That which has been said of the matter of the nerves is also true of the brain. The whole bulk is diminished and the density greater than in earlier years. Some, however, assert that it is even softer than in manhood.
Blandin makes a remark of this kind, in commenting upon Bichat's statement of a greater hardness in the tissue, and says that it might be expected a priori, since there is so strong a correspondence between the two ex tremes of life. There is reason, however, to think that this remark, if true at all, applies only to the cerebral organ of persons very far advanced ; and it is not improbable that dis eased softening has in other cases been mis taken for the natural effect of age. The mem branes investing the brain like the neurilemmes (for they belong to the same system) are also thicker and more resistent. The vascularity of the organ is greatly diminished ; on a di vided surface no red dots are visible as at periods less advanced.
The alterations in the mechanism of the senses must next be considered. The skin, which is the medium between the nerves of tact, and external agents, undergoes great changes in the progress of life. It becomes drier, harder, less flexible, and at the same time looser, in . consequence of the absorp tion of the adipose substance. By the latter qualities the function of the skin is more evidently impaired, in that modification of it more expressly denominated touch, or the sense of tact united with certain muscular feelings in the fingers and hands. By the looseness of the integuments, the slowness and weakness of the .muscles, the stiffness of the digital joints, and that dulness of sensation which exists in this as in every other part of the system more or less, the hand is notably deteriorated in old age.
In the olfactory apparatus we find that, although the cavities and sinuses, through which the Schneiderian membrane is ex tended, are rather increased than diminished in size, the membrane itself is attenuated and less pulpy. The nerve also is mentioned by Rullier to be evidently contracted and wasted.
The sense of taste so closely connected with that just spoken of survives to the extremest limit of existence ; the final cause of which is evi dent. It is too intimately connected with one of the processes of organic life to be easily dispensed with, although one of the functions of the superadded life. It is, however, feebler than at periods less advanced, and requires the excitement of more piquant aliment; this is partly owing to the diminished sensibility of the gustatory nerve itself, and partly to the diminution of the sense of smell, on the per fection of which depends our appreciation of the more delicate species of sapidity. The surface of the tongue is more rugose than in younger subjects, and there is generally a de ficiency of moisture, which is an additional cause of diminished sensation.
The ear, both in its external appendages and in its internal structure, presents certain conditions which very well account for the frequency of deafness among the aged. It is
true the cartilages become harder, more elastic, and therefore more vibratory, but the internal surface of the meatus is often thickened, and obstructed by a dense cerumen. The mem brana tympani is more rigid and therefore less capable of varying with the degree of•the vibra tions. In the internal cavity, although the mastoid cells are enlarged as life advances, the deficiency of the liquor cotunnii in the vestibule, the cochlea, and the semicircular canals, must greatly interfere with the produc tion of hearing. In addition to all these cir cumstances there is probably an idiopathic insensibility of the nerve.
The modifications of the organ of vision are familiar to all who have paid even the most superfidial attention to the science of optics. The cornea is less transparent and less convex, partly from the diminution of the aqueous humour, and partly from the condensation of its texture. The latter change is more marked at the circumference, where a nebulosity is often formed, which has gotten the name of gerontotoxon, or arcus senilis. The'pigmentum diminishes, and the iris grows paler in con formity with the altered colour of the hair. The crystalline lens is denser, less transparent, and often acquires a yellow tint ; the vitreous humour likewise suffers a decrease. The retina is considerably attenuated, but has increased in firmness. The punctum luteum is paler, and not unfrequently altogether effaced; a change which, in the opinion of MeckeVbears a direct ratio to the diminution of the transparency of the cornea. These several alterations are ne cessarily followed by two results—diminished refraction of the rays of light, and torpor of the nervous function, both of which produce pres byopia. That long sight bears a relation with nervous as well as more mechanical causes is, we think, attested by the fact that this kind of vision is modified by temporary excitement of the brain, as in phrenitis.t If we now take a retrospect of the revolu tions which have occurred in the several struc tures enumerated, and endeavour to arrange them under specific heads, it will be found that diminution of bulk, deficiency of fluid, and condensation of substance, comprehend them all or nearly all. The attenuation has been generally ascribed to a preponderance of absorption over deposition, or a reverse of that condition in which incremental growth consists. But we cannot enter upon the question here, and must refer to the article NUTRITION, con tenting ourselves with the remark that it seems a superfluous multiplication of causes to sup pose that absorption increases, when the cessa-_ tion or diminution of deposition fully explains the fact, provided the absorption is only main tained in its usual ratio.