The cartilages, which may be considered as soft and imperfect bones, grow also more rigid as we increase in years; and as they are generally placed near the joints, the motion of these must of consequence become more difficult. Thus, in old age, every action of the body is performed with labour; and the cartilages, which in youth were elastic, and in manhood pliant, will now sooner break than bend, and may be considered as the second cause of our dissolution.
The membranes become likewise as we grow old more dense and more dry. Those, for example, which surround the bones cease to be ductile, and are incapable of extension so early as the age of 18 or 20. It is also the same with the muscular fibres, and though to the external touch the body seems, as we advance in years, to grow more soft, yet in reality it is increasing in hardness. On such occasions it is the skin, and not the flesh, that communicates this perception. The fat which increases when the body is arrived at maturity, being interspersed between the skin and muscles, gives an appearance of softness which the flesh is far from possessing in reality; an undeniable proof of which is to be found in comparing the flesh of young and old animals; the former is tender and delicate; the latter hard, dry, and unfit for eating.
While the body increases, the skin will stretch to any degree of tension; but when the former diminishes, the latter never contracts; and hence the source of wrinkles, which cannot be prevented. Those of the face proceed from this cause, though as to shape they depend in a greater measure on its form, features, and habitual movements. By examining the countenance of a man at the age of 25 or 30, we may discover in it the origin of all the wrinkles it will have in old age; particularly when the features are in a state of agitation by laughing, weepings or any strong grimace. All the little furrows formed by these agitations will one day become wrinkles, which no art shall be able to remove.
In proportion then as we advance in years, the bones, the cartilages, the membranes, the flesh, the skin, and all the fibres of the body grow more solid, hard, and dry. Every part shrinks, and every motion becomes more slow; the circulation of the fluids is performed with less freedom, the perspiration diminishes, the secretions alter, the digestion becomes slow and laborious, the nutritive juices become less plentiful, and no longer serving to convey their accustomed nourishment, are wholly useless, as if they did not exist. Thus the body dies by little and little, all its functions diminish by degrees, and death only at last seizes upon that little which is left.