THE BRAIN THE ORGAN OF MIND.

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(2.) Injury or disease of the brain impairs in some way or other the powers of the mind. A blow on the head will destroy consciousness for the time; a severe hurt will cause a loss of memory. The various disorders of the brain, as inflammation, softening, &c., are known to affect the mental energies. Insanity is often accompanied by evident cerebral disease.

(3.) The products of nervous waste are increased when the mind is more than ordinarily exerted. The alkaline phosphates (triple phosphate of ammonia and magnesia) removed by the kidneys are derived principally from the waste of nervous substance; and they are sensibly increased after great mental exertion or excitement.

Phosphorus

abounds more in the brain than in any other tissue.

(4.) There is an indisputable connexion between size of brain and the mental energy displayed by the individual man or animal. It cannot be maintained that size is the sole circumstance that determines the amount of mental force. But just as largeness of muscle gives greater strength of body, as a general rule, so largeness of brain gives greater vigour of mental impulse. The measurements of the heads of remarkable men have often been quoted. 'All other circumstances being alike,' says Dr. Sharpey, 'the size of the brain appears to bear a general relation to the mental power of the individual, although instances occur in which this rule is not applicable. The brain of Cuvier weighed upwards of 64 oz., and that of the late Dr. Abercrombie about 63 oz. avoirdupois. On the other hand, the brain in idiots is remarkably small. In three idiots, whose ages were sixteen, forty, and fifty years, Tiedemann found the weight of their respective brains to be 193 oz., 25 oz., and 22 oz.; and Dr. Sims records the case of a female idiot twelve years old, whose brain weighed 27 oz. The weight of the human brain is taken at about 3 lbs. (48 oz.).—QUAIN'S Anatomy, Vol. II., p. 432.*

In a paper by Mr. John Marshall, of University College, read before the Royal Society (June, 1863), the author gives a minute account of

(5.) The specific experiments on the nerve cords and nerve centres, to be afterwards quoted, have proved the immediate dependence of sensation, intelligence, and volition on those parts.

No fact in our constitution can be considered more certain than this, that the brain is the chief organ of mind, and has mind for its principal function. As we descend in the animal scale, through Quadrupeds, Birds, Reptiles, Fishes, &c., the nervous system dwindles according to the decreasing measure of mental endowment.

three brains, one the brain of a Bushwoman, the others the brains of two idiots of European descent. The Bushwoman's brain was computed to have weighed in the fresh state 31 oz. One of the idiots was a woman aged forty-two years; she was able to walk, though badly, to nurse a doll, and to say a few words; the weight of her brain was 10 oz. 5 grs. The other was a boy of twelve; he could neither walk nor handle anything, nor articulate a single word; the weight of his brain was 8 oz. These are the two smallest idiot brains whose weight has been recorded.

Mr. Marshall enters into a very minute description of the structure of all the three brains, and his remarks are valuable as showing what other deficiencies, besides weight, attach to the brains of human beings of low mental power. Not merely is the cerebrum in idiots a small organ, having all the proper parts on a smaller scale, but these parts are fewer in number, less complex, and different in relative proportion and position. And in particular, the convolutions of the brain are much less developed, much simpler, than in an average brain. On comparing the two idiots in question, the convolutions of the woman were more developed than those of the boy.

The circumstance of inequality in the richness of the convolutions has been alluded to by physiologists as explaining the cases of great mental power allied with brains not above the average weight. Such differences have actually been observed in the examination of brains. The brain of Cuvier was said to be distinguished in this respect, as well as in weight. But the connexion of force of mind with richness of convolutions is also liable to various qualifications. It does not hold in the comparison of different species, -the sheep's brain is more highly convoluted than the dog's; and there are well authenticated cases of men of superior powers, whose brains, both as to weight and as to convolutions, were below the average. Still, there can be no doubt that generally, though not universally, an increase in one or both of these peculiarities is the concomitant of a higher mental endowment. Both the statistics of the Races of men, and Comparative Anatomy, are decisive to this extent.

We may readily suppose that, with a view to intellectual power, an abundance of nervous elements-fibres and corpuscles-must be accompanied with a felicitous distribution or arrangement of them.

DIVISIONS OF THE NERVOUS SYSTEM.

