224 SPECIAL PHYSIOLOGY. 



irritation of the fifth. (Budge.) The rate of propagation is moreover 

 influenced by the strength of the stimulus, for powerful irritation of 

 the ganglia belonging to the so-called sympathetic nerves, produces 

 sudden reflex movements, although the normal character of the sym- 

 pathetic is to act more slowly under the influence of moderate stimu- 

 lation. 



It has even been observed, in certain experiments, that muscular 

 contraction takes place more slowly, the more distant the point of 

 nerve which is excited; and the amount of difference in time, compared 

 with the difference between the excited points of the nerve touched, 

 enables the rate of propagation to be estimated. The effect of the 

 stimulus upon a muscle has been shown to be greater, according to the 

 length of nerve between the point excited and the muscle. Speaking 

 generally, the force with which a muscle, excited through its nerve, 

 contracts, is proportional to the force or intensity of the stimulus ap- 

 plied to the nerve. When one nerve is excited, a neighboring nerve 

 lying close alongside it, may be affected, and so groups of muscles may 

 be called into play. If a muscle be loaded with a certain weight, the 

 commencement of contraction, when it is excited through a nerve, is 

 somewhat delayed. 



Nerves have been supposed to conduct impressions after the manner 

 of the propagation of vibrations in tense cords, or by undulations in 

 the fluid contents of the nerve-fibres, or through the agency of an im- 

 ponderable ether ; but such views are entirely speculative. \ 



Electrical Phenomena in Nerves. 



Nerves, like muscles, are conductors, though not such good con- 

 ductors, of electricity; like muscles, they have electrical currents pass- 

 ing through them, even through small portions of them, during life, 

 when they are in their normal condition, and in a quiescent state ; 

 and, lastly, like muscles, they are as excitable by electrical currents, 

 as by any other stimulus. But nerve is distinguished from muscle, 

 not only by being a feebler conductor of electricity, but by exhibiting 

 various peculiarities of behavior, especially as regards its intrinsic 

 electrical currents when under the influence of other extrinsic exciting 

 currents or other stimuli. The proper nerve- cur rent, discovered by 

 Du Bois-Reymond, exactly like the muscular current, runs within the 

 nerve, from the interior to the surface ; and there is, by analogy, 

 ground for concluding that, outside the nerve, it also passes, like the 

 muscular current, from the surface to the cut ends; hence, in a sepa- 

 rated portion of nerve, the surface is positive and the ends are nega- 

 tive, currents, as indicated by a galvanometer, passing from the equa- 

 tor in each direction, to the ends of the cut piece. The nerve-current 

 may be shown, by placing a portion of the divided sciatic nerve of a 

 frog, still connected with the leg, with its surface in contact with one 

 cushion of the apparatus already described (p. 138), and its cut end 

 with the other cushion. In a completely separated portion of nerve, 

 the current is equally evident, whether the peripheral or the central 

 cut end be brought against one cushion, whilst the surface touches the 



