374 ABSORPTION. 



every day. The same will happen if the piece of tissue be 

 placed in a saline solution instead of water, or in a solution 

 of colouring or odorous matter, either of which will give 

 their tinge or smell to the blood, and receive, in exchange, 

 the colour of the blood. 



Even so simple an experiment will illustrate the ab- 

 sorption by blood-vessels during life ; the process it shows 

 is imitated, but with these differences : that, during life, 

 as soon as water or any other substance is admitted into 

 the blood, it is carried from the place at which it was 

 absorbed into the general current of the circulation, and 

 that the colouring matter of the blood is not dissolved so 

 as to ooze out of the blood-vessels into the fluid which they 

 are absorbing. 



The absorption of gases by the blood may be thus simply 

 imitated. If venous blood' be suspended in a moist bladder 

 in the air, its surface will be reddened by the contact of 

 oxygen, which is first dissolved in the fluid that moistens 

 the bladder, and is then carried in the fluid to the surface 

 of the blood : while, on the other hand, watery vapour 

 and carbonic acid will pass through the membrane, and be 

 exhaled into the air. 



In all these cases alike there is a mutual interchange 

 between the substances ; while the blood is receiving water, 

 it is giving out its colouring matter and other constituents : 

 or, while it is receiving oxygen, it is giving out carbonic 

 acid and water ; so that, at the end of the experiment, the 

 two substances employed in it are mixed ; and if, instead 

 of a piece of tissue, one had taken a single blood-vessel 

 full of blood, and placed it in the water, both blood and 

 water would, after a time, have been found both inside and 

 outside the vessel. In such a case, moreover, if one were 

 to determine accurately the quantity of water that passed 

 to the blood, and of blood that passed to the water, it 

 would be found that the former was always greater than 

 the latter. And so with other substances ; it almost always 



