FUNCTIONS OF NER VE - F I B R ES. 379 



which, when covering a wire, prevent the electric condition of 

 the wire from being conducted into the surrounding medium. 



Nervous force travels along nerve-fibres with considerable 

 velocity. Helmholtz and Baxt have estimated the average 

 rate of conduction of electrical impressions in human motor 

 nerves at 111 feet per second: this result agreeing very closely 

 with that previously obtained by Hirsch. Dr. Rutherford's 

 observations agree with those of Von Wittich, that the rate 

 of transmission in sensory nerves is about 140 feet per second. 



Nerve-fibres convey only one kind of impression. Thus, a 

 motor fibre conveys only motor impulses, that is, such as may 

 produce movements in contractile parts: a sensitive fibre trans- 

 mits none but such as may produce sensation, if they are propa- 

 gated to the brain. Moreover, the fibres of a nerve of special 

 sense, as the optic or auditory, convey only such impressions 

 as may produce a peculiar sensation, e. g., that of light or 

 sound. While the rays of light and the sonorous vibrations 

 of the air, are without influence on the nerves of common sen- 

 sation, the other stimuli, which may produce pain when applied 

 to them, produce when applied to these nerves of special sense, 

 only morbid sensations of light, or sound, or taste, according to 

 the nerve impressed. 



Of the laws of action peculiar to nerves of sensation and of 

 motion respectively, many can be ascertained only by experi- 

 ments on the roots of the nerves. For it is only at their origin 

 that the nerves of sensation and of motion are distinct; their 

 filaments, shortly after their departure from the nervous cen- 

 tres, are mingled together, so that nearly all nerves, except 

 those of the special senses, consist of both sensitive and motor 

 filaments, and are hence termed mixed nerves. 



Nerves of sensation appear able to convey impressions only 

 from the parts in which they are distributed, towards the nerve- 

 centre from which they arise, or to which they tend. Thus, 

 when a sensitive nerve is divided, and irritation is applied to 

 the end of the proximal portion, i, e., of the portion still con- 

 nected with the nervous centre, sensation is perceived, or a 

 reflex action ensues; but, when the end of the distal portion of 

 the divided nerve is irritated, no effect appears. 



When an impression is made upon any part of the course of 

 a sensitive nerve, the mind may perceive it as if it were made 

 not only upon the point to which the stimulus is applied, but 

 also upon all the points in which the fibres of the irritated 

 nerve are distributed : in other words, the effect is the same as 

 if the irritation were applied to the parts supplied by the 

 branches of the nerve. When the whole trunk of the nerve 



