496 OP THE CIRCULATION OF THE BLOOD. 



passage." A physical principle which has been put forth by Prof. Draper 1 

 seems quite adequate to explain these phenomena. It seems fully capable of 

 proof 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 

 different 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 before it." Now Arterial blood -containing oxygen with which it is 

 ready to part, and being prepared to receive in exchange the carbonic acid which 

 the tissues set free must obviously have a greater affinity for those tissues, than 

 Venous blood, in which both these changes have already been effected. Con- 

 sequently, upon mere physical principles, the arterial blood which enters the 

 Systemic capillaries on one side must drive before it, and expel on the other 

 side of the network, the blood which has become venous whilst traversing it; 

 but if the blood which enters the capillaries have no such affinity, no such motor 

 power can be developed. On the other hand, in the Pulmonary capillaries the 

 opposite affinities prevail. The venous blood and the air iri the cells of the 

 lungs have a mutual attraction, which is satisfied by the exchange of oxygen and 

 carbonic acid that takes place through the walls of the capillaries ; and when 

 the blood has become arterialized, it no longer has any attraction for the air. 

 Upon the very same principle, therefore, the venous blood will drive the arterial 

 before it, in the pulmonary capillaries, whilst respiration is properly going on ; 

 but if the supply of oxygen be interrupted, so that the blood is no longer aerated, 

 no change in the affinities takes place whilst it traverses the capillary net-work ; 

 the blood continuing venous, still retains its need of a change, and its attrac- 

 tion for the walls of the capillaries; and its egress into the pulmonary veins is 

 thus resisted, rather than aided, by the force generated in the lungs. The 

 change in the condition of the blood, in regard to the relative proportions of 

 its oxygen and carbonic acid, is the only one to which the Pulmonary circula- 

 tion is subservient; but in the Systemic circulation, the changes are of a much 

 more complex nature, every distinct organ attracting to itself the peculiar sub- 

 stances which it requires as the materials of its own nutrition, and the nature 

 of the affinities thus generated being consequently different in each case. But 

 the same law may be considered to hold good in all instances. Thus the blood 

 conveyed to the Liver by the portal vein, contains the materials at the expense 

 of which the bile-secreting cells are developed; consequently, the tissue of the 

 liver, which is principally made up of these cells, possesses a certain degree of 

 affinity or attraction for blood containing these materials ; and this is diminished, 

 so soon as they have been drawn from it into the cell around. Consequently 

 theHcJlood of the portal vein will drive before it, into the hepatic vein, the blood 

 which has traversed the capillaries of the portal system, and which has given 

 up, in doing so, the elements of bile to the solid tissues of the liver. 



528. It can be scarcely doubted that it is by some influence exercised over 

 the molecular actions, to which the blood is subject in the capillaries, that the 

 Nervous system can operate on the functions of Nutrition, Secretion, &c. 

 ( 381, 385) ; and this influence can scarcely be considered in any other light 

 than as a peculiar manifestation of vital force ( 352). The following experi- 

 ment made by Dr. Wilson Philip exhibits the effect of "shock" upon the 

 capillary circulation. "The web of one of the hind legs of a frog was brought 

 before the microscope ; and while Dr. Hastings observed the circulation, which 

 was vigorous, the brain was crushed by the blow of a hammer. The vessels of 

 the web instantly lost their power, the circulation ceasing; an effect which can- 

 not arise, as we have seen, from the ceasing of the action of the heart. [Dr. 

 P. here refers to experiments, by which it was ascertained, that the circulation 



1 "Treatise on the Forces which produce the Organization of Plants," pp. 22-41. 



