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opening had been made there for it. And though I could not be 

 quite certain as to my feeing thefe tubes, I do not doubt that there 

 are a great number of them in the Bone ; and the rather, as I 

 think it is to be noted, that the membrane covering the Bone is 

 chiefly formed out of thefe vefTels, and that it is alfo fupported by 

 them. And of this we cannot doubt, when we obferve, that fome 

 trees have very fmall vefTels difperfed among the perpendicular vef- 

 fels, and by means of which the bark is chiefly compofed. 



And in like manner as we cannot with certainty point out the 

 formation of the bark in the tree, becaufe it is formed every year 

 out of the horizontal vefl'els ; fo (according to my hypothefis), we 

 are never likely to draw any other conclufion, than that the mem- 

 branes covering the bones, receive their growth and nourifliment 

 from certain veflels which proceed from the cavity to the circum- 

 ference of the bone, and there continuing to grow, are changed 

 into thin and foft veflels, which protect the bone, in like manner 

 as we obferve the bark of trees are formed out of the wood, and 

 defend it from external injury. 



I know that many think the origin and nourifliment of the bark 

 of trees depend upon the root ; but if it was fo, we fhould obferve 

 thofe parts of the bark next the root to be very thick, and towards 

 the upper part of the tree to grow thinner, and fpread into branches, 

 in like manner as we fee the arteries fpread from the heart, and the 

 •nerves from the brain ; whereas, in the veflels forming the bark of 

 trees, they are the fame in the upper parts as about the roots, and 

 what is more, the veflels in the bark of divers trees, and efpccially 

 the birch, cherry, peach, and goofeberry, do not take an upward courfe, 

 as in the oak, afli, elm, filbert, apple, and pear trees, but run only to 

 the furface round about the tree ; and as the barks of trees whofe vef- 

 fels mount upwards every year, increafe in thicknefs ; for when a tree 

 increafes in thicknefs, then the external part of the bark divides 

 in filfures, and the old and dead bark adheres to the new; and for 

 tills realbn, the older trees are, the thicker their bark, tliougli onh- 



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