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SECT. XL.

OF SLEEP.

THOUGH man in his fleeping state is a much less perfect animal than in his waking hours, and though he confumes more than one third of his life in this his irrational situation, yet is the wisdom of the AUTHOR of NATURE manifeft even in this feeming imperfection of his work.

According to Dr. JOHN BROWN,

I. Life is a forced state.

II. To every animated being is allotted a certain portion of

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III. Every power that acts on the living frame is STI

MULANT, or produces excitement by expending IR

RITABILITY.

*Dr. BROWN fuppofed every living fyftem to have received at the beginning its determinate portion of IRRITABILITY, and therefore, although he spoke of the exhaustion, augmentation, and even renewal, of IRRITABILITY, it was not his intention to induce his pupils to think of it, as a kind of FLUID SUBSTANCE generated in the animal body. According to him IRRITABILITY was an unknown somewhat, fubject to peculiar laws, and whose different fates we were obliged to defcribe by terms borrowed from the qualities of material fubítances. See Part I. page 33; alfo the Life of Dr. BROWN by Dr. BEDDOES, prefixed to a new edition of his works.

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IV. The

IV. The EXCITING POWERS may be referred to 2 claffes. 1. Mechanical, as heat, food, blood, &c.

2. Mental, as thought, and the emotions of the mind, &c *.

V. The application of ftimuli + after a certain time exhaufts the IRRITABILITY, OF EXCITABILITY, of the fyftem.

To recruit which lofs, the all-wife and merciful CREATOR has inftituted the feafon of SLEEP, at which time the fti

*As Dr. BROWN has defined life to be a forced ftate, it is fitly reprefented by a flame, forcibly drawn forth, from fuel little difpofed to combuftion by the conftant application of streams of air poured into it from the different tubes of a machine.

1. If fome of the tubes are fuppofed to convey oxygen, or pure, air, they will denote the highest class of exciting powers, opium, musk, spirits, camphor, wine, &c. the DIFFUSIBLE STIMULI of Dr. BROWN, which bring forth for a time a greater quantity of life than usual, as the blowing in of pure air into a fire will temporarily draw forth an uncommon quantity of flame.

2. If other tubes be fuppofed to convey common, or atmospheric, air, they will represent the ordinary EXCITING POWERS, or ftimuli, applied to the human frame, fuch as heat, light, air, food, drink, &c.

3. While fuch as convey impure or bad air may be used to denote what have formerly been termed SEDATIVE POWERS, such as bad diet, contagious miafmata, foul air, &c.

†The reader will now probably be at no lofs to understand the seeming paradox of the Bru.onian fyftem; that food, drink, and all the powers applied to the body, though they support life, yet confume it; for he will fee, that the application of these powers, though they bring forth life, it wastes the ExCITABILITY, or MATTER OF LIFE, just as the air blown into the fire brings forth more flame, but wastes THE FUEL, or MATTER OF FIRE. This is conformable to the common faying, the more a spark is blown, the brighter it burns and the fooner it is spent.

muli of external objects are excluded by the filence and the darkness of the night; and as cold* accumulates the irritability of the fibre, it is wifely fore-ordained that this season shall be accompanied with a fuitable degree of cold. Common or ordinary fleep, produced by the application and action of ftimuli, from what has been said, feems therefore to be a state, the refult of a law of the animal œconomy, which takes place in order to remove the effects of ftimuli, and to restore, as much as poffible, the excitability of the fyftem; as during this ftate the ftimulus of volition is fufpended, all external objects cease to make impreffion, even cathartics lofe their powers of action, while the atmospheric air is almoft the only external power, which then continues to be applied, at once carrying off what is excrementitious from the lungs, as it affords probably to the system that principle which is expended by the various actions of life.

Befides the very great quantity of the IRRITABLE PRINCIPLE perpetually expended in moving the arterial, venous, and absorbent systems, and the other organs of the body, as described page 351, there is alfo, during our waking hours, a conftant expenditure of it by the action

* Hence the evil of feather-beds; for we should court no more than a fuitable degree of warmth.

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of our locomotive mufcles and organs of sense. Thus the optic nerves, where they enter the eye, and the 'great expansion of the nerves of touch beneath the whole of the cuticle, evince the great confumption of the IRRITABLE PRINCIPLE by these senses. And our perpetual mufcular action in the common offices of life, and in constantly preserving the perpendicularity * of our bodies

during

*When any perfon lofes the power of muscular action, whether he is erect or in a fitting pofture, he finks down upon the ground; as is feen in fainting fits, and other inftances of great debility. Hence it follows, that fome exertion of muscular power is necessary to preserve our perpendicular attitude. This is performed by proportionally exerting the antagonist muscles of the trunk, neck, and limbs; and if at any time in our locomotions we find ourfelves inclining to one fide, we either restore our equilibrium by the efforts of the muscles on the other fide, or by moving one of our feet extend the base which we reft upon to the new center of gravity. But the most easy and habitual manner of determining our want of perpendicularity, is by attending to the apparent motion of the objects within the fphere of distinct vifion. Hence no one who is hood-winked can walk in a straight line for a hundred steps together; for he inclines fo greatly, before he is warned of his want of perpendicularity, not having the apparent motions of ambient objects to measure this inclination by, that he is neceffitated to move one of his feet outwards, to the right or to the left, to fupport the new centre of gravity, and thus errs from the line he endeavours to proceed in. Thus any one who ftands alone on the top of a high tower, if he has not been accustomed to balance himself by objects placed at such distances and with such inclinations, begins to, and endeavours to recover himself. During this time the apparent motion of objects at a distance below him is very great, and the impressions of these apparent motions continue a little time after he has experienced them; and he is perfuaded to incline the contrary way to counteract their effects; and either immediately falls, or applying his hands to the building, uses them to preferve

during the day, evince a confiderable expenditure of the IRRITABLE PRINCIPLE by our locomotive muscles. It follows, therefore, that if the exertion of these organs of fenfe and muscles be for a while intermitted, that a large quantity of the IRRITABLE PRINCIPLE must be accumulated.

As foon as a perfon begins to fleep, as (in hemiplagia, where the limbs on one fide have lost their power of voluntary motion, and the patient is for many days employed in moving those of the other, or as when in the cold fit of an intermittent fever fome parts of the fyftem have for a time continued torpid, and have thus expended lefs than their ufual expenditure of the IRRITABLE PRINCIPLE), a hot fit succeeds; so, owing to the fufpenfion of the voluntary actions, the peristaltic motion of the

his perpendicular attitude, contrary to the erroneous perfuafions of his eyes. Thus on horfeback we accurately observe another person, whom we meet trotting towards us, without confounding his jumping and progreffive motion with our own, because we have been accustomed to them both; that is, to undergo the one, and to fee the other at the fame time. But in riding over a broad and fluctuating stream, though we are well experienced in the motions of our horse, we are liable to become dizzy from our inexperience in that of the water. And when we first go on fhip-board, where the movements of ourselves, and the movements of the large waves are both new to us, the vertigo is almost unavoidable with the terrible sickness which attends it, and after we come from on ship-board, being used to reel about to maintain our perpendicularity, we have at first the same drunken gait as we had on fhip-board.-Dr. DARWIN.

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