in the constituent elements of Living Beings. 243 



In the Philosophical Magazine (March 1846, p. 178) there 

 is a paper by me explanatory of the causes of the circulation 

 of the blood in the capillary vessels. It is merely an abridge- 

 ment of a lecture which for eight years past has been delivered 

 in this university. The doctrine there set forth has been ge- 

 nerally received in America, and introduced into some of the 

 standard works on physiology published in England. The 

 principle on which it essentially depends, and which has been 

 abundantly confirmed by direct experiment, is briefly this— 

 that if there be two fluids occupying a capillary tube, or a 

 porous structure of any kind, under the condition that one of 

 them has a stronger chemical affinity for the substance of that 

 tube or structure than the other, a movement of the liquids 

 will at once ensue, that which has the stronger affinity driving 

 the other before it. On this principle a clear account of the 

 systemic circulation of animals may be given ; for the arterial 

 blood, an oxidizing liquid, having a stronger affinity for the 

 soft tissues with which it is in contact than the venous blood, the 

 affinities of which have been satisfied and therefore no longer 

 exist, necessarily exerts such a pressure that motion must 

 ensue, the arterial blood forcing the venous before it. 



An application of the same principles shows that in the 

 pulmonary circulation the motions must necessarily be in the 

 opposite direction, or from the venous to the arterial side, as 

 is actually the case. It also explains clearly the conditions 

 of the portal circulation, in which the direct action of the 

 heart could hardly be expected to be felt. With the gene- 

 rality which ought to belong to a true theory, it meets all the 

 cases which occur in the lower orders of animal life, such 

 as the greater circulation in fishes, in which there is no 

 systemic heart; the movements which take place in the vas- 

 cular system of insects; and even the extreme case of the rise 

 and descent of sap in plants. 



In this doctrine everything depends on the relationship 

 between the nutritive fluid, or blood, and the solid parts with 

 which it is brought in contact; and whatever changes that 

 relationship must impress a corresponding change on the cir- 

 culation itself. 



From experiments which I made some time ago, I have 

 been led to suppose that the arterial ization of the blood, as it 

 takes place on the cell- walls of the lungs, bears a strong ana- 

 logy to the oxidation of white indigo. The loose hold which 

 the colouring matter of the blood retains on the oxygen, cou- 

 pled but not combined with it, is not unlike what is witnessed 

 in other nitrogenized colouring matters, such as indigo, which 

 oxidizes and deoxidizes with the utmost facility. Charged 



R 2 



