ANIMALS. 370 



tbat 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 com- 

 mon food, for some time, and he found their bones tinctured 

 with alternate layers, in conformity to their manner of living. 

 From all this be naturally concluded, 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 soft- 

 est parts, they were furnished through all their substance, with 

 their proper canals. Nevertheless, these canals are of very dif- 

 ferent 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 impercep- 

 tible ; and the circulation through them is proportionably slow. 

 But, in the decline of life, the blood which flows through the 

 bones, no longer contributing to their growth, must necessarily 

 serve to increase their hardness. 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 diminished, 

 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 

 beginning to be formed, grow also more rigid. The juices cir- 

 culating through them, for there is a circidation through all 

 parts of the body, every day contribute 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 of the joints also must of 

 consequence become more difficult. Thus, in 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, so also that mucous liquor, vvhich is always separated 

 between the joints, and which serves, like oil to a hinge, to give 



