64 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



message sent from the cut end, the head office at Wash- 

 ington could not but refer it to the place where this 

 wire ought to go. 



Law of Peripheral Reference. — Thus we have 

 the law that an impulse received by the brain through a 

 nerve fiber is of necessity referred by the consciousness to the 

 peripheral extremity. . This explains the fact that in 



the case of an amputated limb the patient still has a 

 sense of the presence of a foot or hand; and if the 

 nerve of the stump should become diseased, he will 

 often feel intense pain in foot or hand. 



Nerve Force versus Electricity. — I have used 

 this comparison with a telegraphic system in order to 

 make the mode of action of the nervous system clear. 

 But we must not conclude, therefore, as many do, that 

 nerve force, and indeed life itself, is nothing but elec- 

 tricity. It becomes necessary, therefore, that we should 

 draw attention to some fundamental differences be- 

 tween these two forms of energy : 



i. Wires lying in contact with one another in the 

 same bundle will not conduct true unless insulated. 

 Nerve fibers, on the contrary, conduct true although 

 lying in contact in the same sheath — in a moist condi- 

 tion, and therefore uninsulated. 



2. Cut a wire and press the fresh-cut ends together 

 ■ — they still conduct well. But a cut nerve pressed to- 

 gether utterly fails to conduct nerve influence. 



3. In the case of an electric current there must be 

 a closed circuit. This is fundamental. If the circuit is 

 open anywhere there is no current and can not be. 

 Not so in the case of a nerve current. There is indeed 

 a sensory current and a return motor current. They 

 are connected, too, at the cerebral end, but certainly 

 not at the peripheral ends. Besides, there is often 

 current only one way — i. e., sensation without corre^ 



