THE GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM 99 



p. 25) ; but in the crossed reflex two such synapses are involved (a three- 

 neuron circuit such as the pathway from d.r.2 to v.r.l' through correlation 

 neuron 1 in Fig. 61, p. 134), and the introduction of the second synapse 

 doubles the time. It is, therefore, assumed that it requires in the frog 

 between .01 and .02 second for the nervous impulse to pass the synapse 

 between two neurons in a reflex circuit. 



Turning now to the activities of the nerve-cell body, it will 

 be recalled (p. 45) that here the chroraophilic substance is gen- 

 erally scattered throughout the cytoplasm in the form of the 

 "Nissl bodies." This substance is very similar to that of the 

 chromatin of the nucleus, from which it is said to be derived 

 during the development and functional activity of the neuron. 

 During the resting state of the cell it and other reserve materials 

 accumulate in the cytoplasm; and now, when the cell is stimu- 

 lated to activity, the energy thus stored up may be liberated 

 almost instantly because the chemical substances necessary for 

 the reaction are widely diffused throughout the entire mass of 

 the cytoplasm. 



The function of neurons, as compared with that of most other 

 cells of the body, may, therefore, be described as of the explo- 

 sive type. A word of explanation will render the analogy clear. 

 In ordinary combustion, oxygen is supplied to the surface of the 

 burning material, say a blazing log, and the chemical process 

 of burning goes on only as fast as the superficial parts can le 

 oxidized and removed. But explosive substances are chemic- 

 ally so constituted that as soon as combustion begins oxygen is 

 liberated in the interior of the material and the process of oxi- 

 dation takes place almost instantaneously throughout the 

 entire mass. Similarly in the nerve-cell, the processes of metab- 

 olksm are not dependent upon the slow interchange of substances 

 through the nuclear membrane between the cytoplasm and the 

 nuclear plasm; but the chromophilic substance distributed 

 through the cytoplasm permits of much more rapid responses. 

 The organization of the protoplasm of the nerve-cell is such 

 that a very small stimulus may liberate a large amount of energy 

 with explosive suddenness. The energy thus liberated does 

 not all leave the cell, but part of it is directed into the axon, 

 which is thereby excited to conduct a nervous impulse to the 

 appropriate end-organ or to the next synapse, and thence to a 

 second neuron. 



