that given regions may exert a specific directive attrac- 
tion on the nerves which are destined to supply them. 
As a result of the primary nerve distribution certain nerves are 
directed toward various cutaneous areas and certain nerves are directed 
into the anlages of the muscular apparatus. It is convenient to take up 
separately the distribution of these two sets of nerves. 
The distribution of the cutaneous nerves may be followed with com- 
parative ease during the early stages of embryonic development. Up to 
the period when,a human or a pig-embryo reaches a length of two centi- 
meters the cutaneous nerves may be followed readily in serial sections 
of embryos hardened in Zenker’s fluid and stained in iron hematoxylin 
followed by Congo-red or in other intense stains. The relations of the 
nerves may best be understood by making reconstructions in wax or 
projection drawings. Thus in a previous article in this Journal, Lewis 
and I have pictured the peripheral cutaneous nerves of the body-wall and 
