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is a difficult and delicate task, even for scientifically trained men, to distinguish at all times between imaginations and facts; and this difficulty is materially increased by the circumstance that so much of the results of thought lie in a debateable region where hypotheses are unavoidably mingled with facts in the most complicated and perplexing way. The practical escape from this difficulty is the habit which sound reasoners acquire of classifying their own conclusions according to the inherent and intrinsic weight of each, so that some are held loosely as mere approximations to truth, whilst others are regarded as quite settled affairs. It was this consideration, no doubt, which was present to Faraday's mind when, in one of his charming lectures at the Royal Institution, he said, “Our varying hypotheses are simply the confessions of our ignorance in a hidden form; and so it ought to be, only the igno*rance should be openly acknowledged.' It is a notable instance of the almost invincible power of the ideo-motor influence over the human mind that the practised philosopher, who had arrived at so clear a perception of this important truth, nevertheless had a rather large series of hypotheses, which he habitually and avowedly excepted from his own wise canon. The same remark would apply to one of his most eminent successors in natural philosophy.

If the conclusion of the physiologists, that the presence of a convoluted brain-mass of necessity indicates a power of dealing with ideas, be correct, it follows that all the lower animals which have convoluted brains are also endowed with ideas. Wherever there is clear evidence of the existence of memory, as is unquestionably the case with the horse and the dog, it certainly must be so. It is probable, however, that in the most sagacious of the four-footed and four-handed types of the animated kingdom, these mental functions are altogether of the ideo-motor class, and that their succession is not directed or controlled by conscious purpose and will. If this be the correct statement of the fact, the mental lives of the lower animals must be somewhat of the nature of a long reverie or dream, chequered with episodical promptings of instinct.

A noteworthy instance of a dream-life of this character, in which the reveries appeared to have taken a very mathematical line in a canine brain, was but recently familiar in the scientific circles of London. The well-known spectroscopist and astronomer, Dr. Huggins, had a four-footed friend dwelling with him for many years as a regular member of his household, who was a mastiff of very noble proportions by descent, and who bore the great name of Kepler.' This dog possessed many

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rare gifts, which had secured for him the admiration and regard of a large number of scientific acquaintances, and amongst these was one which he was always ready to exercise for the entertainment of visitors. At the close of luncheon or dinner, Kepler used to march gravely and sedately into the room, and set himself down at his master's fect. Dr. Huggins then propounded to him a series of arithmetical questions, which the dog invariably solved without a mistake. Square roots were extracted off hand with the utmost readiness and promptness. If asked what was the square root of 9, Kepler replied by three barks; or, if the question were the square root of 16, by four. Then various questions followed, in which much more complicated processes were involved—such, for instance, as * add 7 to 8, divide the sum by 3, and multiply by 2.' To such a question as that Kepler gave more consideration, and sometimes hesitated in making up his mind as to where his barks ought finally to stop. Still, in the end, his decision was always right. The reward for each correct answer was a piece of cake, which was held before him during the exercise; but until the solution was arrived at, Kepler never moved his eye from his master's face. The instant the last bark was given he transferred his attention to the cake.

This notable case of canine sagacity, however, in no way militates against the remarks which have recently been made in reference to the ideo-motor character of the quadrupedal mind. Dr. Huggins was perfectly unconscious of suggesting the proper answer to the dog, but it is beyond all question that he did so. The wonderful fact is that Kepler had acquired the habit of reading in his master's eye or countenance some indication that was not known to Dr. Huggins himself. The case was one of the class which is distinguished by physiologists as that of expectant attention. Dr. Huggins was himself engaged in working out mentally the various stages of his arithmetical processes as he propounded the numbers to Kepler, and being, therefore, aware of what the answer should be, expected the dog to cease barking when that number was reached; and that expectation suggested to his own brain the unconscious signal which was caught by the quick eye of the dog. The instance is strictly analogous to the well-known case in which a button, suspended from a thread and held by a finger near to the rim of a glass, strikes the hour of the day as it swings, and then stopsthat is, provided the person who holds the button himself knows the hour! The explanation of this occurrence is that the hand which holds the button trembles in consequence of its constrained position, and in that way sets the button swinging; and as the attention of the experimenter is fixed upon the oscillation, in the expectation that a definite number of strokes upon the glass will occur, his own brain-convolutions take care that the movements of the finger shall be in accordance with that expectation.

The mathematical training of poor Kepler has unfortunately come to an untimely end. The interesting arithmetician died of an attack of typhus fever, to the great sorrow of his large circle of friends, at the beginning of last year, and he now sleeps under the shadow of the telescopes at Tulse Hill. The memory of his high attainments and of the distinguished success with which he upheld the reputation of his name, however, remains. His most intimate friends also enjoy the consolation of an excellent portrait of his thoughtful face, lit up with the exact expression which it bore when he was engaged with his arithmetical problems.

