As the nutritive secretions of the various structures are supplied with materials by the fluids in those structures, it is evident that they must at any time be increased, diminished, or otherwise modified by changes in the quantity and properties of these fluids. It is therefore a natural commencement of the subject to begin with the circulating system.
Nothing is more obvious in the condition of the aged as contrasted with the young than the different ratio between the fluids and the solids, the former being remarkably deficient. There is not only a notable diminution in the quantity of oleaginous or serous secretions, which are generally contained in the cellular parts of the body, but it is manifest that the tissues are per meated by a much smaller proportion of blood. This fluid moreover is very different in quality from what it was in earlier life ; it is less arte rial, its colour has not the same bright red it once presented, it has a large proportion of serum, and its coagulum is less firm in con sistence.* Correspondently with the defi ciency of fluids, many parts which once contained them are shrunk or obliterated. The capillary system becomes infinitely less ex tended than it once was ; many of the extreme branches of the arteries themselves are no longer to be penetrated, and those which remain per vious, are far less distensible than formerly. There is indeed a remarkable change in the coats of these vessels ; they are not only con tracted in diameter, but are become denser and more rigid in texture. In this respect they dif fer from the veins, which in old age are more dilatable than in youth, and consequently con tain a larger quantity of blood. The final cause of this is evident ; in youth the arteries must convey a relatively larger quantity to sup ply the increasing structures ; in the decline of life, when the latter are decreasing, there can no longer be any need for the same supply; the permission, however, of an accumulation in the veins, where it is less likely to be productive of injury, appears to be an accommodation to the diminution of the circulating powers.
If we trace the arteries from their extremities back to the heart, we shall find their calibres every where diminished, their coats less elastic, less capable of adapting themselves to the varying quantity of their contents, in some places resembling the texture of ligament, in some that of cartilage, and in others studded with deposits of osseous matter. The heart
itself presents marks of degeneration no less decided ; its cavities are shrunk, its fibres pale, and but feebly contractile, and fat will some times seem to take the place of the muscular substance. Frequently, also, the coronary arte ries are found ossified, and the same alteration is not uncommon in the valves.
All these facts account for the slow, languid, staggering circulation characteristic of advanced life ; there is less blood to be transmitted to the various organs, and that which is sent is pro pelled with a degree of feebleness that shows how little energy is required in its motion, when so few nutritive actions are transpiring. We have spoken of the altered character of the blood, of its being less arterial and of a darker tint : this change is explained by the alteration in the respiratory system. The lungs are be come lighter, the cells being relatively much larger, and the parenchyma, which consists principally of bloodvessels, being greatly di minished. This alone would not explain why the blood is imperfectly arterialized, because, although the respiratory surface is diminished, less of that fluid enters the organ. But the bronchial membrane is always in a more or less unhealthy condition, being covered with a thick and copious secretion, that constitutes the " old man's catarrh," and prevents a due intercourse between the air and the blood. Besides this circumstance, the expansion of the chest is less perfect in consequence of the diminished elas ticity of the parietes of the chest produced by the ossification of the cartilages and other causes : the muscles also participate with less energy in the respiratory movements. Every thing in the history of advanced life indicates the diminution in the vigour of the circulation and respiration. The apathy and languor of mind, the deficiency of many secretions, and the general decrease of animal heat, but par ticularly in the parts most distant from the heart, are all more or less intimately connected with the failure of these vital actions.