500 BIOLOGICAL PHYSICS 



or other, or both, of which may be in use, according as 

 the sleep is shallow or profound, the former being 

 exemplified in the states of dreaming, and the latter 

 in the dreamless sleep of great fatigue or exhaustion 

 Exhaustion of the nerve energy production, which, 

 under ordinary circumstances, supervenes once in the 

 twenty-four hours, usually synchronising with the "day's 

 decline," determines the process of inhibition under the 

 normal physiological conditions, the material waste entailed 

 in the day's cerebral work and the neuronal output, the 

 lost, or spent, energy and the material waste being com- 

 mensurate with, and representing respectively, the amount 

 of dynamic expenditure and material loss or displacement. 

 As to how inhibition of the process and the machinery of 

 cerebration is effected we must confess to a want of 

 tangible evidence and proved data whereon to base 

 more than the slenderest inference ; nevertheless, taking 

 advantage of what little tangible evidence and data we 

 have, and straining the modicum of inferential light on 

 which we can legitimately lay hold for the purpose, we 

 think we are warranted in offering the opinion that it is 

 effected in some such way as the following : Up to the 

 present the " weight of evidence " favours the view that 

 the neuron is an independent unit histologically, but that 

 it is in contiguity with its neighbouring neurons so inti- 

 mately that, for combined functional purposes, it is 

 virtually in continuity through mutual dendritic contact, 

 the dendrons being possessed of the power of amoeboid 

 movements whereby they can be projected and retracted. 

 From this we would infer that the projection, or extension, 

 of these neighbouring, communicating processes or den- 

 drons coincides with the period of co-ordinated cell or 

 neuronal activity, and similarly that their retraction or 

 relaxation coincides with the period of cell rest or unicell 

 activity, and that these processes, being more or less in 

 active exercise during the period of the " waking day," 

 become exhausted, and require rest, and, therefore, on 

 the withdrawal of all stimulus to further exertion they 

 become for the time being permanently and systematically 

 released and withdrawn for, we may suppose, purposes 

 of material nutrition and renewal of cell energy. The 



