NERVE STIMULI. 



18? 



Intercentral Nerve-Fibres. These, which do not convey 

 impulses between peripheral parts and nerve-centres, but 

 connect one centre with another, form a final group in ad- 

 dition to efferent and afferent nerve-fibres. Many of them 

 connect the sporadic and sympathetic ganglia with one 

 another and with the cerebro-spinal centre, while others 

 place different parts of this latter in direct communication ; 

 as for instance different parts of the spinal cord, the brain 

 and the spinal cord, and the two halves of the brain. 

 These fibres are of very great importance, but as yet their 

 course is imperfectly known. 



General Table. We may physiologically classify nerve- 

 fibres as in the following tabular form which is founded 

 upon the facts above stated. 



Xerve-fibres. < 



Peripheral. 



Afferent. 



Efferent. 



I Sensory. 

 I Reflex. 

 ] Excito-motor. 

 [ Inhibitory? 



I* Motor. 



Vaso-motor. 

 I Secretory. 



Trophic? 

 [ Inhibitory. 



. ( Exciting. 

 Intercentral. j Inhibitory . 



The Stimuli of Nerve-Fibres. Nerve-fibres, like mus- 

 cular fibres, possess no automaticity; acted upon by certain 

 external forces or stimuli they propagate a change, which is 

 known as a nervous impulse, from the point acted upon to 

 their ends; but they do not generate nervous impulses when 

 left entirely to themselves. Normally, in the living Body 

 the stimulus acts 011 a nerve-fibre at one of its ends, being 

 either some change in a nerve-centre with which the fibre 

 is connected (efferent nerves) or some change in an organ 

 attached to the outer end of the nerve (afferent fibres). 

 Experiment shows, however, that a nerve can be stimulated 

 in any part of its course; that it is irritable all through its 