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3. The NERVOUS SYSTEM consists of a central part, or rather a series of connected central organs named the cerebrospinal axis, or cerebro-spinal centre; and of the nerves, which have the form of cords connected at one extremity with the cerebro-spinal centre, and extending from thence through the body to the muscles, sensible parts, and other organs placed under their control. The nerves form the medium of communication between these distant parts and the centre; one class of nervous fibres, termed afferent (incarrying) or centripetal, conducting impressions towards the centre,another, the efferent (outcarrying) or centrifugal, carrying material stimuli from the centre to the moving organs. The nerves are, therefore, said to be internuncial in their office, whilst the central organ receives the impressions conducted to it by the one class of nerves, and imparts stimuli to the other, rendering certain of these impressions cognizable to the mind, and combining in due association, and towards a definite end, movements, whether voluntary or involuntary, of different and often of distant parts.'-QUAIN, Introduction.

The foregoing division of the nervous system into nervecentres and nerve-cords determines the order and method of description both as regards their Anatomy, or structure, and their Physiology, or function.

THE NERVOUS SUBSTANCE.

4. 'The nervous system is made up of a substance proper and peculiar to it, with inclosing membranes, cellular tissue, and blood vessels. The nervous substance has long been distinguished into two kinds, obviously differing from each other in colour, and therefore named the white, and the grey, or cineritious (ashcoloured).

'When subjected to the microscope, the nervous substance is seen to consist of two different structural elements, viz., fibres, and cells or vesicles. The fibres are found universally in the nervous cords, and they also constitute the greater part of the nervous centres; the cells or vesicles, on the other hand, are confined in a great measure to the latter, and do not exist in the nerves properly so called, unless it be at their peripheral expansions in some

of the organs of special sense; they are contained in the grey portion of the brain, spinal cord, and ganglia, which grey substance is, in fact, made up of these vesicles intermixed in many parts with fibres, and with a variable quantity of granular or amorphous matter.'

ness.

The nerve fibres are principally of the class termed white, or tubular nerve-fibres. They are of microscopic minuteIn thickness, they range from the foo to the Taboo of an inch; the medium or average being of an inch. When in the fresh condition, they are homogeneous and transparent, but after separation from the body they acquire a double outline or contour, and are apt also to assume a varicose or beaded appearance. The inference as to their ultimate structure, from these changes, is that each tube consists of (1) an outer structureless membrane, (2) an interior surrounding layer of transparent fatty matter, and (3) a central core or cylinder, which is not fatty, but albuminous in composition. The central band or axis appears alone, or divested of the two envelopes, both in the central connexions of the fibres, and in the ultimate ramifications in the extremities of the body; being, therefore, the essential part of the structure. In thickness, it does not exceed the Tooooo of an inch.

These tubular nerve-fibres are finest on the superficial layers of the brain, and in the nerves of special sense; they are largest in the motor nerves. From the foregoing statements of their size, we may judge of the immense multiplication of the nervous elements. Estimates have been made of the number of fibres in individual nerves. The third cerebral nerve (the common motor of the eyes) is supposed to have as many as fifteen thousand fibres; the small root of the fifth (governing mastication) nine or ten thousand; the nerve of the tongue five thousand; these being all motor nerves, which have the largest fibres. It would be interesting to estimate the probable number of fibres of the nerve of sight, which, besides being a sensitive nerve, is much thicker than any of those just quoted; there cannot probably be less than one hundred thousand fibres, and there may be many more.

The number of nerve fibres forming the white substance of the brain must be counted by hundreds of millions.

In the grey substance of the nerve centres, the nerve fibres are supposed to be continuous with the cells or vesicles. At their other extremity in the organs of sense, in the muscles, and in the body generally, their mode of termination. appears to be varied. Sometimes they end in loops, sometimes in meshes of network; not unfrequently sub-dividing into minuter nerves (besides dropping their two investing sheaths). In other cases, they seem, according to the majority of Anatomists, to end free in fine points, or else in little swellings of various structure.

It is important to note that each fibre is continued unbroken and independent from the central nervous masses to the peripheral extremity; there are no loose ends; and although the nervous cords frequently unite, as well as subdivide, in their course, the ultimate fibres are never fused with one another.

The nerve cells, vesicles, or ganglionic corpuscles, are little bodies, of a variety of forms; being round, oval, pear-shaped, tailed, and star-like or radiated. They consist of pulpy matter, with an eccentric roundish body or nucleus, enclosing one or more still smaller nuclei, surrounded by coloured granules. They vary in size from 30 to 300 of an inch in diameter.

FIG. 1.

Nucleated nerve-corpuscles magnified 170 diameters.

a and b from the

cortical grey matter of the cerebellum; c and d from the spongy grey matter