224 



THE INTELLIGENCE SERVICE 



That depends on the energy set free by the cell and on the resistance 

 of the circuit. 



Of course the receptor must be modified to suit different kinds 

 of stimuli. A telegraph key or a bell push is a convenient kind of 

 mechanism for closing a circuit mechanically, but it would not 

 answer as well for electrical, thermal, sound or light vibrations. 

 Special means for closing the circuit have to be devised to suit 

 different kinds of stimuli. For example, sound waves may be 

 caused to close an electrical circuit by microphone, e.g., telephone 

 transmitter (see following chapter). 



The neuron may be likened to this electrical model (Fig. 54a). 

 The nerve cell is similar to the galvanic unit, F is the axon or 



XEACTING OR. EFFECTOR. UNIT : RECEIVING OR. KBCEPTOI^ UNIT 



I 



S 



N, 



'prr 



I 



! SYNAPSE C, 



^ 



KEY 



ELECTRDMAGNET 



Fig. 55. — A. ])iagram showing a receiving (A',) and a reuctim) Neuron (N.,), eacli with 



dendrites at its extremities, and their eoiiiiection to one another tlirougli a Synapsis {*'). 



B. Electrical Model to illustrate the functional contintiity of two neurons. See text. 



nerve fibre, the key the receptor mechanism and M the effector 

 mechanism. 



A second circuit of the nature of a telephone relay could be added 

 to the first, S being an electro-magnet which closes the second 

 circuit when the key of the first is depressed (Fig. 55). S may be 

 termed the synapse joining an effector and receptor neuron. The 

 student will notice that there is no material continuity between the 

 two neurons and that no energy passes from one to the other. 



4. " All or nothing." It is obvious in the electrical model that 

 connection is either made or not made. The energy available 

 from Cj is a fixed quantity independent of the energy used to 

 close the circuit, and similarly the energy in the system of which 

 C^ is the cell is independent of the energy used in the electro- 

 magnet S. 



It is true also for the nervous system that the maximum motor 

 effect is produced, if any effect is produced at all. It is a case of 

 " all or nothing." The existence of the " all or nothing " effect 



