OF THE DECAY OF TILE SYSTEM.
ALTHOUGH we have found it difficult to ascertain the Although we have found it difficult to ascertain the efficient cause of sleep, its final cause is obviously to af ford rest to the nervous system, by which it may repair the waste that necessarily occurs during our waking hours. This reparation, in the first instance, is applicable to the nervous system only ; but it is also experienced in the organic functions, although in a less direct manner. Still, however, under the most favourable circumstances, the process of expenditure is more rapid than that of repara tion ; and both the structure of the body and its functions begin to exhibit symptoms of that tendency to decay, which forms a necessary part of our constitution. We shall offer a few remarks upon the progress by which the body, after having arrived at its state of maturity, passes through the different stages of decline, until its powers be come filially extinguished.
The progression from youth to maturity, and from ma turity to decay, is visible in all the solid parts—as the membranous matter, the bones, and the muscles. In in fancy, membrane contains a large proportion of jelly and water, whereas, as age advances, these constituents are much diminished in quantity, so that at length it consists almost entirely of albumen ; to which, in some parts, is added a portion of earthy matter. The mechanical pro perties of membrane experience a corresponding change ; at first it is soft and relaxed, it gradually becomes firmer and more elastic, until at length it is so rigid and inflexi ble as to be no longer adapted for its appropriate func tions. From the important part which membrane acts, both in the circulation and in locomotion, we may easily conceive how much they must be impaired by any cause which materially changes its structure and composition; and, with respect to the former of these, we may conceive how it must affect every part of the system, by altering the distribution of the blood, or the quantity which is sent to the different parts of the body. An important alteration is likewise induced upon the bones; they contain a larger proportion of the phosphate of lime, by which they appear to be rendered harder and more brittle ; but, what is more important with respect to the animal economy in general, some parts, which are tendinous or cartilaginous in infan cy, become osseous as age advances; and, in this way, many of our motions become less free, and some parts connected with the vital functions, as the valves of the heart, and even the arteries themselves, acquire a texture which renders them no longer adapted for the purposes of the circulation. Nor is the mechanical structure of the brain and nerves exempt from its appropriate changes. Their substance becomes firmer, it contains a less propor tion of fluid, and has less blood sent to it. And we observe that a corresponding change takes place in the functions of the nervous system. In infancy it appears merely adapted to receive external impressions, conveyed to it either from the organs of sense or from the action of some of the organic functions; and some time elapses before the intellectual powers are able to unfold themselves. In youth they acquire the state of greatest activity, while, as age advances. although the understanding is improved by the knowledge which it has been gradually acquiring through life, still the functions of the brain and nerves are exercised with more difficulty, and at length become en tirely dormant.
Every one must be aware that the successive stages of growth, maturity, and decline, are necessarily connected with our constitution ; and it would appear that the most `active agent in bringing about the successive steps of this process is the circulation. Some ingenious speculations on this subject were formed by-Cullen ; lie supposes that in youth the arteries arc in a state of plethora, and are en dued with a capacity for powerful action. Hence the growth of the body is rapid, the secretory vessels furnish ing a copious supply of materials; this enables the diges tive agents to elaborate a large quantity of blood, which, being sent to the muscles and nerves, enables them to exert their powers with unusual energy. But after some time this operation tends to counteract itself; the vessels become extended to the utmost limit which they are able to sustain, while the quantity of matter added to them renders them less moveable, and, consequently, less capa ble of propelling their contents. In the most perfect state of the human body, when it has completed its growth, and when all its functions are in their most vigorous state, the force of the arteries may be conceived to be precisely balanced by the resistance which the vescls oppose to it ; but this balance is soon destroyed by the increasing strength of the arteries, so that the plethoric state which they formerly experienced becomes transferred to the veins, in consequence of their being more distensible than the arteries, and being less liable to have their texture af fected, as Less under the immediate influence of the active operations of the system. This state of venous plethora seems to be proved by many observations that have been made on the state of the body in old age, and explains very satisfactorily many facts in pathology. The consequences that must ensue from this diminished activity of the arterial system are apparent in every part of the system. When the blood is sent with less force and in less quantity to any part, the deficiency is first experienced in the capil laries; and, as these are the most active organs of the circu lation, a diminution of their number, or in their capacity, must directly impair the functions of every part of the sys tem, and induce a state of diminished activity of both the corporeal and mental powers.
There is another circumstance which indirectly tends to hasten the decay of the system, the want of a due cor respondence between the different functions, and especial ly between those of assimilation and absorption. In the perfect state of the system, these functions always main tain their due' relation to each other, and, while the ab sorbents are employed in removing the old materials, the secreting arteries furnish a due supply of fresh matter. But as life advances, this balance of action is destroyed, and, although the more ususal effect is a deficient action of the secretory vessels, yet, either irregularity disturbs the general order of the functions, and accelerates the progress towards decline, or even induces premature dis solution. Thus, even under the most favourable circum stances, and where every cause of injury is the most care fully excluded, the animal body is, after a certain period, doomed to destruction ; its powers cease to act; its fabric is destroyed; and the matter of which it is composed enters into new combinations.
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