CAPILLARY RELATIONS OF ARTERIAL AND VENOUS BLOOD TO THE TISSUES. 35 



the systemic circulation, and then proceed to show how the same principles bear on 

 the other varieties also. For, as has already been remarked, it is essential for a true 

 theory of these movements, not only to explain them in an isolated case, but also in 

 every instance ; to explain not only the systemic, but also the pulmonary and the portal ; 

 to explain not only the varieties of circulatory movement in one given individual, but 

 also in every tribe, no matter whether they be low or high in the scale of creation, or 

 whether they belong to the animal or vegetable world. 



108. Let us, therefore, take as our guide the great principle laid down in the pre- 

 ceding chapter (84-88), That if. two liquids communicate with one another in a capillary 

 tube, or in a porous or parenchymatous structure, and have for that tube or structure dif- 

 ferent chemical affinities, movement will ensue ; that liquid which has the most energetic 

 affinity will move with the greatest velocity, and may even drive the other liquid entirely 

 before it. 



109. The arterial blood which moves along the various aortic branches, and is dis- 

 tributed to every part of the system, contains oxygen, which it has derived during 

 its passage through the lungs. Its colour is crimson. As soon as it has reached its 

 destination in the minute capillary vessels, it begins to carry on its proper process of 

 oxydation, attacking in a measured way the various tissues through which it is flowing, 

 burning out their effete carbonaceous matter, perhaps also burning their hydrogen into 

 water. The direct 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, which may be looked upon as its active principle, 

 and gaining in exchange the results of the combustion. From being crimson, it turns 

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



110. Let us farther confine our thoughts to what must take place in a single cap- 

 illary tube, or in one small portion of a porous structure ; for whatever reasoning holds 

 in this case, will also hold for any number of capillary tubes, or any mass of parenchy- 

 matous structure. On the arterial side of such a tube we have the crimson arterial 

 blood ; on the venous side we have the dark venous blood two different fluids ; but 

 what is the relation which obtains between each of these liquids, and the walls of the 

 tube or the substance of the parenchyma in which they are placed ? Must it not be 

 that the arterial blood, bearing its oxygen, ready to burn out any carbon or hydrogen 

 in its way, substances of which the tube or structure is composed, possesses an intense 

 affinity for those structures, an affinity which is at last exhibited by its actual destruc- 

 tion of them? The arterial blood, therefore, has an intense affinity for any of the 

 tissues with which it is brought in contact. 



111. In the next place, how is it with the venous blood, which occupies the other 

 extremity of the tube ? The affinities of the oxygenized the arterial blood have been 

 satisfied ; it has effected the combustion of the tissues through which it has gone, it has 

 changed into inert venous blood. From being red, it has turned dark. The very 

 change which has come upon it, or which, rather, it has undergone, is sufficient to as- 

 sure us that, so far as its chemical affinities for the surrounding structures are concerned, 

 those affinities are at an end. The venous blood, therefore, has little affinity for any 

 of those structures with which it is brought in contact. 



