370 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI. 



which it passes by capillary action to every part of the 

 system. 



The cause of the movement of the sap in flowering 

 plants, both of the rise of the crude sap and the descent 

 of the elaborated sap, is the light of the sun, which ef- 

 fects the decomposition of carbonic acid. 



From this explanation of the causes of the movement 

 of sap in plants, I turn to the circulation of the blood in 

 animals. 



In man there are three chief circulations the sys- 

 temic, the pulmonary, the portal. Bearing the above- 

 mentioned general principle in mind, I presented the 

 following explanation of these circulations: 



I., The Systemic. The arterial blood, which moves 

 along; the various aortic branches, and is distributed to 



O 7 



every part of the system, contains oxygen which it has 

 received during its passage through the lungs. Its color 

 is crimson. As soon as it has reached its destination in 

 the minute capillary vessels, it begins to carry on its 

 proper process of oxidation, attacking in a measured 

 way the tissues through which it is flowing. The di- 

 rect result of this operation is an evolution of heat. 

 But while this chemical change in the tissues is going 

 forward, the arterial blood itself is also suffering a 

 change in giving up its oxygen and gaining in exchange 

 the results of combustion. From being crimson, it turns 

 dark ; from being arterial, it changes into venous blood. 

 Now, under these circumstances, what must take place 

 in every capillary or each small portion of a porous struct- 

 ure ? On the arterial side, we have the crimson arterial 

 blood ; on the venous side, dark venous blood two dif- 

 ferent liquids. What, then, is the relation that obtains 

 between each of these liquids and the walls of the tube, 

 or the substance of the parenchyma in which they are 



