This led to the conclusion that there is no give and take between two independent nervous systems, the cerebro-spinal and the sympathetic, as had been taught formerly, but only one nervous system, the cerebro-spinal, which sends special medullated nerve-fibres, characterized by their smallness, to the cells of the sympathetic system, from which fibres pass to the periphery, usually non-medullated. These fine medullated nerves form the system of white _rami communicantes_, and have since been called by Langley the preganglionic nerves. Further investigation showed that such white rami are not universally distributed, but are confined to the thoracico-lumbar region, where their distribution is easily seen in {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, belonging 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 musculature 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 associate the topographical arrangement of cell-groups in this system with the outflow of the different kinds of nerve-fibres to the peripheral organs.