KEVERY,AND TRANCE. 47 



have established. But there is still a very important question, and which, 

 indeed, constitutes the chief difficulty of- the subject, and that'whieh none of 

 them have attempted to answer, or, at least, have satisfied themselves upon, 

 while making such attempt. I mean, whence comes it to pass that ideas can 

 at all exist in the brain during sleep, or that all the internal senses are not as 

 much locked up as the external senses, and the faculty of the will? 



In the -course of the present lecture it will be my. endeavour to account for 

 this most curious phenomenon. But we must first follow up, in the series in 

 which they appear to arise, the train of circumstances which accompany sleep 

 and dreaming. The entire study is highly interesting, but requires close 

 attention, in order to its being fully comprehended. And when we have ad- 

 vanced thus far, we shall obtain a clew, if I mistake not, to those equally 

 abstruse and intimately connected subjects, sleep-walking, revery, and winter 

 sleep; as well as to various other obscurities that ramify from the same source. 



The fibres distributed over the moving organs of animals, I have already 

 had occasion to observe, in a preceding lecture,* are of two sorts : those of 

 the nerves, which are, called sensitive fibres, and those more properly belong- 

 ing to the muscles, which are called irritative fibres ; which last, however, 

 are always accompanied by a greater or less number of the former ; by which, 

 indeed, they become endowed with the sense of touch, as well as are rendered 

 capable of contributing to the other external senses, and of maintaining a 

 communication with the brain, from which the sensitive fibres issue, or in 

 which they terminate. 



Both these kinds of fibres become fatigued, exhausted, and torpid, in pro- 

 portion to the length and violence of their exertion, and recover their power 

 alone by rest. The weariness and flaccidity of the muscles of the arms or 

 legs after extreme exercise, or exercise to which they have not been accus- 

 tomed, may be adduced as a sufficient proof of the truth of this position.f 

 In like manner, we neither hear, nor see, nor taste, nor feel, with the same 

 accuracy, after any or all the organs of these various functions have been 

 long upon the full stretch of action, with which we do on their first exertion 

 in the morning. Increase or prolong this action, and their power will be still 

 farther obtunded, till at length, like an over-wearied limb, they become per- 

 fectly inert and insensible, and give no account of whatever is passing around 

 us ; and it is this general torpitude or inaction of all the external senses, 

 which we call SLEEP. By the exercise of the will, or by any other strong 

 stimulus, this sleep or sensorial torpitude may be postponed; and, vice versa, 

 by the consent of the will, it may be accelerated. 



This, however, is sleep in its first or simplest shape alone : it is that which 

 I shall take leave to call SLUMBER, and '-& the mere sleep, or torpitude of the 

 organs of external sense'; the will being drowsy, indeed, but still continuing 

 in some degree awake: whence the sleeper, if he lie or sit in any uneasy 

 position, exercises his muscles, which are still under the control of the will, 

 and the position is changed. The other internal senses also, as those of 

 memory, imagination, and consciousness, are in like manner, in a greater or 

 less degree, awake ; whence the mind is yet filled with ideas, that crowd upon 

 one another with about an equal degree of regularity and confusion : and, if 



* Scries i. Lecture x. p. 107. 



t The principles ol 1 the theory here advanced were first given to the world, by the author, as far back as 

 1805, in the comment subjoined to his translation of Lucretius, where the poet is treating of the cause and 

 phenomena of sleep ; and may be found in vol. ii. p. 137141 of that work. Several of the doctrine* 

 there laid down have been since advanced in various limns by different writers, though in some cases, very 

 probably, without their having perused his explanation. Thus the immediate cause of sleep, advanced in 

 the present passage, is that chicrJy rested upon by the author of the article on sleep in Dr. Rees's Cyclo- 

 pedia, though he also adverts to an occasional increased action in the vessels of the brain as a concurrent 

 cause. And thus much of the explanation which will here be found to follow, respecting the nature and 

 phenomena of dreaminir, have still more lately been offered to the world by Dr. Spur/.heim, and adopted 

 from him by Mr. (Jurmichacl of Dublin, with the exception that they have interwoven such views with their 

 peculiar doctrine of a plurality of organs in the brain ; which, lor reasons that will be given in a subse- 

 quent lecture (Series in. Lecture xiii.), the present author cannot admit; and docs not conceive is by 

 any means necessary on the present occasion. Such coincidences of opinion, however, and especially ir 

 they should he accidental, and not derived from his comment on Lucretius, give a considerable degree of 

 confirmation to tho general basis on which the theory rcsta. The lecture, as now published, was delivered 

 In the spring of 1611 



