236 ELEMENTS OF BIOLOGY 



The impulse is accompanied by a wave of electrical negativity, 

 measurable by a galvanometer set in circuit between the passing im- 

 pulse and a resting portion of the nerve. This fact reveals the 

 changing electrical conditions of the boundary as the excitation is 

 propagated along the fibre. That the impulse is primarily a phe- 

 nomenon of the surface of the fibre is shown by the fact that the 

 speed of the impulse is proportional to the surface and not to the 

 volume of the fibre. 



A nerve is either totally stimulated, or else does not react at all. 

 This characteristic is known as the All-or-None Law. Moreover, a 

 succession of stimuli, each too weak to set up a response, will, if 

 applied in rapid succession, initiate an impulse. These characteristics 

 are regarded as evidence that the excitation at the point the impulse 

 is initiated must set up a diflference of potential of a certain value 

 before the excitation is propagated, but once that value is reached, 

 the entire mechanism is totally engaged. A nerve fibre will transmit 

 stimuli in either direction, but in the living animal the functioning 

 nerve tracts will transmit stimuli in one direction only. The single 

 direction transmission in a reflex arc is known to be due to the 

 properties of the synapses between the axones of one nerve and the 

 dendrites of the next. The synapse may then be compared with a 

 one-way valve. The mechanism of the synapse is still elusive; little 

 is known of its nature. 



Anaesthesia. Since the middle of the last century the relief of 

 pain has been accomplished by the administration of certain anaes- 

 thetic agents. The subject of anaesthesia is so important from both 

 the theoretical and practical points of view that it merits discussion 

 here. Anaesthesia may be defined as a reversible decrease of irritabil- 

 ity. It may be accomplished by a wide variety of agents and condi- 

 tions, fat solvents such as ether, chloroform, and the alcohols, 

 water-solvent substances, as chloral and magnesium salts, alkaloids 

 such as the derivatives of opium, low temperature, and other con- 

 ditions. An ideal anaesthetic is one that produces a complete loss of 



