FUNCTIONS OF NERVOUS TISSUE. 351 



note), analogous in its mode of transmission to Electricity, it is not difficult to 

 understand that the reversal of the usual direction of its action may produce 

 the effects in question ; especially when it is borne in mind, that the direct and 

 inverse electric currents (as shown by Prof. Matteucci) exert opposite influences 

 upon the nervous excitability. 1 



353. The manifestations of nerve-force are almost invariably excited by the 

 agency of stimuli, which operate upon the peripheral origins of the afferent 

 nerves, and upon the central origins of the efferent. In the cases already re- 

 ferred to, the stimulus whose influence, conveyed to the central gangalia, excited 

 sensations or respondent movements, was that of mechanical Motion; and this 

 is equally effectual when applied to the trunks of the afferent nerves, as by 

 pinching or pricking them. But other physical agencies produce corresponding 

 results. Thus Neat, applied to a sensory surface, excites the nerve-force of the 

 afferent nerves, by whose instrumentality such a change is produced in the sen- 

 sorial centres, as renders us conscious of the external alteration. The power of 

 Light, moreover, to excite nerve-force, is clearly indicated by its influence upon 

 the optic nerve through the instrumentality of the retina. 3 So, again, the stimu- 

 lating agency of Chemical Affinity is made apparent by the pain which results 

 from the application of strong reagents to the extremities or to the trunks of 

 the sensory nerves. But of all the physical forces, Electricity has the most 

 powerful influence in exciting nerve-force, and seems to have the most direct re- 

 lation to it. If an electric current be made to pass far a short distance only 

 along the trunk of a sensory nerve, or through its peripheral ramifications, it 

 excites in the sensorium the peculiar sensations produced by ordinary impres- 

 sions conveyed through that nerve; so, again, if the current be transmitted for 

 however short a space through a motor nerve, it excites contractions in the mus- 

 cles which are supplied from its branches. It was formerly supposed that the 

 electricity was the immediate cause of the changes thus induced in the senso- 

 rium or in the muscular apparatus ; but it has been clearly proved that such is 

 not the case, the passage of the current along a portion of the trunk being suffi- 

 cient to excite the nerve-force in the remainder. On the other hand, we have 

 adequate proof that the force transmitted from the central ganglia to the muscles, 

 is of the same nature with the preceding; for all the stimuli which excite sen- 

 sations when applied to the trunks of the afferent nerves, excite motions when 

 applied to the trunks of the efferent. This force seems to be developed by 

 changes which take place in the vesicular matter of those ganglia ( 357 362); 

 but the stimulus to that development may be of two very different kinds. For, 

 on the one hand, the " motor impulse" (as, for the sake of convenience, it may 

 be designated) may be excited by a change originating in any part of the body, 

 and transmitted to the ganglion through the afferent nerves; as in "reflex" 

 actions of various kinds. But, on the other, it may be called forth by an 

 agency purely mental, which acts directly upon the vesicular matter of the gan- 

 glion, producing movements whose source is thus not peripheral, but central. 



1 The direct current is that which is transmitted along a motor nerve in the direction of 

 its ramifications ; the inverse, that which is transmitted from the periphery towards the 

 centre. The "direct" current rapidly weakens and at last destroys the excitability of a 

 nerve ; so that, when it has traversed the lumbar nerve of a frog for twenty or thirty 

 minutes, there are no further contractions, either on opening or closing the circuit. On 

 the other hand, the passage of the "inverse" current augments the excitability within 

 certain limits ; and restores that which has been exhausted by the direct current. (See 

 Prof. Matteucci's "Lectures on the Physical Phenomena of Living Beings," translated by 

 Dr. Pereira, p. 242, Am. Ed.} 



2 It seems to be the want of this intermediate ganglionic apparatus (g 343), which 

 prevents Light from acting upon the trunks of the sensory and motor nerves, after the 

 manner of Heat. 



