seven hours sleep, the fluid exhaled from the skin of a healthy man was about double what was lost in the same time while he was awake. (Med. Stat. Sect. 4.)

But as less caloric is obtained by respiration during sleep than when we are awake; if the body be not well covered, it is more easily chilled, and the circulation depressed, than during exercise. Hence it is, that when exhausted by previous exertion, the caloric obtained by breathing is still more rapidly abstracted by exposure to cold and damp night air during sleep, than it is replaced,-by which the body is predisposed to a chill, which ushers in all the different forms of fever and other constitutional maladies. M. Quetelet states in his recent statistical work on man, that the number of inspirations are diminished during sleep, in the ratio of 6 to 7 when awake; and the pulsations at the wrist, in the ratio of 3 to 4 per minute:—that among 300 individuals, at different ages, the extreme values were as represented in the following table :-(see p. 71.)

But that the diminished frequency of the heart's

action during sleep is owing to the cessation of

THE FREQUENCY OF THE PULSE.

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voluntary motion, has been demonstrated by Dr. Knox, who found that in a healthy young man of regular habits, aged 20, the average number of pulsations at 7 in the morning, was 74 beats greater than at 10 in the evening, independent of

He has further shown, that the excitability of the heart diminishes regularly from an early hour in the morning until midnight, when it is at zero. And I have found that the mean temperature under the tongue, ceteris paribus, is from 1 to 2 or 3o higher early in the morning than at midnight.

As might naturally be supposed, Dr. Knox observed, that the action of the heart was greatly reduced by sitting in a cold room :*-that its pulsations are most frequent in infancy, and diminish on till old age, when they are at a minimum :-that they are more accelerated by food and exercise in the morning and during the forenoon, than in the afternoon, and least of all in the evening,-more in weak than in strong individuals; but that this does not apply to wine and spirituous liquors :-finally, that the action of the heart is augmented more by muscular exertion, than by fever or any other cause. (Ed.

* Blumenbach says the pulse is slower in cold than in tropical climates, where the mean temperature under the tongue is about 2o higher than in England, as shewn by Dr. J. Davy.

Med. and Surg. Journal for 1813-14-15.) Nor is this latter fact at all surprising, when we reflect that from three to four times more caloric is disengaged by respiration during active exercise, than when we are at rest. Nor is it difficult to comprehend, why the action of the heart is more strong and frequent in the morning than evening, when we reflect that more animal heat is expended by exercise during the day, than is obtained from the atmosphere by respiration.

It is said by M'Nish, that birds sleep less than mammalia, and the latter less than man, who requires more during winter than summer, and more in cold than in hot climates. However this may be, it is certain, that during the long nights of winter, for some time before and after Christmas, the cock frequently crows at intervals, as if not disposed to sleep so long. During the prime of life, men require from six to eight hours sleep, if we except those of large chests, vigorous constitutions, and sanguine temperament, who are able to endure great muscular and intellectual exertion with much less repose, than individuals of the phlegmatic temperament. During the most active period of their lives, Alexander, Julius Cæsar, Napoleon, Washington, the Duke of Wellington, Lord Brougham, and many other distinguished men, have not slept above four or five hours in the twenty-four. But nothing tends more certainly to injure the constitution and shorten life, than a want of sufficient sleep,

IMPERFECT DURING OLD AGE.

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which is more essential to men of studious habits, and to such as take much laborious exercise, than to the idle and inactive. Owing to imperfection in the nutritive process, sleep is less sound and refreshing in old age, than during the earlier periods of life.

Whatever diminishes the circulation of arterial blood through the brain, and thus impairs its vital activity, tends to produce drowsiness. For example, it is often induced by a full meal, which causes a determination of blood and vital heat from the brain and other organs to the stomach, for the purpose of supplying it with gastric juice, during the process of digestion ; causing sleepiness, languor, and chilliness, in feeble constitutions. Hence the impropriety of taking late and hearty suppers, (especially after a full dinner,) the digestion of which during sleep requires a large supply of blood in the stomach, at a time that it should be employed exclusively in repairing the previous waste of the solids, which, as we have seen, is the principal object of repose. For the same reason, (that is, owing to diminished nutrition) sleep is less sound after full than light suppers; and more of it is required to refresh and renovate the body.

Sleep is frequently induced by a glass of negus or warm spirits and water, which also produce a determination of blood from the brain to the stomach,―and by hearing a dull monotonous discourse, which nearly suspends the train of one's

own thoughts, without being interesting enough to maintain the activity of the brain. Sleep is also induced by exposure to the air of a heated room, which causes a determination of blood to the surface, at the expense of the brain, muscles, and other important organs. Hence it is, that the most delightful of all soporifics is the warm bath, especially after exposure to cold and fatigue, as it removes the stiffness, soreness, and aching, which sometimes prevent sleep when most required. But excessive warmth is unfavourable to sound sleep; first, by increasing the circulation of blood through the brain; and secondly, by raising the temperature of the solids nearly to an equilibrium with that of the arterial blood; by which the combination of its particles with the solids is diminished. Hence it is, that when oppressed with too much covering, we feel languid and unrefreshed on rising in the morning; and that men sleep more soundly in temperate than in hot climates, where nothing more conduces to healthful repose than cooling ablutions, or the tepid bath, before going to bed.

Whatever greatly diminishes the nutritive process, tends to prevent natural sleep, which is therefore always imperfect, if not wholly interrupted during fever, and many other forms of disease. It is also frequently prevented by over activity of the nervous system, caused by mental anxiety and too intense thinking, which interfere with the nutritive process, and induce a feverish