FUNCTIONS OF NERVE-FIBRES. 377 



will give rise to sensation (sensitive nerves), or (6) they may 

 convey an impression which travels out again from the nerve- 

 centre by an efferent nerve-fibre, and produces some effect 

 where the latter is distributed (see section on Reflex Action), 

 or (c) they may convey an impression which will produce a 

 restraining or inhibitory action in the nerve-centre (inhibitory 

 nerves, p. 113). 



Centrifugal or efferent nerves may be (a) for the convey- 

 ance of impulses to the voluntary and involuntary muscles, 

 (motor nerves), or (6) they may influence nutrition (trophic 

 nerves), (p. 310), or (c) they may influence secretion (some- 

 times called secretory nerves) (p. 325). 



With this difference in the functions of nerves, there is no 

 apparent difference in the structure of the nerve-fibres by 

 which it might be explained. Among the cerebro-spinal nerves, 

 the fibres of the optic and auditory nerves are finer than those 

 of the nerves of common sensation ; but, with these exceptions, 

 no centripetal fibres can be distinguished in their microscopic 

 or general characters from those of centrifugal nerves. 



Nerve-fibres possess no power of generating force in them- 

 selves, or of originating impulses to action : for the manifesta- 

 tion of their peculiar endowments they require to be stimu- 

 lated. They possess a certain property of conducting impres- 

 sions, a property which has been named excitability ; but this 

 is never manifested till some stimulus is applied. Thus, under 

 ordinary circumstances, nerves of sensation are stimulated by. 

 external objects acting upon their extremities ; and nerves of 

 motion by the will, or by some force generated in the nervous 

 centres. But almost all things that can disturb the nerves 

 from their passive state act as stimuli, and agents the most 

 dissimilar produce the same kind, though not the same degree 

 of effect, because that on which they act possesses but one 

 kind of excitable force. Thus all stimuli chemical, me- 

 chanical, and electric, when applied to parts endowed with 

 sensation, or to sensitive nerves (the connection of the latter 

 with the brain and spinal cord being uninjured) produce sen- 

 sations ; and when applied to the nerves of muscles excite con- 

 tractions. Muscular contraction is produced by such stimuli 

 as well when the motor nerve is still in connection with the 

 brain, as when its communication with the nervous centres is 

 cut off by dividing it ; nerves, therefore, have, by virtue of 

 their excitability, the property of exciting contractions in 

 muscles to which they are distributed ; and the part of the 

 divided motor nerve which is connected with the muscle will 

 still retain this power, however much we may curtail it. 



Mechanical irritation, when so violent as to injure the tex- 



