INTRODUCTION 3 
the ventral roots, for the cells of the sympathetic system are entirely 
efferent in nature, not afferent ; therefore, the fibres entering into them 
from the central nervous system leave the spinal cord by ventral, not 
dorsal roots. 
Following out this clue, I then found that in addition to this 
thoracico-lumbar outflow of efferent ganglionated visceral nerves, 
there are similar outflows in the cranial and sacral regions, belong- 
ing in the former case especially to the vagus system of nerves, and 
in the latter to the system of nerves which pass from the sacral 
region of the cord to the ganglion-cells of the hypogastric plexus, 
and from them supply the bladder, rectum, etc. To this system of 
nerves, formerly called the nervi erigentes, I gave the name pelvic 
splanchnics, in order to show their uniformity with the abdominal 
splanchnics. These investigations led to the conclusion that the 
organic system of nerves, characterized by the possession of efferent 
nerve-cells situated peripherally, arises from the central nervous 
system by three distinct outflows—cranial, thoracico-lumbar, and 
sacral, respectively. To this system Langley has lately given the 
name ‘autonomic.’ These three outflows are separated by two gaps 
just where the plexuses for the anterior and posterior extremities 
come in. 
This peculiar arrangement of the white rami communicantes set 
me thinking, for the gaps corresponded to an increase of somatic 
musculature to form the muscles of the fore and hind limbs, so that 
if, as seemed probable, the white rami communicantes arise segmentally 
from the spinal cord, then a marked distinction must exist in 
structure between the spinal cord in the thoracic region, where the 
visceral efferent nerves are large in amount and the body muscu- 
lature scanty, and in the cervical or lumbar swellings, where the 
somatic musculature abounds, and the white rami communicantes 
scarcely exist. 
I therefore directed my attention in the next place to the 
structure of the central nervous system in the endeavour to asso- 
ciate the topographical arrangement of cell-groups in this system 
with the outflow of the different kinds of nerve-fibres to the 
peripheral organs. 
This investigation forcibly impressed upon my mind the 
uniformity in the arrangement of the central nervous system as far 
as the centres of origin of all the segmental nerves are concerned, 
