Blumenbach takes a similar view, and considers that sleep arises from want of circulation, or diminished afflux of arterial blood to the brain. (See Institutions of Physiology, p. 178. transl. by Elliotson, 2d edit.)

Bichât referred sleep to an ultimate law of nature, establishing, ashe conceived, the necessity for alternations of action and rest in the animal functions, whilst he maintained along with others, that the organic are exempt from this law. (See Recherches Physiologiques, p. 34.)

Whytt does not offer any explanation of the cause of sleep; but in his valuable treatise on the Vital and Involuntary Motions, he shews that the heart and other involuntary organs experience a diminution of sensibility during sleep, in consequence of which their action is retarded. (See Vit. & Inv. Mot. section 12.)

The most obvious objection to the doctrines of Cullen, Darwin, Brown, and others, which assume either a change in the quality, or deficiency in the quantity of the nervous power, is the suddenness with which this state is often removed; as when sleep and mental weariness are instantaneously dispelled by an impression on the mind. An objection that will not apply to the doctrine about to be submitted.

The inconsistency of representing sleep as an affection exclusively confined to voluntary functions, and yet referring it to altered circulation, seems to have been perceived by Bichât alone; who accordingly evades this dilemma by waving the inquiry into its cause, and referring it to an ultimate law of nature.

The facts that suggested this explanation, were too striking to be overlooked; and it is singular that they did not point out the evident subjection of involuntary as well as voluntary organs, to periodical change of action.

Whether sleep be ascribed to too much or too little blood in the brain; still, in either case, a periodical change of circulation is admitted.

If diminished circulation be deemed the cause of sleep, then the blood must be sent less forcibly to the head, to produce the change in question; and to account for this, the heart, which is

also an involuntary organ, must be allowed to be subject to periodical diminution of action.

If increased fulness of the cerebral vessels be supposed the cause of sleep, then a diminished resistance or periodical relaxation in these vessels must be admitted to take place, in order to account for their fulness; for it will hardly be contended, that the action of the heart is augmented, or the impulse of the blood stronger during sleep than waking.

Thus, in either case, the origin of sleep is referred to the class of involuntary organs; and a periodical remission of their action must be admitted to occasion it.

In fact, if the doctrine about to be submitted be correct, it will be seen that there is some foundation for both these opinions; and it would appear, that the partial manner in which the subject has been viewed is the chief cause of the contrariety of opinion that prevails.

Sleep will be found to result from full and slow circulation in the brain; proceeding from a spontaneous relaxation of the cerebral vessels, occurring simultaneously with a retarded pulse, from diminished action of the heart.

The periodical recurrence of this state, results from the general laws of fibrous contraction, to which the heart and capillary vessels are subject no less than other moving organs. And the influence which this change of circulation in the brain exerts over the faculties of the mind, is similar to the influence of altered circulation in every other function.

In order to substantiate the truth of this doctrine, however, the same method of proof is called for, that was pursued in regard to the doctrine of hunger. In the first place, the cause alleged must be independently proved to have a real existence; and not be hypothetically assumed to explain the phenomena. Secondly, the phenomena must be shewn to accord completely with the cause alleged. And, lastly, any objections to which this explanation appears liable, must be fairly met and answered.

The cause assigned for the diurnal revolutions of sleep and waking, being the same as that to which the periodical returns of hunger and thirst were ascribed; namely, the alternate remission and renewal of activity in the circulating system, its reality has, it is hoped, been already shewn. But, since the periodical change of vascular action, is a fact not obvious at all times, owing to numerous circumstances that may disturb its regularity, it is of some moment to notice the statements of those who have been esteemed the most accurate observers of nature.

The names of Haller, Hunter, Whytt, and Bichât, will ever command respect; and each of these writers has noticed the diurnal fluctuations of the pulse. They concur in stating that it is generally somewhat languid and feeble in the morning; becomes fuller and stronger about mid-day; grows rapid and irritable towards evening, the usual period of febrile exacerbation; and at length becomes full and slow at night as sleep approaches.

Here then, we have the successive stages of activity in the vascular system distinctly marked, and these stages of action succeeded by a state of comparative relaxation or rest.

