CAUSES OF ITS COLOUR. 167 



According to Nasse, the tendency of the blood-corpuscles of dif- 

 ferent animals to sink, decreases in the following order: the horse, the 

 cat, the dog, the rabbit, the goat, the sheep, the ox, birds, the 

 pig ; so that in the horse the corpuscles sink the most rapidly, and 

 in the pig (at all events in the winter) the most slowly. 



As the density and form of the blood-cells stand in definite 

 relations to their sinking tendency, so also is there a certain 

 dependence between the colour of the blood-corpuscles and their 

 form. 



It has been already shown (in the first volume), that the 

 colouring matter of the blood exists only in the cells, and that 

 consequently the colour of the blood is primarily dependent on the 

 blood-cells. In reference to the colour of the individual blood- 

 cells, we always remark, on a careful microscopic examination, some 

 which are paler and darker than the rest, although the number of 

 those presenting an intermediate tint very greatly preponderates ; 

 in the blood of the portal vein, we always find some which have a 

 speckled appearance, showing that the pigment is riot distributed 

 uniformly in them, as in all other blood-corpuscles. This difference, 

 therefore, depends upon the absolute amount of heematin which 

 they contain; but the colour of the cells must be relatively pale or 

 intense, according as they are dilated or collapsed by the absorption 

 or the loss of water. The gases, especially oxygen, probably exert 

 a chemical action on the pigment, and thus influence the coloration 

 of the corpuscles. The colour of the individual cells has, however, 

 only a secondary influence on the coloration of the mass of the 

 blood, but the peculiar tint of the blood is especially modified by 

 their number as well as their form. It need scarcely be remarked, 

 that blood which is poor in corpuscles is of a bright red colour, 

 while blood which is rich in them must be of a darker colour ; 

 but notwithstanding this, it by no means happens (as is shown by 

 the beautiful investigations of Popp) that blood which is poor in 

 cells, should invariably be pale, and that blood abounding in them 

 should be dark-coloured. Hence there must exist yet other causes 

 which exert an essential influence on the colour of the mass of the 

 blood causes even more important than the colour and number of 

 the cells. We are indebted to the genius of a Henle, for the first 

 indication of the connexion between the colour 1 of the mass of the 

 blood and the form of the red corpuscles. We had been pre- 

 viously satisfied with the idea, that everything relating to the 

 colour of the blood pertained to chemistry, which however could 

 yield no information on the point. The striking changes which are 



