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 oiervc-Jihres, from a physiological 

 point of view, is the following ; 



o 



S 2i (h) Vascular (vaso-motor), excite contraction of blood-vessels. 



'5 «: I (c) tSecretory, excite secretion. 

 bD i 

 S > 

 'C g destroy their action. 



^(a) 3Iotor (efferent), excite contraction of muscles. 



(d) Ivhlhitory, aft'ect other nerve-centres so as to moderate or 



s 

 O 



o 



u 



(o) Sensory 



Je) ConnectiiKj, vi-hich connect motor-cells in nerve-centres. 



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

 influences which cause sensations of a 

 vague character (not permanent)." 

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

 influences which cause visual, auditory, 

 gustatory, olfactory, or tactile sensa- 

 tions." 

 {b) Afferent, " con\ej to nerve-centres influences which cause no 

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

 nervous activity.'' 

 c3 \(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 



