THE NERVOUS SYSTEM. 457 



which, with sufficient suddenness, increases their irritability, but they 

 are incapable of originating of themselves the condition necessary for 

 the manifestation of their own energy. The stimulus produces its effect 

 upon the termination of the nerve stimulated, being conducted to it by 

 the nerve-fibre. The effect of the stimulus upon a nerve therefore de- 

 pends upon the nature of its end-organ. A length of a nerve-trunk 

 when freshly removed from the body, if stimulated midway between its 

 extremities, will, as shown by the deflection of a needle of the galvano- 

 meter at either end, conduct the electrical impressions in either direction, 

 and it may be considered therefore only an accidental circumstance as it 

 were, whether when in situ it has conducted impressions to the central 

 nervous system from the periphery, or from the central nervous system 

 to the muscles or other tissues. The same fibre cannot be used for the 

 one purpose at one time, and for the other at another, simply because of 

 the nature of its terminal organs. Thus, when a cerebro-spinal nerve- 

 fibre is irritated in the living body as by pinching, or by heat, or by 

 electrifying it, there is, under ordinary circumstances, one of two effects 

 either there is pain, or there is twitching of one or more muscles to 

 which the nerve distributes its fibres. From various considerations it is 

 certain that pain is always the result of a change in the nerve-cells of 

 the brain. Therefore, in such an experiment as that referred to, the ir- 

 ritation of the nerve-fibre is conducted in one of two directions, i, e., 

 either to the brain, which is the central termination of the fibre, when 

 there is pain, or to a muscle, which is the peripheral termination, when 

 there is movement. 



That this is the true explanation is made highly probable, not merely 

 by the absence of any essential structural differences in the two kinds 

 of nerve-fibre, but also by the fact, proved by direct experiment, that if 

 a centripetal nerve (gustatory) be divided, and its central portion be 

 made to unite with the distal portion of a divided motor nerve (hypo- 

 glossal) the effect of irritating the former after the parts have healed, is 

 to excite contraction in the muscles supplied by the latter. (Phillippeaux 

 and Vulpian.) 



The effect of this simple experiment is a type of what always occurs 

 when nerve-fibres are engaged in the performance of their functions. 

 The result of stimulating them, which roughly imitates what happens 

 naturally in the body, is found to occur at one or other of their extremi- 

 ties, central or peripheral, never at both; and in accordance with this 

 fact, and because, for any given nerve-fibre, the result is always the same, 

 nerves are commonly classed as sensory or motor. 



Classification. The classification of nerve-fibres into sensory and 

 motor is not altogether accurate, and the terms Centripetal or afferent, 

 and Centrifugal or efferent are more properly used in connection with 

 nerve-fibres in lieu of the corresponding terms, because the result of 



