AC.JK. 



71 



that the eye collects rays of light and con- 

 centrates them on its internal surface, but are 

 utterly ignorant of the changes which the re- 

 tina, the optic nerve, and the brain undergo in 

 producing that condition of our sentient exis- 

 tence which we call vision. It is true that we 

 are aware that certain states of these parts are 

 incompatible with sight ; but why they are so 

 is quite beyond our knowledge. We are, as it 

 regards our acquaintance with the adaptation of 

 nervous tissue to the production of sensation, 

 in the same predicament as a man who watches 

 the working of a steam-engine, and knows that 

 a certain quantity of fuel, of water, of valvular 

 compression, &c. is necessary to its motion, 

 but has no idea of the laws of caloric, vapori- 

 zation, constitution of elastic fluids, &c. Our 

 science demonstrates the fitness of the external 

 and internal ear for receiving, propagating, 

 multiplying, and diffusing vibrations, but why 

 the contact with the auditory nerve produces 

 sound, is an all but impossible inquiry; as well 

 as the reason why the sensation may be absent 

 when the organ is in perfect order, and the 

 nerve to all appearance unchanged ; or why the 

 sensation may occur without vibrations, as in 

 dreaming, and many nervous disorders. The 

 same may be said of the skin ; it is well 

 adapted for coming in contact with the points 

 or superficies of bodies, but who can say why 

 the nerves spread over it occasion certain feel- 

 ings ? These remarks are premised merely to 

 shew that it must not excite surprise, if we are 

 unable to point out completely the changes 

 which age produces in the human body, corres- 

 pondently with the changes of its sentient fa- 

 culties. 



It has already, in all probability, struck the 

 mind of the reader, that the great develop- 

 ment of the cerebral system in the infant is 

 inconsistent with the principle which we have 

 been endeavouring to demonstrate, viz., that the 

 growth of the human body consists essentially 

 in the elevation of the organs, subservient to the 

 animal functions, from a rudimentary state. 

 The more we grow, the smaller is the proportion 

 of the brain to the rest of the fabric. But it 

 is no less true that the Junctions of the brain 

 grow with our growth. How then are we to 

 reconcile these opposite facts ? We must cer- 

 tainly discard the opinion,-that the bulk of the 

 organ is proportionate to its power, and exa- 

 mine the composition and the relations of its 

 various parts to each other. 



Limited as our knowledge is of the requisite 

 conditions of nervous substance for its func- 

 tion, we are notwithstanding aware of two ex- 

 tremes of softness and hardness, which comprise 

 those states of the tissue which are compatible 

 with the exercise of its peculiar faculty. Patholo- 

 gisls well know that ramollissement and indu- 

 ration of the brain may produce the same lesion 

 of function, viz., abolition of sensation ; while it 

 is equally well known that approximations to 

 the same conditions will produce impairment of 

 this faculty. Now in infancy the brain is ex- 

 tremely soft, almost pultaceous, while in old 

 age it is extremely hard in comparison, and the 

 similarity of the two ages in many respects, 



but particularly as it regards the functions of 

 the nervous system, is matter of universal ob- 

 servation. It might then a priori be suspected 

 that one of the changes in cerebral growth 

 would be a tendency to a certain intermediate 

 degree of consistence, and this is found actually 

 to be the case. 



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 theirwork,De Penitiori Cere- 

 bri Structur^,* 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 bones 

 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 is 

 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. -f- 

 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.} 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-r 



* See the notes to Milligan's Translation of 

 Magendie's Physiology. 



f This observation refers to the cineritious matter 

 of the convolutions. In certain other parts this still- 

 stance diminishes after birth. Thus, in the full- 

 grown foetus, the medulla oblongatais grey through- 

 out, but soon begins to whiten, first in the corpora 

 pyramidalia, and afterwards in the olivaria. The 

 outer surface of the tuber annulare, and of the 

 crura cerebvi, is also grey at the commencement of 

 extra-uterine life; but they lose this colour after 

 a few weeks. In the thalami nptici and corpora 

 striata there is no distinction of white and grey 

 matter, the latter alone being visible. See Meckel, 

 Manuel d'Analomie, t. ii. p. 717. Till the 

 functions of these parts in mature age are belter 

 understood than at present, it would be useless to 

 speculate upon the physiological relations of the 

 changes which they undergo in earlier periods. 



J Diet, de Med. et Chir. Pratique, art. Aliena- 

 tion Mentale. 



