formed, the hair grows grey, the teeth fall, the body bends, &c. The first alterations of this state are perceptible before the age of forty; they increase by pretty slow degrees to sixty; by more rapid degrees to seventy— at which time caducity begins, and proceeds constantly increasing decrepitude follows; and death generally closes the scene, before ninety or an hundred years of old age. DIFFERENCE IN STRUCTURE OF MEN AND ALL the bones, cartilages, and muscles, and all the other parts that compose the body, are less solid, and more soft in women than in men, more time is required for those parts to assume that solidity, which causes death, and consequently women ought to be longer growing old than men. This indeed happens, and it may be observed by consulting the tables made on the mortality of mankind, that, when women have passed a certain age, they live longer than men of the same age; and hence, also, it may be inferred, that such men who in appearance are weaker than others, and approach nearer the constitution of women, ought to live longer than those that seem to be stronger and more robust: and in like manner it may be believed, that those of either sex, who have been very late in receiving their growth, are such as should live longer; for in both cases the bones, cartilages, and all the fibres, arrive later at that degree of solidity, that must produce their destruction. This cause of natural death is general and common to all animals, and even to vegetables. An oak perishes, because the oldest parts of the wood, which are in the centre, become so hard and compact that they cannot receive any more nourishment; the humidity they contain having no longer a circulation, and not being replaced by new sap, ferments, corrupts, and alters gradually the fibres of the wood, which turning red, and losing their organization, crumble, at last, into dust. ANIMAL GROWTH AND DURATION. THE total duration of life may, in some respects, be measured by that of the time of growth. A tree or animal that receives in a short time its whole growth, perishes much sooner than another that takes up more time in growing. Among animals, as well as vegetables, the growth in height is that which is first perfected; an oak ceases to grow tall for a long time before it ceases to grow thick. Man grows in height till he is sixteen or eighteen years old; and the entire expansion of all the parts of his body in thickness is not complete till he is thirty years old. Dogs receive their growth in length in less than a year, and it is not till the second year that their thickness is perfected. The man who is thirty years in growing, lives ninety or an hundred years; the dog that is but two or three years in growing, lives but ten or twelve years. The same may be said of the greater number of animals. Fishes that do not cease growing till after a great number of years, live for ages, and this long duration of their life must depend on the particular constitution of their bones, which never assume as much solidity as the bones of terrestrial animals. In general, large animals live longer than small, because they are longer growing. |