THE BLOOD. 37 



Now, the above described changes, which the compounds 

 of iron when exposed to the same influences as the blood 

 corpuscles are themselves submitted to, precisely correspond 

 with those alterations which it is known and ascertained 

 that the blood corpuscles do themselves experience, and 

 therefore there is every probability in favour of the strict 

 accuracy of Liebig's explanation of the chemical changes 

 which the blood corpuscles pass through during respiration 

 and circulation. 



Thus it has been long known, that in the lungs the coloured 

 blood corpuscles give off carbonic acid and imbibe oxygen : 

 it has also been ascertained that during their circulation 

 they lose a portion of their oxygen and acquire carbon. 



Venous blood, then, exposed to the air gives out carbonic 

 acid and absorbs oxygen, but arterial blood submitted to the 

 same influence gives out oxygen and acquires carbonic acid, 

 the seat of these changes being the red corpuscles. 



It will be seen on reflection, that, according to the views 

 just propounded, the surplus amount of oxygen which exists 

 in the peroxide becomes disengaged in the reduction of 

 that oxide to the state of protoxide : during circulation 

 in the capillaries, this surplus is chiefly expended in the 

 elaboration of the different secretions which are continually 

 being formed in the various organs of the body. 



Such is the corpuscular theory of respiration. Hereafter 

 we shall have to speak of a corpuscular theory of nutrition 

 growth and secretion. 



In connexion with Secretion. It is very probable that the 

 use of the red corpuscles is not limited to the mere office of 

 carrying oxygen from the lungs to be distributed to all parts 

 of the system, and of carbon back again to the lungs to be 

 eliminated, but that they have an ulterior and additional 

 function to discharge. Thus some observers suppose that 

 they exert some influence over the constitution of the blood 

 itself, elaborating from the materials continually thrown 

 into it by the thoracic duct a further quantity of fibrin. There 

 is more reason to believe, however, that it is the white 

 corpuscles which are principally concerned in this process of 



