In the analysis of the above example, only the structural and methodological aspects are emphasized. No attempt is made to legislate for neurologists or to instruct them how they should define and use their terms.
Section B. Order and the nervous system.
We know that, structurally, not all parts of the nervous system are of equal phylogenetic age. The ventral part of the brain, the thalamus (in the rough), which is of most interest in this connection, is older than the cerebral cortex. By the term 'thalamus' I denote all the subdivisions and most important appendages which we need not mention by their technical names. In man, both the thalamus and the cerebral cortex are much enlarged and have a complex structural cyclic interconnection. The cerebral cortex is a term applied to a superficial layer of grey nervous tissue covering the cerebral hemispheres. It is called the 'new brain' by Edinger. The higher correlation centres in the cerebral hemispheres can act only through the agency of the lower centres, the brain-stem, and the thalamus. In other words, the cerebral cortex, the functioning of which is connected chiefly with the higher associations, is of such structure that no nervous impulse can enter it without first passing through the lower centres of the ventral parts of the brain 11 nd brain-stem.
In the lower vertebrates, which lack the cerebral cortex, the subcortical mechanisms are adequate for all simple exigencies of life and simple association processes, these sub-cortical mechanisms being older phylogenetically than the cerebral cortex, yet younger than some still more ventral parts.
The brush-like connections between the nerves are called synapses, and although a nerve-fibre seems to be capable of transmitting nerve impulses in both directions, the nervous impulse can seemingly pass the synapse in only one direction; so a nervous polarity is established whenever synapses are present.1
The following diagram and explanations are taken from Professor
Herrick's Introduction to Neurology, pp. 60, 61, 62, 63, 69, 70. In the
quotations I retain the spelling but change Herrick's Fig. 18 to my Fig. 1;
nil but one word of the italics are mine.*
*I quote here from the Second Edition of the Introduction to Neurology an account of the classical theory of reflex circuits which is quite satisfactory for my purpose. In his later work (see my bibliography), Professor Herrick forcibly points out the limitations of the reflex theories as partial patterns, as opposed to the activity of the organism-as-a-whole. In the Fifth Edition (1931), the chapter on reflex circuits has been entirely rewritten, and the non-el attitude is expressed very clearly. I am much indebted to Professor Herrick for drawing my attention to tills rewritten chapter.