The changes which take place in the system at the different epochs of life consist of changes in the physical condition of the body, and are intimately connected with and mainly dependent on the operation of a principle of consolidation, the influence of which, commencing at the first moment of existence, continues, without intermission, until the last moment of life. By this principle the body is changed, first from the state of a fluid into that of a solid ; and next, from a soft and tender solid, into a solid which slowly, imperceptibly, but never theless uninterruptedly, increases in firmness and hardness.
When first the human embryo becomes distinctly visible, it is almost wholly fluid, consisting only of a soft gelatinous pulp. [Fmrta.] In this gelatinous pulp solid substances are formed, which gradually increase, and are fashioned into organs. These organs, in their rudimentary state, are soft and tender, but, in the progress of their development, constantly acquiring a greater number of solid particles, the cohesion of which progressively increases, the organs at length become dense and firm. As the soft solids augment in bulk and density, bony particles are deposited, sparingly at first and in detached masses, but accumulating by degrees : these, too, are at length fashioned into distinct osseous structures, which, extending in every direction, until they touch at every point, ultimately form the connected bony frame-work of the system. This bony fabric, like the soft solid, tender and yielding at first, becomes by degrees firm and resisting, fitted, as it is designed, to be the mechanical support of the body, and the defence of all the vital organs.
While the osseous system is thus extending in every direction, and everywhere increasing in compactness, the progressive consolidation of the body is equally manifest in all the tissues which are composed of the cellular membrane as well as in all those which possess a fibrous nature. The membranes, the ligaments, the tendons, the cartilages, gradually increase in firmness and elasticity, and proportionally diminish in flexibility and extensibility ; and this change takes place, to a considerable extent, in the muscular fibre also, as is manifest from the toughness of the flesh of animals that are used for food, the degree of which every one knows is in proportion to the age of the animal ; and from the conversion in extreme old age, in many parts of the body, of muscle into tendon, a denser material being substituted for the proper muscular fibre.
The steady and increasing operation of the principle of consolidation is still more strikingly manifest in the deposition, as age advances, of bony matter in tissues and organs to which it does not naturally belong, and the functions of which it immediately impairs and ultimately destroys. The textures in which these osseous depositions most commonly take place are membranes, tendons, cartilages, and the coverings of the viscera, but above all the coats of the blood-vessels, in consequence of which these highly flexible, elastic, and moveable organs become firm, rigid, and immoveable. But even when not converted into bone, several of these structures lose their flexibility with advancing age, and acquire an increasing degree of rigidity. This is strikingly manifest in all the parts of the apparatus of locomotion ; in the joints, the mechanical contrivances for facilitating motion, and in the muscular fibre, the generator of the power by which motion is produced. The joints in old age are less pliable, less elastic, and more rigid than in youth ; first, because the ligamentous and cartilaginous structures of which they are composed are more dense and firm ; and, secondly, because the oily matter which lubricates them, and which renders their motions easy and springy, is secreted in less quantity, and of inferior quality. Induration and proportionate deterioration take place then in the muscular fibre, the origin of the motive power, and in the joint, the instrument by which the operation of the motive power is facilitated; and consequently the movements become slower, feebler, less steady, less certain, and less elastic.
But among all the changes induced in the body by the progress of age, none is more remarkable, or has a greater influence in diminishing the energy of the actions of the economy, and in causing the ultimate termination of all those actions in death, than the change that takes place in the minute blood-vessels. The ultimate divisions, or the
smallest branches of the arteries and veins, the capillary vessels, as they are termed, are exceedingly abundant in the early periods of life, and are as active as they are numerous. The capillary arteries, the masons and architects of the system, by the agency of which all the structures are bnilt up, and all the parts of the body grow and are developed, are numerous and active in the early stages of life, while they are carrying on and completing the organisation of the frame. But from infancy to childhood, from childhood to youth, from youth to maturity, and from maturity to old age, the number and activity of these vessels progressively diminish. Their coats, like other soft solids, increase in density and rigidity ; their diameter contracts, many of them become completely impervious and ultimately disappear.' The diameter of the capillary veins, on the contrary, enlarges. The coats of the veins, originally thinner than those of the arteries, instead of thickening and contracting, seem rather to grow thinner and more dilatable : hence their fulness, their prominence, their more tortuous course, and their greater capacity. At the two extreme periods of life the quantity of blood contained in these two sets of vessels is completely inverted. In infancy, the proportion of blood contained in the capillary arteries is greater than that contained in the capillary veins ; in youth, this disproportion is diminished ; at the period of maturity, the quantity in one set, nearly if not exactly balances that in the other; in advanced age, the preponderance is so great in the veins, that these vessels contain probably two-thirds of the entire mass. This difference in the distribution of the blood, at the different epochs of life, affords an explanation of several important phenomena connected with health and with disease. It shows, for example, why the body grows with so much rapidity at the early periods of life ; why it remains stationary at the period of maturity ; why it diminishes in bulk as age advances ; why a plethoric state of the system affects the arteries in youth, the veins in age ; why haemorrhage, or a flow of blood, is apt to proceed in the young from the arteries, and in the aged from the veins; and so on.
The growth of the heart does not keep pace with the extension of the sanguiferous system, nor does its force increase with the augmenting density and resistance of the solids ; hence there is a disturbance of the balance between the forces of propulsion and of extension which increases with advancing age ; the diminished energy of the heart being indicated by the languor and Blowhole of the pulse, often not exceeding fifty pulsations in a minute, and sometimes sinking oven lower than this. Renee, not only in lean blood sent to the several organs, but that which is sent is less completely acted upon bv the air in respiration on account of the diminished quantity which in transmitted through the pulmonary system of revels; hence, the diminution of all the secretions, and hence, finally, the failure of the function of digestion, the source of the materials from which the blood itself is prepared and eta leases repleniabed.
Upon the whole, then, it is clear that two great changes take place in the physical condition of the body in the progress of age ; first, a gradual diminution in the quantity of the fluids, both of the entire mass contained in the system, and of the proportionate quantity contained in each organ ; and secondly, a progressive augmentation and induration of the solids. With this change in the physical condition of the body is uniformly combined a no less important change in its vital action. Progressively and proportionally as the solid parts increase in density and rigidity, they decrease in irritability and mobility ; that is, they are less sensible to the influence of stimulants, and the power of contraction resident in the muscular fibre is leas excitable.