S44 ON SLEEP, DREAMING, 



the same activity is said to become worn out and rendered 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 general 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 farther, however, I will just hint that Dr. Cullen sup- 

 poses the nervous fluid or power to be disposed by nature to an alternating 

 state of torpor and mobility.* He does not admit that it is ever exhausted 

 and restored as a secretion ;f and hence in sleep it is only suspended : and in 

 consequence of this suspension the exercise of sense and volition is sus- 

 pended, also4 Narcotics do not, therefore, in his view, exhaust, 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 rnedicatrix naturae, the instinctive 

 effort of nature to guard against snch suspension of vital power as essen- 

 tially mischievous, and, when carried to an extreme, fatal : and hence, nar- 

 cotics are with him directly sedative, but only indirectly stimulant. He sup- 

 poses 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 pheno- 

 mena that successively arise, take a glance at a few of the conjectures by 

 which dreaming 1 has hitherto been accounted for. 



Among the Greek philosophers we meet with two explanations that are 

 worthy of not'iQe ; that of Epicurus, because of its ingenuity, and that of 

 Aristotle, because it has descended to the present times. 



According to the Epicurean hypothesis of sensation, all the organs of ex- 

 ternal sense are stimulated to their appropriate functions, by the friction of 

 an effluvium or emanation thrown off from the body perceived. This doc- 

 trine, which still holds good, and is uniformly employed in modern times to 

 explain the senses of taste and smell, was equally extended by Epicurus to 

 those of sight and hearing: the former being supposed to depend upon an 

 effluvium of exquisitely fine films, images, or SPECIES, as they were technically 

 called, perpetually issuing in every direction from every existing substance, 

 somewhat in the manner in which snakes and grasshoppers cast off their 

 skins annually, but almost infinitely finer, and altogether invisible. And as 

 these rush against the eye, they were conceived to convey to it a perfect image 

 of the object from which they are ejected. While sound was supposed to 

 be excited in like manner by particles of a peculiar kind thrown off from the 

 sonorous body, and rousing the ears by their appropriate stimulus. 



These effluvia of every kind were conceived to be so exquisitely attenuate 

 that they can pass, as light, heat, or electricity does, through a variety of 

 solid bodies, without being destroyed in their passage. The efflnv.ia or pelli- 

 cles of vision were supposed not unfrequently to arise from the very bodies 

 of those that have been long buried; and to be capable not only of trans- 

 piercing the soil in which they are inhumed, and of stimulating the organs of 

 external sight, but of winding through the substance of the flesh, and of sti- 

 mulating the soul itself in the interior of the animal frame, especially when 



Dreaming. See Transactions of the Association of Fellows and Licentiates of the Kin's and Queen's 

 College of Physicians in Ireland, vol. ii. p. 48, 8vo. 1819, Dubl. His explanation of drea:iiitig is that of 

 Gall and Spurzheim, which the reader will find adverted lo subsequently. 

 Mttttia Medlpa ii.^25. j ib. p.2<23. J Th. p. 220.. 



