II^EMORRHAGIES. 211 



With respect to the second limitation, I expect that the 

 reason of it will be understood from the following consider- 

 ations. 



It has been already observed, that the extension and growth 

 of the body require the plethoric state of the arterial system ; 

 and nature has provided for this, partly by the constitution of 

 the blood being such, that a great portion of it is unfit to pass 

 into the exhalants and excretories ; partly by giving a certain 

 density and resistance to the several exhalants and excretories 

 through which the fluids might pass out of the red arteries ; and 

 partly, but especially, by a resistance in the veins to the free 

 passage of the blood into them from the arteries. 



DCCLXVI. With respect to this last and chief circum- 

 stance, it appears from the experiments of Sir Clifton Wintring- 

 ham, in his Experimental Inquiry, that the proportional den- 

 sity of the coats of the veins to that of the coats of the arteries, 

 is greater in young than in old animals : From which it may be 

 presumed, that the resistance to the passage of the blood from 

 the arteries into the veins, is greater in young animals than in 

 old ; and, while this resistance continues, the plethoric state of 

 the arteries must be constantly continued and supported. As 

 however the density of the coats of the vessels, consisting chiefly 

 of a cellular texture, is increased by pressure ; so, in proportion 

 as the coats of the arteries are more exposed to pressure by dis- 

 tention than those of the veins, the former, in the progress of 

 the growth of the body, must increase much more in density 

 than the latter ; and, therefore, the coats of the arteries, in re- 

 spect of density and resistance, must come, in time, not only to 

 be in balance with those of the veins, but to prevail over them ; 

 a fact which is sufficiently proved by the experiments of the 

 above-mentioned ingenious author. 



By these means, the proportional quantities of blood in the 

 arteries and veins must change in the course of life. In younger 

 animals, the quantity of blood in the arteries must be propor- 

 tionally greater than in old ones ; but by the increasing density 

 of the arteries, the quantity of blood in them must be continu- 

 ally diminishing, and that in the veins be proportionally in- 

 creasing, so as at length to be in a proportionally greater quan- 

 tity than that in the arteries. When this change happens in 



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