Page:Mind (New Series) Volume 15.djvu/256

 242 CRITICAL NOTICES : greater or less degrees of resistance of conduction-paths is modified not only by augmentation, but also by inhibition, of impulses within the central nervous system. And careful studies of the simplest processes of central conduction, namely, those of the reflex-arcs of the spinal cord, then revealed other important differences between them and the simple conduction-process of peripheral nerve-fibres. It was found that in traversing such a simple reflex-path the nervous impulse always meets with a resistance such as it does not encounter in the peripheral nerves ; that this resistance is only -to be overcome by an impulse of a certain strength or by the sum- mation of a series of feebler impulses ; that it causes a delay in the passage of the impulse ; that it is much greater for impulses passing up from the motor nerves than for those arriving by the sensory nerves, so that an impulse can as a rale pass only in the .forward or efferent direction ; that this resistance is variable by a number of influences ; that transmission through the central path gives the impulse a rhythmical character, and that the process of central conduction, unlike peripheral conduction, is liable to fatigue. Since none of these peculiar features are found in the conduction- .processes of peripheral nerve-fibres, and since fibres and cells were supposed to make up the whole nervous system, it was natural to attribute them to the influence of the cells lying upon the course of the fibres of the network. The taking of this step produced what may be called the nucleated-network view of the central nervous system. About twenty years ago, while this view pre- dominated, great improvements of methods enabled histologists to show that nerve-fibres and what had up till that time been called nerve-cells are alike only parts of cells, that every complete iierve-cell or neurone consists of both a nucleated body and one or more thread-like prolongations of the protoplasm of the body, and that every nerve-fibre is but such a part of a nerve cell. It was further shown that a fibre soon dies if separated from the cell-body. The nerve-cells therefore appeared as the structural and the vital or trophic units of the nervous system, and to the nerve-cell so conceived the term neurone is sometimes applied. Now many kinds of evidence, direct and indirect, converge to prove that all those peculiarities of central-conduction enumerated above are due, not to the influence of those parts of the neurones which contain the nuclei, but rather to the peculiar nature of the structural and .functional connexions between the members of the chains of neu- rones that constitute the central conduction-paths. Such con- nexions have been happily named synapses by Sir M. Foster, and, .though the details of their structure remain obscure, the importance of the part they play is now widely recognised by British physiolo- gists (as was shown in the discussion on the nerve-cell at last year's meeting of the British Association). The neurone is thus, for those who accept this conclusion, no longer merely a histological tfact, but has been raised by experimental observation to the rank of a physiological -theory which, when combined with the con-