The Three Divisions of the Outlying Neurones
As suggested above, the outlying neurones are connected with the brain and spinal cord by neurones whose cell bodies lie within the central nervous organs. These connecting neurones, represented in continuous lines in [Fig. 1], do not pass out in a continuous series all along the cerebrospinal axis. Where the nerves pass out from the spinal cord to the fore and hind limbs, fibres are not given off to the ganglia. Thus these connecting or “preganglionic” fibres are separated into three divisions. In front of the nerve roots for the fore limbs is the head or cranial division, between the nerve roots for the fore limbs and those for the hind limbs is the trunk division (or thoracico-lumbar division, or, in the older terminology, the “sympathetic system”); and after the nerve roots for the hind limbs the sacral division.
This system of outlying neurones, with postganglionic fibres innervating the viscera, and with preganglionic fibres reaching out to them from the cerebrospinal system, has been called by Langley, to whom we are indebted for most of our knowledge of its organization, the autonomic nervous system.[2] This term indicates that the structures which the system supplies are not subject to voluntary control, but operate to a large degree independently. As we have seen, a highly potent mode of influencing these structures is through conditions of pain and emotional excitement. The parts of the autonomic system—the cranial, the sympathetic, and the sacral—have a number of peculiarities which are of prime importance in accounting for the bodily manifestations of such affective states.