As has been already stated in an earlier paragraph, the sudden stoppage of the circulation of blood through the brain simultaneously and summarily puts an end to all manifestations of mind. This is so absolutely the case that firm mechanical pressure upon the arteries of supply instantaneously suspends all thought, feeling, and consciousness. Nature itself has, however, a more gentle way of exhibiting this crucial experiment. If the circulation of the blood through the brain is gradually reduced, instead of being summarily stopped, the same state of insensibility slowly supervenes. In other words, the animal goes to sleep. Observations made upon the lower animals show that during sleep the brain-pulp becomes contracted and pale in consequence of the diminution of its current of blood; and that, with the return of the waking and conscious state, the brain-pulp is again swollen out to its original dimensions, in consequence of its re-engorgement with blood. The vital current during sleep is sufficiently maintained to furnish nourishment for the repair of the exhausted brain, but it is not in sufficient force to keep up its functional activity. During any energetic exertion of the attention or will, on the other hand, exactly the opposite condition is brought about. A strong current of blood is then turned on upon the pulp of the cerebral ganglia, and is directed towards those particular parts of the structure whose functions it is intended to quicken. Physiologists conceive that the physical state upon which attention depends is simply increased force of blood thrown upon certain definite portions of the brain organisation. There is, indeed, a special arrangement in the living mechanism by which diminished or increased. flow can be brought about in reference to any part. The vessels which carry the supply of blood are actually diminished or enlarged according to the effect which is to be brought about. There are a series of fine nerve-threads and delicate muscular bands supplied to the blood-vessels them. selves to manage the proper adjustment of their dimensions according to the effect which is desired. When sleep is to be produced, the blood-vessels which proceed to the brain are narrowed by the contraction of their walls; and when the mind-operations are to be aroused and set to work the same blood-vessels are relaxed and enlarged, so that more of the blood sent out under the stroke of the beart

may
find its

way through their channels.*

In two of the books which have been named at the head of this article, the authors have travelled over very much the same ground. In both the object has avowedly been to explain how far physiological science has advanced towards an explanation of the nature and action of mind. In both instances the task has been honestly and well performed, so well, indeed, that it quite justifies this somewhat late notice of the books. The chief difference between them, perhaps, is that the Mental Physiology' is the more easy to read, and the more abundant and rich in its passages of illustration; whilst the · Physiology of Mind’ is more technical and more physiologically profound, although not so technical or so profound as to require more effort of attention than is readily given by persons of good culture and average intelligence. Both books are the natural outgrowths of larger works conceived in earlier years, but it is one reason for the great value of the result that has been secured in both cases, that it has been reached by two entirely different lines of investigation. The authors have arrived at common ground by quite opposite routes. The · Mental Physiology' has separated itself, by the mere force of its own accumulating weight, from Dr. Carpenter's larger work upon human physiology, which still ranks as an established text-book of the subject. The • Physiology of Mind' is an offset from a book called • The Phy

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The contraction of the smaller arterial vessels is effected through the influence of nerves supplied from the distinct sympathetic nervesystem of organic life, which has its chief centres in the visceral plexuses, and not in the spinal cord and brain. But their dilatation is produced by the operation of other nerve-threads derived from the system of the spinal cord. The power of mental action over the dilatation of the small blood-vessels is manifested in the familiar occurrence of blushing under emotion. VOL. CXLIX. NO. CCCV.

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* siology and Pathology of Mind,' which was also first published many years ago, and which was at that time a bold attempt to investigate some of the more obscure portions of the science of mind by means of the phenomena of insanity. The physiological branch of this treatise has, however, now grown so ripe that it has broken asunder from its pathological stem, and taken root for itself in the form in which it has recently appeared. For both books, it is not too much to say that their purpose has been amply and ably worked out. In both, clear forcible language is used in the construction of the argument, and in both a very complete impression is ultimately given of the existing state of this branch of physiological knowledge.

The great facts relating to the physiology of mind, which have been definitely established by the recent progress of scientific discovery, may be briefly expressed in the following condensed propositions.

With every expression of a mental state, and with every action of the mind, some structural change occurs in the substance of the brain. It is in that sense that the brain is the organ of the mind.

The change which occurs in the brain is of a destructive character. A complex unstable substance, formed out of the blood and deposited in the brain-globules, is decomposed and destroyed by the agency of oxygen. The nerve-influence and mind-action are energies evolved as a consequence of that decomposition. The brain-pulp is burned for the production of brain-force.

The combustible brain-pulp is deposited in minute membranous sacs, or globules, to which an abundant network of blood vessels is distributed. Through these blood-vessels both the oxygen, which is the agent effecting the corrosive decomposition of the pulp, and the nourishment which repairs the corrosive destruction, are conveyed to the brain. The blood circulation both wastes and sustains the brain, and in that way promotes its mental functions.

The globules of the brain are produced in quite incalculable numbers, and are in a state of continuous reproduction, growth, maturation, and decay. The cerebral globules are essentially living organs, which build up the structure of the brain by the multiplication of their own minute forms. The transmission of nerve-influence and mind-force between the several aggregations of globules, and between globule and globule, is effected by means of a destructive decomposition of the pulp of the nerve-threads which meander about amongst them in all conceivable directions. With every effective current of nerve

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