PHYSIOLOGY. 129 



ed on the heart, and therefore to the activity of the whole sys- 

 tem ; and from experiments which shew that from such an 

 affection of the brain, where the repletion of the ventricles is in- 

 terrupted, deliquium animi, epilepsy, and other disorders do so 

 certainly ensue, I am ready to conclude that not only the in- 

 creased impetus is the cause of preternatural waking, but that 

 a certain force is necessary to support the vital activity of the 

 brain in all its different degrees. I have thus gone over the se- 

 veral causes of sleep and waking as well as I can." 



CXXX. As most of the causes (CXXVIII.) are evidently 

 such as diminish motion in the brain, and those of CXXIX. 

 are such as increase it, it is from thence probable, that the 

 nervous fluid in the brain is truly capable of different states or 

 degrees of mobility, which we shall call its states of EXCITE- 

 MENT and COLLAPSE, but without intending by these terms to 

 express or determine any thing with regard to the nature of the 

 nervous fluid, or wherein its different states consist. " I take 

 it for granted, that considering the weakness and manifest mis- 

 takes of the other hypotheses (CXXV. CXXVI.), you will 

 readily think with me that the states of waking and sleep de- 

 pend on something in the matter of the nervous fluid itself, on 

 its having more or less mobility, and, in some cases, being capa- 

 ble of being moved with more ease and vigour, while in other cases 

 it is unfit for being moved with either. Now, merely to avoid long 

 expressions I shall choose shorter ones, and shall speak of the 

 moveable state of the nervous fluid under the name of its EX- 

 CITEMENT, and the deficiency or lesser degree of it I shall call 

 its COLLAPSE. I had the hint of this term from Dr. Hal- 

 ler : now you must consider these words merely as terms em- 

 ployed to express what I take to be a matter of fact, and not as 

 importing a matter of theory, or expressing any thing with re- 

 gard to the nature of the nervous fluid or of its different 

 states : whatever hypotheses I have fancied to myself, I con- 

 sider them as hypotheses still, and I dare not trust you with 

 them, unless you take them as they pass in my mind, and be 

 very certain never to apply them in particular cases. However, 

 I am willing to bestow a few minutes upon a lusus ingenii ; 

 if you carry it further, it is at your peril. This subject may 

 receive some illustration in this way : There is seemingly dif- 



VOL. i. i 



