Licence: Public Domain Mark
Credit: The nervous system and its conservation. Source: Wellcome Collection.
Provider: This material has been provided by the Augustus C. Long Health Sciences Library at Columbia University and Columbia University Libraries/Information Services, through the Medical Heritage Library. The original may be consulted at the the Augustus C. Long Health Sciences Library at Columbia University and Columbia University.
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![direct effect of external stimuli or by the chemical changes occurring in the perikarya. The impulses that run along the afferent (receptive) channels are originated in a relatively direct manner by external forces. Those that traverse the adjustors and run out to the effectors may be thought of as the afferent currents which have continued through the synapses of the central axis, but we shall probably do well to regard them rather as relays, that is, impulses started anew from perikarya excited to dis- charge. This view, according to which the energy of the transmission is reinforced by each neuron in the series, is the older doctrine and is still dominant. Attention must now be called to the relative ease with which neurons will respond to stimuK applied at different points. It is most necessary for the orderly and purposeful working of the nervous system that nerve-fibers shall not be easily excited save at their specialized beginnings or endings. It is, in fact, difficult to stimulate a nerve at any intermediate point in its course. When we do use sufficiently positive means to make the stimulation effective the result is such as to remind us that it would be undesirable to have it a common incident. A blow on the cl])ow may be forcible enough to start impulses in the afferent fibers of the nerve which lies near the surface at that spot, and it instantly seems to the subject that he has sensations pertaining to the wrist and fingers. He has excited to activity the axons which have habitually brought impulses from the hand, and they still awaken the same associations as though they had come over the whole distance. This is a very significant matter and it will be wise to place further emphasis upon it. When an electric door-bell rings, the householder is usually justified in the assumption that some one is out- side ringing it. Yet it might happen that a short-circuit at some point along the course of the wires should cause the signal. The ringing of the bell when occasioned in this abnormal way would have the same quahties to the](https://iiif.wellcomecollection.org/image/b21206545_0046.jp2/full/800%2C/0/default.jpg)
