262 OF THE BLOOD: 



the different absorptive powers of light possessed by blood charged respec- 

 tively with oxygen and carbonic acid. Venous blood recently taken from 

 the living animal, and containing only a small amount of oxygen, is dark, 

 and in thin layers dichromatic, whilst its spectrum is materially different 

 from that of arterial blood into which it is immediately changed, becoming 

 brighter and monochromatic on shaking it with air or oxygen. The change 

 in color which results from the addition of certain reagents to the blood is, 

 however, partly to be attributed to an alteration in the form of the corpus- 

 cles, the tint being darkened by whatever tends to distend the corpuscles, so 

 as to render them flat or biconvex, whilst it is brightened by whatever tends 

 to empty them, so as to render them more deeply biconcave. Thus arterial 

 blood is rendered dark by the addition of water, the corpuscles being swollen 

 and more or less deprived of their coloring matter. Oxygen will not then 

 restore the form of the corpuscles or the bright hue of the blood, but this is 

 quickly effected by the addition of solutions of neutral salts. In like man- 

 ner venous blood can be reddened by strong saline solutions, even without 

 the contact of oxygen. The varying circumstances under which either arte- 

 rial or venous blood may be at one time dark and at another scarlet in color, 

 have been very fully discussed by Wells x and Bernard. 2 The latter observes 

 that in the Foetus, and in the asphyxiated animal, the blood is everywhere 

 dark or venous. On the contrary, during sleep, in the hiberuauts during 

 the period of repose, and in animals dying by anaemia, the blood is every- 

 where scarlet or arterial. With high temperatures of the surrounding me- 

 dium, whether this be air or water, there is little difference in the color of 

 the two kinds of blood, the arterial being less and the venous more florid 

 than usual. 3 The difference between them is, on the other hand, best marked 

 in cold weather. The Blood contained in the veins of muscles is dark, and 

 always of a deeper shade in proportion to the energy and duration of the 

 previous muscular contraction, whilst that returning from the glands is always 

 brighter in proportion to the activity with which secretion is being performed ; 

 and in the latter instance, with the alteration in color, the amount of fibrin 

 is found to be diminished, or so modified as to form a softer clot, from which 

 a larger quantity of serum separates. It is remarkable that if the dark- 

 colored clot of ordinary venous blood be immersed in the serum of the scarlet 

 venous blood, it rapidly assumes a brighter tint, and conversely the clot of 

 scarlet venous blood changes to a dark color when immersed in the serum 

 of deeply-colored venous blood ; from which we may conclude that the pri- 

 mary changes are effected upon the Liquor Sanguinis, and not upon the cor- 

 puscles. The influence of the Nervous System in inducing alterations in the 

 color of the blood returning from the salivary glands has already been pointed 

 out ( 105). 



196. Although but little difference except as regards the relative propor- 

 tion of oxygen and carbonic acid gases present can be detected between 

 samples of blood drawn from various parts of the Arterial system of the 

 same animal, 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 diversified character. The blood of the 

 Vena Portie. for example, differs considerably from the blood of the Hepatic 

 vein, and both of these differ from the blood of the Jugular. So, again, the 

 blood of the Splenic as well as that of the Renal vein differs from all the 



1 Phil. Trans., 1797. 2 Lepons, xi to xix, 1859. 



3 Seo an account <>f Davy's and Crawford's Experiments, in Gulliver's Lectures, 

 Mod. Times and Gaz , 18G3J vol. i, p. 1. 