The circumstances, however, that may disturb or modify these changes are numerous, for the vascular system is subject to the influence both of corporeal and mental impressions. Diversity of occupation and pursuits; difference in habits of life, as to activity or indolence, temperance or indulgence; in short, any thing that operates powerfully on either mind or body, may so affect the pulse as to render it difficult to verify the uniformity with which these stages occur.

Moreover, in the state of perfect health the changes are too slight almost to attract attention, and have therefore generally escaped notice, except during indisposition, when the mobility of the vessels being augmented, their range of action is increased, and their fluctuations become more perceptible. In the state of fever, for instance, these changes become very conspicuous, and the utmost importance attaches to the observance of the period at which the febrile exacerbation recurs.

The reality of the cause being then apparent, its adequacy to produce the effects ascribed to it, now calls for consideration. The influence which altered circulation in the brain exerts over the faculties of the mind, presents in fact nothing peculiar but is in every respect analogous to the effects of altered circulation in all other functions. As the power of motion and the faculty of sensation have been shewn to fluctuate with every change of circulation in their respective organs; so likewise the susceptibility for mental impressions, and the activity of the mind in the performance of intellectual operations, both vary with every change of circulation that the brain undergoes.

Thus, when circulation is impeded, as in the cold fit of an ague, the powers of perception are blunted, and the energies of the mind are impaired.

On the other hand, when circulation is much increased, as in the hot fit of fever, the reverse of this occurs; the susceptibility of impression is now rendered painfully acute, the temper becomes irritable, and delirium is apt to ensue from an uncontrollable rapidity in the succession of ideas.

Moreover, to complete the analogy, the influence which external agents possess in altering the circulation of the sentient organ, and which bodily exertion manifests in augmenting the circulation of the organs of locomotion, appears not less conspicuous in the effects of mental impressions on the circulation of the brain, and in the changes produced in this organ by the continued efforts employed in intellectual exertion.

Thus, for instance, as external irritation brings blood to the part, and causes redness of the surface; so the mental impressions of shame or anger, as formerly explained, determine blood to the brain, as indicated by the flushing of the face and redness of the eyes.

Again, as bodily exertion is attended with increase of circulation in the organs of locomotion; so long continued exertion of the intellectual powers causes a sense of weight and fulness in the head, which denotes congestion of blood in the organ of mind; and pain, after a certain period is equally apt to result from both.

If attentively considered, it will also be found, that exertion of the intellectual powers, exhibits successive stages, similar to those attendant upon bodily exertion, presenting the same fluctuations of activity, and connected with corresponding changes of circulation.

These variations of activity in the mental powers are most perceptible when the mind is long and vigorously exerted; but violent exertion is far from that which is most favourable in disposing to sleep, for reasons that will presently appear.

The successive stages of activity in the intellectual function, are marked as in other organs by symptoms of gradually increasing mobility.

In the first stage, the powers of mind are not fully developed, but with exercise the brain soon attains to the possession of its greatest vigour and activity. After these have been for a certain period energetically employed, the mobility of this organ, like that of others, is liable to become inordinate, and confusion of ideas is the result: and, eventually, the power of efficiently exerting the intellectual faculties, is partially impeded or wholly suspended; and pain in the head, as already stated, is the consequence of continuing this effort too long.

In short, this periodical change of function exhibits similar symptoms in the brain and in other organs; and appears connected with the same changes of circulation, as formerly pointed out. The effects of these changes only are variously manifested in different parts; in the organs of sensation, by change of sensibility; in those of motion, by change of mobility; and in the organ of mind, by change of the mental faculty.

Although the successive stages of mental activity are most distinctly marked, when the mind is long and vigorously exerted, yet the state of circulation thus produced, is by no means that which is most favourable to the production of sleep.

This condition of the mental faculty was stated to proceed from a spontaneous relaxation of the cerebral vessels, occurring simultaneously with a retarded pulse from a similar remission of

action in the heart.

Now in promoting this general relaxation in the capillary system, the increase of circulation attendant upon immoderate exertion of mind is no way favourable. On the contrary, as restlessness and pain in the limbs result from excessive bodily exertion, so over-exertion of the mind produces wakefulness and pain in the head.

The changes that result from violent exertion, may be regarded