476 ELEMENTARY PHYSIOLOGY Lilss. 



7. The Physiological Properties of a Nerve. — It will 

 be observed that in all the experiments described in the 

 first part of the preceding section there is evidence that, 

 when a nerve is irritated, a something which is spoken of as 

 a nervous impulse and consists, probably, of a change 

 in the arrangement or condition of its molecules, is propa- 

 gated along the nerve-fibres. If a motor or a sensory 

 nerve be irritated at any point, contraction in the muscle, 

 or sensation, (or some other corresponding event) in the 

 central organ, immediately follows. . But if the nerve be 

 cut, or even tightly tied at any point between the part 

 irritated and the muscle or central organ, the effect at 

 once ceases, just as cutting a telegraph wire stops the 

 transmission of the electric current or impulse. When a 

 limb, as we say, " goes to sleep," it is frequently because 

 the nerves supplying it have been subjected to pressure 

 sufficient to interfere with the nervous conductivity of 

 the fibres, that is their power to transmit nervous impulses. 

 We lose voluntary control over, and sensation in, the 

 limb, and these powers are only gradually restored as 

 that nervous conductivity returns. 



Having arrived at this notion of an impulse travelling 

 along a nerve, we readily pass to the conception of a 

 sensory nerve as a nerve which, when active, brings an 

 impulse to the central organ, or is afferent ; and of a 

 motor nerve, as a nerve which carries away an impulse 

 from the organ, or is efferent. It is very convenient to 

 use these terms to denote the two great classes of nerves ; 

 for, as we shall find (p. 483), there are afferent nerves 

 which are not sensory in the sense of giving rise to a 

 change of consciousness, or sensation, while there are 

 efferent nerves which are not motor, in the sense of in- 

 ducing muscular contraction. The nerves, for example, 

 by which the electi-ical fishes give rise to discharges of 

 electricity from peculiar organs to which those nerves are 

 distributed, are eff"erent, inasmuch as they carry impulses 

 to the electric organs, but are not motor, inasmuch as 



