The Nervous System. 255 



in one direction only, we know that removed from the 

 body, it is just as easy to transmit nervous impulses in 

 one direction as the other; and experimental inquiry has 

 ascertained that it is possible to join a sensory to a motor 

 nerve, and reverse its function,* showing that it is possible 

 in the body to alter the actual current through the nerve. 



We have spoken of this as if the exciting influence were 

 due to an electrical current ; as a matter of fact, we have no 

 knowledge of the influence which is at work in producing 

 motion as the result of stimulating one nerve, sensation of 

 another, secretion of a third, and so on ; but we do know 

 that the irritability of nerves is readily manifested on the 

 application of electrical stimuli, and we may, therefore, at 

 at once consider the changes thus brought about. 



Electrical Phenomena of Nerves. — If a nerve be removed 

 from the body and suitably applied to an instrument which is 

 capable of measuring delicate electric currents, the galvano- 

 meter, the needle of the instrument will be found to be 

 deflected, showing the passage of a current ; it is spoken of 

 as the current of rest, and by many physiologists grave 

 doubts have been expressed as to whether this is a natural 

 current in nerves or not. It is practically identical in 

 direction with the natural muscle current described on 

 p. 246. If while the current of rest is passing a shock be 

 sent into the nerve from an induction coil, the needle 

 of the galvanometer is found to indicate a momentary 

 current in the opposite direction to the current of rest. 

 This momentary opposite current is spoken of as negative 

 variation or the current of action. If now, instead of 

 passing into the nerve a shock from an induction coil, we 

 pass into it a continuous current of voltaic electricity, 

 certain phenomena will occur, to explain which we must 

 suppose the nerve experimented upon to be in connection 

 with a muscle. 



The electrical stimulation employed is the make and 

 break of a constant (voltaic) current, such as is produced 

 by connecting two wires with a suitable battery, and an 



* This is not generally accepted as correct by physiologists. 



