REVERY, AND TRANCE. 



267 



case of natural sleep ; and in reality to constitute the imnfiediate, whilst 

 sensorial exhaustion only constitutes the remote, cause of this phaenome- 

 non. They appeal to the lethargic effect of a full stomach in infants, and 

 of drunkenness in adults, which they refer to congestion in the brain, in 

 consequence of a greater influx of blood into this organ : and hence they 

 reason that a similar sort of pressure is produced by some means or other 

 in every case of sleep. 



But what are the means of pressure thus referred to ? And here a con- 

 siderable difficulty is felt by every school of physiologists : and two distinct 

 schemes are devised to get rid of it. Bjr the one we are directed to the 

 arterial system, which we are told, becomes pecuharly excited and over- 

 charged in the organ of the brain during wakefulness, from the activity of 

 the internal senses.* By the other we are directed to the absorbent 

 system, which from the same activity is said to become worn out and ren- 

 dered torpid in the same organ ; and, hence, to be incapable of carrying 

 off the fine fluid which is perpetually exhaling from the secernent vessels 

 into the ventricles of the brain. 



Nothing, however, can be more unfounded than both these conjectures, 

 and it is difficult to determine which of the two is the most so. But we 

 are in no want of either of them, for we are in no want of the pressure 

 which they are invented to account for. The principle of exhaustion 

 alone will, I trust, be found sufficient to answer every purpose as a gene- 

 ral cause of natural sleep ; and, were it possible for us to add that of local 

 pressure, the sleep would no longer be natural, but morbid. 



Before we proceed further, however, I will just hint that Dr. Cullen 

 supposes the nervous fluid or power to be disposed by nature to an alter- 

 nating state of torpor and mobility.! He does not admit that it is ever 

 exhausted and restored as a secretion ;J and hence in sleep it is only sus- 

 pended : and in consequence of this suspension the exercise of sense and 

 volition is suspended also.§ Narcotics do fiot, therefore, in his view, ex- 

 haust, but only suspend the nervous power or fluid, and thus induce sleep, 

 which consists in such suspension. The apparently stimulant power of 

 narcotics, he derives from the vigilant exertion of the vis medicatrix 

 naturae, — the instinctive efl^ort of nature, to guard against such suspension 

 of vital power as essentially mischievous, and, when carried to an ex- 

 treme, fatal : and hence, oarcotics are with him directly sedative, but only 

 indirectly stimulant. He supposes both sleep and waking to take place 

 upon each other merely by a law of alternation : an explanation that will 

 satisfy few. 



But the chief attention of physiologists, both ancient and modern, has 

 been directed to the subject of dreaming, which has usually but erroneously 

 been regarded as a distinct process from that of sleeping. Let us next, 

 therefore, as briefly as may be, and before we enter into a direct analysis 

 of the phaenomena that successively arise, take a glance at a few of the 

 conjectures by which dreaming has hitherto been accounted for. 



Among the Greek philosophers we meet v/ith two explanations that are 



* This explanAtion is partly, though not chiefiy adopted by the author of the elaborate 

 article on sleep, in llees^ Cyclopaedia ; and has since been tully embraced by Mr. Car- 

 michael in his learned Essay on Dreaming. See Transactions of the association of Fellows, 

 and Licentiates of the King's and Queen's College of Physicians in Ireland, vol. ii. p. 48. 

 8?o. 1819. Dubl. His explanation of dreaming is that of Gall and Spurzheim, which the 

 reader will find adverted t« subsequently. 



Materia Medica, ii> 226, % Id. p. 223. § Id. p. 226. 



