It is now necessary to add the presence in the spinal cord of small sensory "intermediary neurons" (Sehalt neurons), through which the sensory nerve fibres coming from the spinal ganglion cells in the posterior tract are connected with the centre for the peripheral motor neurons. With the centre for these neurons are connected also the ganglion cells in the cerebellum by means of their nerve fibres.

It is well to remark that the whole of this description refers to a structure of which future investigation will give us a more satisfactory explanation. The facts which I put forward here aim more at giving an idea of, than at really explaining the whole clearly, but an idea which, in the absence of a better, may be of some use.

The cerebro-spinal neurons are connected together by their dendrites and axons. A more central neuron is connected with a more peripheral neuron, in that the fine, root-shaped, branched end apparatus of one, resembling dendrites, embraces the ganglion cell of the other, and in some way transmits nervous impulses to the second cell. Such connections between neurons are known as synapses.

The white matter of the cerebrum, i.e., its collections of nerve fibres, includes the system of coronal fibres which connect the grey matter of the cortex (i.e., the ganglion cells) with similar cells in the mid-brain and the collections of grey matter in the medulla oblongata; we find here both cortico-fugal and cortico-petal fibres. We have further connections from the cortex with the pons and the cerebellum. In the pyramids we have nerve fibres to all the peripheral motor neurons. There are moreover fibres between the ganglion cells of the cortex, both between those which belong to the same side (unilateral) and those which belong to different sides of the body (bilateral), i.e., connections by commissural fibres. I would also remind the reader, concerning cerebral connections : - that the olivary bodies contain part of the (motor) fibres from the anterior tract of the spinal cord, and also communicate chiefly with the cerebellum and with the optic thalami; that the cerebellum (many multipolar ganglion cells) communicates with the centres in the spinal cord by sensory fibres in the lateral tracts, and with the brain by similar fibres in the crura of the pons and cerebrum, and through them with the frontal, parietal, and temporal lobes of the cerebrum, with the corpora quadragemina, and through these with many of the smaller collections of grey matter in the cerebrum; that the corpora quadragemina are connected with the cortex of the cerebrum, with its internal grey matter and with the spinal cord, with the optic tract and with the motor nerves of the eye; that the optic thalami are connected both with the grey matter in the spinal cord and with the cerebrum, especially with the cortex, and also with the optic nerves.

The sympathetic nervous system is made up of a row of ganglia on each side of the spinal cord and of plexuses with their respective ganglia, found chiefly in the cavities of the body. Certain of its fibres are not white nerve fibres with neurilemma sheath, medulla, and axis-cylinder, as in the cerebro-spinal nerve-fibres, but plain grey fibres with an occasional nucleus.

The sympathetic nervous system possesses a certain autonomy, but is subject to, and closely bound up anatomically with, the cerebro-spinal nervous system. A good deal is still unsolved regarding the sympathetic nerves.

This system also consists of neurons, which may be divided into two different systems, of which the central represents the connection with the cerebro-spinal nervous system and really belongs to this system.

The neurons of the first (the central) system have their ganglion cells in the grey matter of the spinal cord (in the lateral horn), between the last cervical vertebra and the lower third of the lumbar cord. The "spinal" fibres ("preganglionic") go from the ganglion cells of the lateral horn of the spinal cord through the anterior roots, and via a white or medullated communicating branch to a (sympathetic) ganglion cell in a ganglion of the sympathetic chain, and either end round this ganglion cell in a root-shaped end apparatus or pass through it to some other ganglion, where they end round a sympathetic ganglion cell. This makes the cell station for the peripheral sympathetic neuron. From these centres go grey or non-medullated sympathetic ("post-ganglionic ") nerve fibres by special grey rami communicantes to the spinal nerves and to the periphery, where they innervate the non-striated muscle fibres in the blood vessels, alimentary canal, uterus, etc.

The centripetal sympathetic neurons which probably exist are little known, but it is supposed that most of them go from the peripheral sympathetic centres to the ganglia in the chain, and from there through the posterior roots to the ganglion cells in the grey matter of the cord.

For every peripheral sensory neuron there is a certain minimal stimulus varying somewhat under different conditions, which just produces an effect, i.e., a sensation; this minimal effective peripheral stimulus is the German "Reizschzvelle" ("the threshold of stimulation "). For an increase in the stimulation to be felt, according to the Weber-Fechner law, this increase must have a certain quantitative relation to the original stimulus (1 in 40 for tactile sensation in the hand, 1 in 100 for sensation of light). For the stimulus to reach the nearest central "contact neurons" it must be somewhat stronger; the minimal stimulus necessary for this is the German "Neuronschwelle" ("the threshold of neuron stimulation "),

During its passage through the nerve tracts the stimulus meets a certain resistance, which we take to be greatest where irritability, which varies in the different parts of the neuron, is least, and greatest of all where it passes from one neuron to another. The stronger the stimulus, the more effect does it produce and the more neurons does it affect.