THE NERVOUS SYSTEM. 83 



nerve be severed, it is the central and not the peripheral extremity which 

 responds to irritation, although the sensation is referred to the periphery. 

 There are, however, certain peculiarities in the effect. Thus the various 

 stimuli, which might cause, through an ordinary sensitive nerve, the sense 

 of pain, would, if applied to the optic nerve, cause a sensation as of flashes 

 of light; if applied to the olfactory, there would be a sense as of something 

 smelt. And so with the other two. 



Hence the explanation of so-called subjective sensations. Irritation in 

 the optic nerve, or the part of the brain from which it arises, may cause a 

 patient to believe he sees flashes of light, and among the commonest 

 troubles of the nerves of special sense, is the distressing noise in the head 

 (tinnitus aurium), which depends on some unknown stimulation of the 

 auditory nerve or centre quite unconnected with external sounds. 



Conduction in Motor Nerves. Conduction in motor nerves pre- 

 sents a remarkable contrast with the foregoing. Thus the effect of 

 applying a stimulus to the motor nerve is always noticeable, at the periph- 

 eral extremity, in the contraction of muscles supplied by it. If a motor 

 nerve be severed, irritation of the distal portion causes contraction of 

 muscle, but no effect whatever is produced by stimulating that part of 

 the nerve which is still in direct connection with the nerve-centre. 



Contractions are excited in all the muscles supplied by the branches 

 given off by the nerve below the point irritated,- and in those muscles alone: 

 the muscles supplied by the branches which come off from the nerve at a 

 higher point than that irritated, are not directly excited to contraction. 

 And it is from the same fact that, when a motor nerve enters a plexus and 

 contributes with other nerves to the formation of a nervous trunk pro- 

 ceeding from the plexus, it does not impart motor power to the whole of 

 that trunk, but only retains it isolated in the fibres which form its con- 

 tinuation in the branches of that trunk. 



FUNCTIONS OF NERVE-CENTRES. 



The functions of nerve-centres may be classified as follows: 1. 

 Conduction. 2. Transference. 3. Reflection. 4. Automatism. 5. Aug- 

 mentation. 6. Inhibition. 



1. Conduction. Conduction in or through nerve-centres may be 

 thus simply illustrated. The food in a given portion of the intestines, 

 acting as a stimulus, produces a certain impression on the nerves in the 

 mucous membrane, which impression is conveyed through them to the 

 adjacent ganglia of the sympathetic. In ordinary cases, the consequence 

 of such an impression on the ganglia is the movement by reflex action 

 (p. 85, Vol. II. ) of the muscular coat of that and the adjacent part of 

 the canal. But if irritant substances be mingled with the food, the 

 sharper stimulus produces a stronger impression, and this is conducted 



