378 THE EFFECTS OF ALLOTROPISM. [Moicm XXVII. 



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 oxidiz- 

 ing 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 mo- 

 tion 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 generality 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 circula- 

 tion in fishes, in which there is no systemic heart ; the 

 movements which take place in the vascular system of 

 insects ; and even the extreme case of the rise and de- 

 scent of sap in plants. 



In this doctrine everything depends on the relation- 

 ship 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 circulation itself. 



From experiments which I made some time ago, I have 

 been led to suppose that the arterialization of the blood, 

 as it takes place on the cell-walls of the lungs, bears a 

 strong analogy to the oxidation of white indigo. The 

 loose hold which the coloring matter of the blood retains 

 on the oxygen, coupled but not combined with it, is not 

 unlike what is witnessed in other nitrogenized coloring 

 matters, such as indigo, which oxidizes and deoxidizes 



