ch. vii.] SOURCES OF TERRESTRIAL ENERGY. 413 



knowledge is likely to bridge over; it is nevertheless un- 

 questionable both that every change in consciousness is con- 

 ditioned by a chemical change in ganglionic tissue, and also 

 that there is a discernible quantitative correspondence be- 

 tween the two parallel changes. Let us glance for a moment 

 at certain facts which will serve to illustrate and justify 

 these propositions. 



Those changes of consciousness which are variously classi- 

 fied as thoughts, feelings, sensations, and emotions, cannot 

 for a moment go on save in the presence of certain assign- 

 able physical conditions. 



The first of these conditions is complete continuity of 

 molecular cohesion among the parts of nerve-tissue. A 

 nerve which is cut does not transmit sensori-motor im- 

 pulses; and even where the continuity of molecular equili- 

 brium is disturbed, without overcoming cohesion, as in a 

 tied nerve, there is no transmission. It is in the same way 

 that pressure on the cerebrum instantly arrests consciousness 

 when a piece of the skull is driven in by a blow, and slowly 

 arrests it when coma is produced by congestion of the 

 cerebral arteries. Now the need for complete continuity of 

 molecular equilibrium, both in the white and in the grey 

 tissue, is a fact of no meaning unless a molecular rearrange- 

 ment is an indispensable accompaniment of each change in 

 consciousness. 



Secondly, the presence of a certain amount of nutritive 

 material in the cerebral blood-vessels is essential to every 

 change in consciousness ; and upon the quantity of material 

 present depends, within certain limits, the rapidity of the 

 changes. While rapid loss of blood causes fainting, or total 

 stoppage of conscious changes, it is also true that lowered 

 nutrition, implying deficiency of blood, retards the rate and 

 interferes with the complication of mental processes. In a 

 'state of extreme anaemia not only does thinking go on 

 slowly, but the manifold compounding and recompounding 



