294 



PHYSIOLOGY OF THE INVERTEBRATA. 



molecular, wave of stimulation. Nerve-fibres, then, are 

 functionally distinguished from muscle-fibres — and also from 

 protoplasm — by displaying the property of conducting in- 

 visible, or molecular, waves of stimulation from one part of 

 an organism to another, so establishing physiological con- 

 tinuity between such parts, without the necessary passage 

 of waves of contraction." (Romanes.) 



Nerve-fibres may be functionally divided into five groups 

 — ^motor, sensory, vascular, secretory, and inhibitory. When 

 a nerve-fibre is stimulated from some nerve-centre, it may 

 give rise to the contraction of a muscle or a blood-vessel, in- 

 creased secretion from a gland, or a diminution or arrest of 

 some other kind of nervous action. 



In all these cases, " the nervous influence travels outwards 

 from a ganglion or nerve-centre towards the periphery, thus 

 presenting an analogy to ordinary motor nerves." Perhaps 

 the best classification of nerve-fibres, from a physiological 

 point of view, is the following ; 



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 O 



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^(a) Motor (efferent), excite contraction of muscles. 

 (J) VaiciAar (vaso-motor), excite contraction of blood-vessels, 

 (c) Secretory, excite secretion. 

 {d) Inhibitory, affect other nerve-centres so as to moderate or 



destroy their action, 

 ^(c) Connecting, which connect motor-cells in nerve-centres. 



(i) General, "convey to nerve-centres in brain 

 influences which cause sensations of a 

 vague character (not permanent)." 

 (o) Sensory -{ (2) Special, "convey to nerve-centres in brain 

 influences which cause visual, auditory, 

 gustatory, olfactory, or tactile sensa- 

 tions." 

 (6) Afferent, " convey to nerve-centres influences which cause no 

 sensation, and which may or may not be followed by further 

 nervous activity." 

 \(c) Connecting, "which connect sensory cells in nervous centres." 



The centrifugal nerve-fibres convey influences outwards 

 from a nerve-centre ; while the centripetal nerve-fibres con- 

 vey influences inwards towards a nerve-centre. It should be 



/ 



