ANIMALS. 233 



lust dimensions, it then also begins to receive an additional bulk, 

 which rather loads than assists it. This is formed from fat ; which 

 generally, at the age of thirty-five or forty, covers all the muscles, and 

 interrupts their activity. Every action is then performed with greater 

 labour, and the increase of size only serves as a forerunner of decay. 



The bones, also, become every day more solid. In the embryo 

 they are as soft almost as the muscles of the flesh ; but, by degrees, 

 they harden, and acquire their natural vigour; but still, however, the 

 circulation is carried on through them, and, how hard soever the bones 

 may seem, yet the blood holds its current through them as through all 

 other parts of the body. Of this we may be convinced, by an expe- 

 riment, which was first accidentally discovered by our ingenious coun- 

 tryman Mr. Belcher. Perceiving at a friend's house, that the bones 

 of hogs, which were fed upon madder, were red, he tried it upon vari- 

 ous animals by mixing this root with their usual food ; and he found 

 that it tinctured the bones in all; an evident demonstration that the 

 juices of the body had a circulation through the bones. He fed some 

 animals alternately upon madder and their common food, for some 

 time, and he found their bones tinctured with alternate layers, in con- 

 formity to their manner of living. From all this he naturally con- 

 cluded, that the blood circulated through the bones, as it does through 

 every other part of the body ; and that, how solid soever they seemed, 

 yet, like the softest parts, they were furnished through all their sub- 

 stance with their proper canals. Nevertheless, these canals are of 

 very different capacities, during the different stages of life. In infancy 

 they are capacious; and the blood flows almost as freely through the 

 bones as through any other part of the body; in manhood their size is 

 greatly diminished; the vessels are almost imperceptible; and t.h 

 circulation through them is proportionally slow. But, in the decline 

 of life, the blood, which flows through the bones, no longer contri- 

 buting to their growth, must necessarily serve to increase their hard- 

 ness. The channels that every where run through the human frame, 

 may be compared to those pipes that we every where see crusted on 

 the inside, by the water, for a long continuance, running through them. 

 Both every day grow less and less, by the small rigid particles which 

 are deposited within them. Thus as the vessels are by degrees dimi- 

 nished, the juices also, which were necessary for the circulation 

 through them, are diminished in proportion ; till at length, in old age, 

 those props of the human frame are not only more solid but more 

 brittle. 



The cartilages, or gristles, which may be considered as bones be- 

 ginning to be formed, grow also more rigid. The juices circulating 

 through them, for there is a circulation through all parts of the body, 

 every day contributes to render them harder; so that these substances, 

 which in youth are elastic and pliant, in age become hard and bony 

 As these cartilages are generally placed near the joints, the motion 

 ol the joints also must, of consequence, become more difficult. Thus, 

 ;n old age, every action of the body is performed with labour; and 

 the cartilages, formerly so supple, will now sooner break than bend. 



" As the cartilages acquire hardness, and unfit the joints for motion, 

 t>o tlso, that mucous liquor, which is always separated between l\n* 



