28 NERVOUS SYSTEM. 



In the state ot rest certain currents are constantly travers- 

 ing nerves, going from the surface to the interior, and acting 

 as if the nerve fibres were the seat of two enclosed elements, 

 the extremity being positive and the centre negative. In 

 fact, whenever by means of a galvanometer, a communica- 

 tion is made between the external surface and the surface 

 of the section of a nerve, a current is observed to pass from 

 the periphery towards the centre. This electrical phe- 

 nomenon, called the electro-motor y force of the nerve, disap- 

 pears or becomes feeble whenever the fibre is subjected to an 

 irritation, or whenever it acts as a conductor, or in fact when- 

 ever it performs its proper function ; a disappearance of the 

 electro-motor power is called negative oscillation. It has been 

 surmised that at this moment nutrition is arrested, and with 

 this ensues the normal current of a state of rest. The de- 

 duction can easily be drawn in what way the. fatigue of the 

 nerve may be brought about, and why an irritation too long 

 maintained may cause destruction, which latter may also be 

 accompanied with pain. 



But, on the other hand, direct experiment shows that the nerve 

 in functional activity does more , there is produced a development 

 of heat, the existence of which Schiff has just demonstrated iu the 

 nerve-centres, influenced by fear, or excitement of the senses, or 

 from every cause which may produce cerebral activity. It may be 

 that the negative oscillation indicates that electricity of the nerve 

 in a state of repose is transformed i^ito heat in the active state. 

 (In regard to this see farther on an analogue of the negative oscil- 

 lation, in the study of the muscles, and also the transformation of 

 one force into another force.) 



3. Action of the Nervous System. What constitutes the 



special function of the nerve 

 apparatus, both fibre and cell ? 

 This consists essentially in 

 a phenomenon called reflex. 

 When a nerve fibre is irri- 

 tated, this irritation is trans- 

 mitted to globules more or 



Diagram of aSple reflex action * le8S dista t ' and . f m , the ^ 



ter to the peripheral parts. 



Most generally this irritation is upon a tactile corpuscle or 



* 1, Surface (epithelium). 2, Muscle. A, Centripetal fibre. B, Central nerve 

 cell. C, Centrifugal fibre. A, B, C, form the nerve arc, which presides over the 

 reflex action : the diastaltic arc of Marshall Hall. A represents the tisodicjibre ; 

 B, the central excito-motor ; and C, the txodic jibre. 



