480 THE NERVOUS SYSTEM. 



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 r 1 1 feet per second : this result agreeing very 

 closely with that previously obtained by Hirsch. 



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 transmits none but such as may produce sensation, if 

 they are propagated 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 sensation, the other stimuli, which 

 may produce pain when applied to them, produce, when 

 applied to these nerves of special sense, only morbid sensa- 

 tions 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 

 experiments 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 centres, 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 irrita- 

 tion is applied to the end of the proximal portion, i.e., of 

 the portion still connected with the nervous centre, sensa- 

 tion is perceived, or a reflex action ensues ; but, when the 



