ITS PHYSICAL, CHEMICAL, AND STRUCTURAL CHARACTERS. 181 



and carbonic acid respectively. Of such change, however, there is no adequate 

 evidence; and there are many indications that we are to look for the source of 

 the difference of color, rather in modifications in the form of the corpuscles, 

 affecting their power of transmitting and reflecting light, than in any chemical 

 alterations of their contents. It is true that if arterial blood be exposed to car- 

 bonic acid out of the body, it will acquire the dark hue of venous blood ; whilst, 

 conversely, venous blood exposed to oxygen will acquire (on its surface at least) 

 the florid hue of arterial blood. But for these changes to take place, it is neces- 

 sary that the normal proportion of saline matter should exist in the serum in 

 which the corpuscles float, and that the corpuscles themselves should not have 

 ruptured and discharged their hgematin. For if arterial blood deprived of its 

 fibrin be diluted with twice or thrice its volume of water, it assumes a dark venous 

 tint, which is not affected by the passage of a current of oxygen through it ; yet 

 the red color is restored by the addition of a saturated solution of a neutral salt, 

 even without the contact of oxygen. On the other hand, venous blood is red- 

 dened by the addition of a strong saline solution, without any exposure to oxygen ; 

 and it is not readily darkened again by the passage of carbonic acid through it. 

 Again, a scarlet clot is darkened by washing it with distilled water, and is only 

 very slowly reddened by exposure to oxygen ; whilst a black clot becomes at once 

 scarlet when it is washed with salt, and is not blackened again by carbonic acid. 

 Further, if the corpuscles be treated with water until they burst, so that the 

 hsematin is diffused through the liquid, scarcely any effect is produced upon the 

 hue of the solution, either by carbonic acid, by oxygen, or by salines ; such slight 

 alteration as does occur being fairly attributable, either to the presence of a few 

 corpuscles still unruptured, or to the influence which the absorption of these 

 gases may produce upon the coloring matter, without entering into chemical 

 combination with it. 1 Hence it is obvious that the light or dark color of the 

 blood affords no indication whatever of its state of oxygenation, since the change 

 from the one to the other may be effected by other agents; and if we examine 

 into the nature of their influence, we find that the blood is darkened by whatever 

 tends to distend the corpuscles, so as to render them flat or bi-convex, whilst it 

 is brightened by whatever tends to empty them, so as to render them more deeply 

 bi-concave than usual. And observation of the effects of oxygen and carbonic 

 acid, respectively, upon the form of the corpuscles, confirms the idea that this is 

 the mode in which these agents affect their color ; for the former causes their 

 contraction, and renders their cell-walls thick and granular, so as to increase their 

 power of reflecting light ; whilst the latter, producing a dilatation of the corpus- 

 cles, thins their cell-walls, and enables them to transmit light more readily. 

 That an increase in the opacity and reflecting power of the corpuscles tends to 

 heighten the color of the blood, is shown by an experimenter of Scherer's; who 

 found that, when defibrinated blood had been darkened by the addition of water, 

 its original bright color was restored by the addition of a little milk, oil, or finely 

 powdered chalk or gypsum. 3 



167. No difference can be detected between samples of blood drawn from 

 various parts of the arterial system of the same animal; but very important 

 variations exist, as might be expected, in the composition of the blood drawn from 

 the several parts of the venous system, since the changes to which it has been 

 subjected in the several organs through which it has passed are of a very diversi- 

 fied character. The blood of the vena portae, for example, differs considerably 



1 It has been shown by Peligot, that the colors of solutions of the salts of the protoxide 

 of iron are considerably modified by passing a current of protoxide of nitrogen through 

 them, although no chemical change is thereby induced. 



2 See, on this subject, the Reports by Scherer in "Canstatt's Jahresbericht" for 1844 

 and subsequent years, and the works therein referred to; also Mulder's "Chemistry of 

 Animal and Vegetable Physiology" (translated by Prof. Johnston), pp. 338-344. 



