The great intellectual principle which pervades every intellectual function, the association of ideas, is also carried on in the brain; and it is this principle which seems to require that the receptacle of our ideas should be so large and extensive. We mean not to revive the old system of material vestiges as the effect of impression, and the sensible prototypes of our ideas; but it involves no contradiction to suppose, that a nervous fibre, whose mobility has been once excited, will more readily yield to the same stimulus, when repeated in a less degree; and ideas once connected will, by the same increased mobility, be excited by any impression, mental or corporeal, on the neighbouring parts, as the voice involuntarily pursues the air which another has begun to sing. The variety of distinct impressions communicated to the mind through the medium of the nerves, necessarily, therefore, requires an extended bulk of the common sensorium; and, though a part of the cerebrum may be occasionally destroyed without apparent injury to any function, we need only reflect how few and simple are the ideas of the greater number of mankind, compared with those of Bacon, Newton, Boyle, or Locke. Yet we see, after a partial compression of the brain, some of the powers of mind greatly weakened; and it is singular that these powers are chiefly such as are concerned in succession, and consequently association; as in counting numbers, or pursuing any successive train of ideas.

It has been doubted, whether the mind can of itself commence a train of thought wholly new, or excite ideas different from those received through the senses. We have, as already observed, never been able to discover any such in all the wanderings of a morbidly excited imagination, all the reveries of fancy, all the eccentric images in dreams or low delirium: yet the mind can voluntarily again raise ideas formerly impressed; and this faculty is styled reminiscence, or memory: a power which is greatly assisted by association.

Though we consider mind as an immaterial principle, yet, as its instrument is the brain, the state of mind must often appear to partake of the diseases of the body: and the restoration of mind equally follows the returning health of body. The reason is, that we do not see the mind act but through the medium of the corporeal organs; and every agent will be powerful in proportion to the power of his means. We. therefore, in considering the effects of astringents, referred the change in the state of the nervous fluid to that of the solid; and so again in mental diseases we shall endeavour to show the connection of mental powers with the same fluid. In some instances the change is so sudden, that the fluid itself must be primarily affected, as in the attack of fevers; and the mind is weakened in proportion. This did not escape Shakspeare, who has described the effects of an ague fit with the spirit of a poet, and the precision of a philosopher. Speaking of Caesar, Cassius says,

"He had a fever when he was in Spain; And, when the fit was on him, 1 did mark How he did shake ! ' Tis true, this god did shake: His coward lips did from their colour fly: And that same eye, whose bend doth awe the world, Did lose its lustre I did hear him groan;

Aye; and that tongue of his, that bade the Romans Mark him. and write his speeches in their books, 'alas !' it cried,' Give me some drink, Titinius,' As a sick girl."

This sudden change in the nervous energy, as evident in fevers, and in breathing noxious vapours, has been styled by Dr. Cullen "collapse;" and we shall adopt the term more from shortness than any desire of connecting it with any real alteration, or as referring to any theory. Yet, as we know no instance in nature of a sensible fluid circulating with the rapidity of the nervous; and as we do know that a power not obvious to our senses, which possesses some of its peculiar properties, really exists, we perceive no theoretical temerity in adopting its terms. It extends, as we have said, the bounds of language; and facts expressed in this language may be without violence transferred to any other, when future discoveries may extend our knowledge of this mysterious power, whose cause we know not, but whose influence is general and extensive.

To penetrate further into the regions of metaphysics is not suitable to our present object. What we have now introduced, we shall have occasion hereafter to apply. We shall conclude this article with some more particular account of the circulation through the brain than the pages of Dr. Motherby and his associate have supplied. To connect the whole, a little repetition is unavoidable.

The course of the carotid arteries has been already explained; and the peculiarity in the circulation chiefly relates to the venous system. The veins are large and tortuous, unconnected with cellular substance, without valves; freely anastomosing, and terminating in reservoirs rather than in large trunks. The apparent great object of this apparatus is to confine a certain portion of blood in the head; and, at the risk of a morbid accumulation, to prevent the source of the nerves from wanting at any time a supply. Even respiration, as we shall find, contributes to the same purpose.

The mode in which the veins open into the sinuses confirm the same principle. We are informed by Vicq dazyr that they open in a direction opposite to the current of blood; and, though the universality of this distribution has been doubted, it is admitted that they very frequently penetrate obliquely through the coats of the sinus, as the ureters into the bladder; or that little; valves occasionally cover their apertures; a structure which produces the same effects.

Where the veins penetrate the dura mater, this membrane and the pia mater are united by a fatty substance, which has been mistaken for the glands of Pacconius; but these are in the cavity of the longitudinal sinus, and act still further as valves, to save the arteries from the danger of being exhausted; since these glands are placed at the entrance of the vein into the sinus. The course of the smaller veins is circuitous, and with difficulty, explained without numerous plates. We shall not attempt a description, for it admits of no application.