GASES OF THE BLOOD. 89 



unless the portal vein is tied before the liver is removed 

 from the body, hepatic venous blood is very liable to 

 regurgitate into the portal vein, and thus vitiate the result 

 of the analysis. Guarding against this source of error, 

 recent observers seemed to have determined that hepatic 

 venous blood contains less water, albumen, and salts, than 

 the blood of the portal vein ; but that it yields a much 

 larger amount of extractive matter, in which, according to 

 Bernard and others, is one constant element, namely, grape- 

 sugar, which is found, whether saccharine or farinaceous 

 matter have been present in the food or not. 



Besides the rather wide difference between the composi- 

 tion of the blood of these veins and of others, it must not be 

 forgotten that in its passage through every organ and tissue 

 of the body, the blood's composition must be varying con- 

 stantly, as each part takes from it or adds to it such matter 

 as it, roughly speaking, wishes either to have or to throw 

 away. Thus the blood of the renal vein has been proved 

 by experiment to contain less water than does the blood of 

 the artery, and doubtless its salts are diminished also. The 

 blood in the renal vein is said, moreover, by Bernard and 

 Brown-Sequard not to coagulate. 



This then is an example of the change produced in the 

 blood by its passage through a special excretory organ. But 

 all parts of the body, bones, muscles, nerves, etc., must act 

 on the blood as it passes through them, and leave in it some 

 mark of their action, too slight though it may be, at any 

 given moment, for analysis by means now at our disposal. 



On the Gases contained in the Blood. 



The gases contained in the blood are carbonic acid, 

 oxygen, and nitrogen, 100 volumes of blood containing 

 from 40 to 50 volumes of these gases collectively. 



Arterial blood contains relatively more oxygen and less 

 carbonic acid than venous. But the absolute quantity of 

 carbonic acid is in both kinds of blood greater than that of 



