From a careful comparison of the size and weight of the brain at different ages, it was ascer tained by the Wenzels, and is demonstrated in tables contained in their work,De Penitiori Cere bri Structurh,* that although the organ increases very sligthly in bulk after the third year, its weight does not attain its maximum till after the seventh, so that up to this time there is a progressive increase of density. After the seventh year there is no great difference either in size or density. (The size of the brain must not be confounded with that of the head, which after the period that we speak of, is determined by the growth of the external table of the skull, correspondently with the projection of the hones of the face.) There must, therefore, be some other change than that of density, to account for the augmentation of intellectual power in the suc ceeding periods, and herein our information most at fault. Nevertheless we are not altogether without intimations of organic changes. The majority of physiologists are agreed that the function of the cortical substance is of a higher character than that of the medullary. The lower we descend in the animal series, the less we find of the cineritious matter, which is not apparent at all in the invertebrata, nor indeed in fishes. It is, therefore, not without proba bility conceived that this matter is more imme diately concerned in thought ; and, conformably with this view, we find its colour more strongly marked, as boyhood stretches on to manhood.t We may mention as corroborative of this cir cumstance, that M. Foville, an eminent in vestigator of the pathology of mental diseases, asserts that the principal lesions in the brains of maniacs occur in the grey tissue.X The convolutions again afford us some hints upon the subject before us. Intelligence is in direct proportion to their extent, and we accordingly find that these parts are deeper as age advances. As the existence of the posterior cerebral lobes, and of the corpus rhomboideum in the cere. helium, is observed only at the top of the animal scale, we might expect that in the pro gress of age there would be a change in the relations of these parts to the whole mass, but we cannot find that any researches have been prosecuted in elucidation of these points.
Most of what we have predicated of the pro gressive actions in the brain, is likewise appli cable to the cerebellum and spinal marrow, and nerves. The latter parts, however, are more forward in their organization, being de voted to the more primitive functions of sensa tion and voluntary motion, while the former is the instrument of the faculties more eminently intellectual. The proportion of the cerebellum to the brain at birth, is, according to Meckel, as 1.23 ; the former weighing nearly 31 drachms, the latter 9 or 10 oz. A month after birth the ratio is 1.17 ; after six months, 1-8.
The proportion between the spinal marrow and the brain at birth, and for five months after, is 1.107 or even 1.112 ; the brain at the former period weighing 9oz. 4dr., and the spinal marrow 45gr., while at the latter period the cerebral organ weighs 21oz., and the spinal 12dr. In the foetus of five months the propor tion is 1.63, of three months 1.18. In the adult it is 1.40. The diminishing ratio of the brain to the spinal marrow is in obvious har mony with the elongation of the vertebral column, and with the general growth of the members. The medulla oblongata is propor tionally larger in early than in advanced life; the corpora pyramidalia and olivaria being dis tinct and prominent; a fact which corresponds with the development of the brain.
The longitudinal dimensions of the corpora quadrigemina at birth exceed those of the adult period ; after the former period they increase only in their transverse diameter.
The concretions of the Pineal glands have not begun to be formed till the seventh year. They are sometimes wanting in very advanced age, according to the observations of Meckel and the Wenzels, which we have had oppor tunities of verifying by our own dissections. The number of these bodies increases with the progress of life, and their colour is paler in youth and old age than in intermediate periods.* So much then for the nervous organs of sensation. Our attention must next be directed to the mechanism intermediate to the nerves, and the excitants of sensation. The simplest kind of sensation is that which informs or re minds us that we are possessed of bodily parts, such as members and internal organs. The me chanism employed, if there be any, is unknown. Nerves are distributed through the tissues, we feel those tissues, and conclude that these feel ings result from relations between the nerves and the other textural molecules with which they are in contact. These feelings must of course vary with age because the tissues alter, but whether the susceptibility is increased we cannot say, and only venture to remark that the proba bility of this being the case is suggested by the fact, that adults are more subject to perver sions of sensibility than children; witness the various nervous, hypochondriacal, and hys terical disorders with which adults are almost exclusively visited.
The next order of sensations in respect of simplicity are those of tact, or those by which we are made acquainted that foreign bodies are in contact with our skin. It is perhaps in some respects only a modification of the first mentioned sensation, but it requires the pre sence of something not belonging to us. It is true that other parts than the skin may convey the notion of an external body being applied to them, but they do not afford any perception of the qualities of the body ; it is merely the affection of themselves which is produced by that body. We are, aware that all sensation may be analysed in the same manner with similar results, but it is enough for our present purpose that the sensation excited on the skin is less selfish, if we may use the term in this sense, and ought to be so, in order that it may serve its office of supplying some knowledge of the external world. Doubtless the organization of the epidermis and of the skin itself, as well as the greater distribution of nervous matter, occa sion the difference. The dermoidal tissue in modifying the external cause stands in the same relation to the nerves of tact, as the eye to the optic nerve, or the nose to the olfactory. The organ of tact is affected by age; the skin in very early life appears less susceptible of im pressions, and differs in its tissue, the papillae being less developed. A change, however, is soon effected in this respect, and as we advance towards manhood, it becomes less gelatinous and more fibrous. It must be confessed, how ever, that the modifications which it undergoes in reference to its function of sensation, are not well defined. This circumstance is owing to the variety of sensations to which it ministers, such as (in addition to what we have men tioned) feelings of heat and cold, dryness and moisture, &c. and, secondly, to its being also an organ for other and very different func tions, such as transpiration, secretion, and ab sorption.