AGE.— (Lat. (etas. Gr. ;ix‘mcc. Germ. Alter. Fr. age. Ital. eta.) This word, in its most extended sense, may express any period of duration. In reference to the human body it is used to denote either the whole time occu pied by this system in passing through its several stages from birth to decay, or, in a more limited signification, that particular por tion of existence commonly designated old age. It is in the former of these meanings that we employ the prefix to the following arti cle ; in other words, we propose to give an account of the organic and functional changes which the human system undergoes, from the commencement of extra-uterine life to the period of its dissolution by natural decay.
The term of human existence has been va riously divided, and in many instances with a view to adapt its divisions to certain fanciful notions respecting the power of num bers; but the only rational principle on which we can distinguish certain definite periods, must be that of observing alterations in the condition of the whole body or of its several organs, and the correspondence which they bear to particular epochs. The old Aristo telian division of human life into three stages, growth, maturity, and decline, is founded on this principle ; for, viewing man as a whole, the conditions in which he is an imperfect, a complete, or a declining member of his species, are well marked. But these conditions are capable of subdivision according to the changes which particular organs have undergone ; in other words, man, in the progress of his per .fectionnenient, makes certain acquisitions in his structures and functions, and in his decline suffers certain losses and impairments ; the more striking of these additions to, or sub tractions from his resources, suggest the well known division of existence into infancy, boy hood, puberty or adolescence, manhood, old age, and decrepitude. It is not our intention to discuss the subject of age by describing the characteristics of the stages last enumerated ; we think it better to take a view of the general revolutions which transpire in the human economy during growth, maturity, and decline, and under each of these heads to mention the changes which particular organs undergo in the course of time, without limiting ourselves to distinct stages, the determination of which must be, to a certain extent, arbitrary.
The consideration of the alterations which take place in the body during its progress from infancy to manhood might very properly be preceded by some remarks on those ultimate processes which are essential to growth, viz.— nutrition, secretion and absorption ; but, for information upon this interesting subject, the limits prescribed to this article compel us to re fer the reader to that upon NUTRITION, in which the processes alluded to will be viewed in rela tion not only to the development, but also to the maintenance, and to the decay of the tissues.
On comparing a young with an adult animal we are at first struck by the difference in bulk ; but immediately afterwards our attention is attracted by the difference in their respective capabilities of action,—a difference not merely proportionate to that of size. A closer ex amination informs us, that in the infant many of the parts of the body are absolutely incomplete, as organs or instruments, and we proceed to in vestigate whether this imperfection holds with all the organs or only with some of them ; and if the latter be the case, whether the parts thus existing only in a rudimentary state belong to a particular class. Now, the organs and func tions of man, in common with those of other animals, are divided into those which he shares with organic beings in general, and those which distinguish him as an animal; the former subserving his own independent existence, the latter his existence in relation to external ob jects of his consciousness ; these more or less subjected to the control of volition, those re moved, under ordinary circumstances, from the government of this principle. Hence these two classes have been variously named organic and animal, nutritive and relative, automatic and voluntary ; and, as life is a term employed to designate the collective functions according to some physiologists, or the cause of them ac cording to others, we have organic life and animal life, &c., &c. But the animal functions are truly supplemental; they could not subsist but by virtue of the organic ; while, on the other hand, the latter are perfectly capable of a separate existence, as in the vegetable world, or in those conditions of animal life in which its characteristics are all but suspended, such as profound sleep and apoplexy. Yet, al
though the functions of relation are thus de pendent on those of nutrition, it is evident, at a moment's glance, that the latter viewed col lectively in an animal structure, would present an aspect altogether incomplete, and different from that which we notice in the system of a vegetable. In the one case they were obviously intended to act only for themselves and for one another ; in the other they have an ulterior object to fulfil, but for which they would not have been called into existence and opera tion; this object is the production and support of the functions that constitute the animal. If we now look at the new-born infant in con trast with the full-grown man, we at once per ceive that the essential difference between them has reference to the life of relations ; in other words, the immaturity of the former is not de termined by the state of the vegetative organs, which, as organs, are perfect, but by the unde veloped conditions of the parts which are to receive impressions from, and to re-act upon surrounding objects. Thus, on the one hand, we observe that the food adapted to the little being is rapidly converted into chyle, that the blood, after undergoing its requisite changes, performs its circuit freely and effectively, and that the activity of the nutritive, secernent, and absorbent processes is evidenced by the quick increase of growth, and by the abundant fluids contained in the various tissues. But, on turning to the relative functions, we find the case altogether reversed ; sensation is dull, faint, and flitting ; voluntary motion scarcely ex ceeds the amount necessary for obtaining nutri ment from the parent ; while the demonstra tions of intelligence are the very lowest com patible with our belief in the possession of such a principle by the being in question. An examination of the organs devoted to these several actions leads to results in accordance with what we observe in the functions them selves ; in the one class the organization is complete, in the other much remains to be accomplished. If the apparatus of digestion be inspected, the parts employed in deglutition, viz., the tongue, pharynx, and oesophagus, will be found fully formed ; in the stomach the parts required for accommodating the aliment during its stay and for mixing certain fluids with it, are properly developed; no deficiency is observable in the structure of the liver and pancreas ; and the chyliferous vessels are pervious, extensile, and perhaps contractile. If we proceed to the organs of circulation, similar conditions are observable. In the heart the seve ral cavities, valves, and fibrous arrangements are duly proportionate to each other, and possess such qualities of firmness, pliancy, distensibility and contractility as are required for receiving, expel ling, agitating, and keeping in separate compart ments the two different kinds of blood ; the arteries are found resistant enough to hold the blood within their calibres, and at the same time elastic enough to adapt themselves to the varying quantity of their contents, while the veins are found so organized both as to the muscularity of their coats, and to the perfection of their valves, as to be quite capable of con veying the fluid back to the heart. Not less complete is the apparatus of respiration, whether we regard the development of the diaphragm, or the elasticity of the thorax, or the cellular and tubular arrangements in the lungs and their appendages. For affording the necessary conditions for the occurrence of those molecular motions vvhich constitute deposition and absorption, and upon which secretion also depends, we find an infinite number of capillary tubes well formed for supplying the fluids from which new particles may be taken,and to which old ones may return, and so disposed as not to interfere with the action of any supposable chemical affinities. If we next direct our at tention to the organs of the animal functions, an opposite set of facts will directly meet us. In the locomotive system, the bones are dis covered imperfectly ossified, the muscles de ficient in fibrin, and the tendons and ligaments in firmness and density. Of the organs of sensation it may be said, in general terms, that the mechanism employed in the application of the appropriate stimulus is, for the most part, incomplete, while a difference is also observa ble in certain properties of the nervous sub stance.