106 ON THE POISON 



years, enables us to enter upon this part of the fubjec^ 

 with Ibme degree of confidence, and if it Hiould be 

 thought I have failed in determining this queftion with 

 fufficient precifion, the view here taken of the fubjefet 

 may not be altogether deftitute of ufe. It is an opi- 

 nion at leafl: as old as Pliny *, that the blood is a 

 living fluid, but it was referved for the late celebrated 

 ph) fiologift, Mr. John Hunter, to place this opinion 

 among the number of thofe truths that can no longer 

 be difputed. How the life of this fluid begins, and 

 in what the living principle itfelf confifts, are matters 

 concerning which we iliall probablv remain for ever 

 ignorant 3 but it has been eflablilhed beyond all con- 

 troverfy, tliat the life of the blood immediately de- 

 pends upon the a8ion of the atmofpheric air, to which 

 it is expofed in its palfage through the lungs. The 

 human heart, and in general the heait of all animals 

 with warm blood, has two cavities or ventricles, and 

 the blood, before it is returned to the right ventricle 

 of the heart, has performed two circles, a lefler be- 

 tween the heart and the lungs, and a larger between 

 the heart and the rell of the body. While the blood 

 pafies through the lungs, it undergoes a very remark- 

 able change in its colour, and other properties : a 

 certain portion of the atmofpheric air is attracted and 

 ahforbed, while the remainder carriesoff by expiration, 

 th?A matter in tlie blood, whicii is cither ufelefs or 

 noxious to the bodv. The atmofphere wc live in, it 

 is now well known, is a compound fluid, one fourth 

 part of which is called pure or oxygen air, and the 

 remainder, and larger portion, noxious or azotic air ; 

 but it is the former part only, which is attraRed by 

 the blood as it pafles through the lungs, and con- 

 tributes to the fup])ort of animal life, from whence 

 olfo, t/ie red colour of the blood-, and the heat of animals 

 is derived, independently of the dire6f proofs of thefc 



'* In treating on the blood, he obferves— Magna et in eo vitalitatis 

 portio. EmilTus J'piritum fenun trahit, tamen taftum non fcniit. 



1'lin. Sccund. Nat. Hilt, lib, xi. cap. 38. 



facts 



