The bones also, and the other solid parts of the body, encrease in solidity. The membranes become cartilaginous, or gristly, the cartilages become bony, the fibres become more hard, the skin dries up, wrinkles are gradually formed in it, the hair grows grey, the teeth fall out, the visage becomes haggard, and the body stoops. The first approach of these alterations is perceived before the age of forty; by slow degrees they advance till that of sixty, and by rapid ones till that of seventy: after which period, decrepitude soon follows, and continues to augment to the age of ninety, or a hundred, when the life of man is generally terminated. Having already traced the causes of the formation, growth, and expansion of the human frame, we shall now proceed to consider those of its decay. At first the bones of the foetus are only small threads, of a ductile matter, and of little more substance than the flesh; by degrees they acquire solidity, and may be considered as a kind of small tubes, lined both within and without by a thin membrane which supplies the osseous matter. A pretty exact idea might be formed of the growth of bones, by comparing them with the manner in which the wood and solid parts of vegetables are produced. These bones, or, as we have said, tubes, are covered at both ends by a soft substance, and in proportion as they receive nutritious juices, the extremities extend from the middle point which always preserves its original station. The ossification begins at the middle and gradually follows the extension until the whole is converted into bone. Having acquired their full growth, and the nutritious juices no longer being necessary for their augmentation, they serve the purpose of increasing their solidity; in time the bones become so solid as not to admit the circulation of these juices which are highly essential to their nourishment; and this being stopped, they undergo a change like that perceived in old trees; and this change is the first cause that renders the decay of the human body inevitable. 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.