THE NERVOUS SYSTEM. 81 



fibre, is simply and uninterruptedly transmitted along it, without being 

 imparted or diffused to any of the fibres lying near it. In other words, 

 all nerve-fibres are mere conductors of impressions. Their adaptation to 

 this purpose is, perhaps, due to the contents of each fibre being complete!^ 

 isolated from those of adjacent fibres by the membrane or sheath in which 

 each is enclosed, and which acts, it may be supposed, just as silk or other 

 non-conductors of electricity do, which, when covering a wire, prevent the 

 electric condition of the wire from being conducted into the surrounding 

 medium. 



Velocity of Nerve-force. The change which a stimulus sets upon a 

 nerve, of the exact nature of which we are unacquainted, appears to travel 

 along a nerve-fibre in both directions in the form of a wave. Nervous 

 force travels along nerve-fibres with considerable velocity. Helmholtz and 

 Baxt have estimated the average rate of conduction in human motor 

 nerves at 111 feet (nearly 29 metres) per second; this result agreeing very 

 closely with that previously obtained by Hirsch. Kutherford's observations 

 agree with those of Von Wittich, that the rate of transmission in sensory 

 nerves is about 140 feet per second. 



Conduction in Sensory Nerves. Centripetal nerves appear (p. 

 80, Vol. II.) able to convey impressions only from the parts in which they 

 are distributed, toward 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 sensory 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 distrib- 

 uted: 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 is irritated, the sensation is felt at all the parts which 

 receive branches from it; but when only individual portions of the trunk 

 are irritated, the sensation is perceived at those parts only which are sup- 

 plied by the several portions. Thus, if we compress the ulnar nerve 

 where it lies at the inner side of the elbow-joint, behind the internal 

 condyle, we have the sensation of "pins and needles," or of a shock, in the 

 parts to which its fibres are distributed, namely, in the palm and back of 

 the hand, and in the fifth and ulna half of the fourth finger. When 

 stronger pressure is made, the sensations are felt in the fore-arm also; and 

 if the mode and direction of the pressure be varied, the sensation is felt 

 by turns in the fourth finger, in the fifth, and in the palm of the hand, or 

 in the back of the hand, according as different fibres or fasciculi of fibres 

 are more pressed upon than others. 

 VOL. II. 6. 



