CHAP, ii.] RESPIRATION. 565 



it takes up the oxygen, regains its bright scarlet colour and its 

 characteristic absorption spectrum, the single band being replaced 

 by the two. Thus if a solution of oxyhaemoglobin in a test- 

 tube, after being reduced by the action of a drop or two of 

 ammonium sulphide solution and thus shewing the purple colour 

 and the single band, be shaken up with air, the bright scarlet 

 colour at once returns, and when the fluid is placed before the 

 spectroscope, it is seen that the single faint broad band of the 

 reduced haemoglobin has wholly disappeared, and that in its 

 place are the two sharp thinner bands of the oxy haemoglobin. If 

 left to stand in the test-tube the quantity of reducing agent still 

 present is generally sufficient again to rob the haemoglobin of the 

 oxygen thus newly acquired, and soon the scarlet hue fades back 

 again into the purple, the two bands giving place to the one. 

 Another shake and exposure to air will however again bring back 

 the scarlet hue and the two bands ; and once more these may dis- 

 appear. In fact, a few drops of the reducing fluid will allow this 

 game of haemoglobin taking oxygen from the air and giving it up 

 to the reducer to be played over and over again ; at each turn of 

 the game the colour shifts from scarlet to purple, and from purple 

 to scarlet, while the two bands exchange for the one, and the one 

 for the two. 



348. Colour of Venous and Arterial Blood. Evidently we 

 have in these properties of haemoglobin an explanation of at least 

 one-half of the great respiratory process, and they teach us the 

 meaning of the change of colour which takes place when venous 

 blood becomes arterial or arterial venous. 



In venous blood, as it issues from the right ventricle, the 

 oxygen present is insufficient to satisfy wholly the haemoglobin 

 of the red corpuscles; the haemoglobin is, to a large extent, 

 reduced, hence the purple colour of venous blood. When ordinary 

 venous blood, diluted without access of oxygen, is brought before 

 the spectroscope, the two bands of oxyhaemoglobin are seen. This 

 is explained by the fact that in partly reduced haemoglobin, which 

 we may conveniently regard as a mixture of oxyhaemoglobin and 

 (reduced) haemoglobin, the two sharp bands of the former are 

 always much more readily seen than the much fainter band of the 

 latter. Now in ordinary venous blood there is always some loose 

 oxygen, removable by diminished pressure or otherwise ; the haemo- 

 globin is only partly reduced, there is always some, indeed a 

 considerable quantity, of oxyhaemoglobin as well as (reduced) 

 haemoglobin. It is only under special circumstances, as for instance 

 after death by what we shall presently speak of as asphyxia, that 

 all the loose oxygen of the blood disappears ; and then the two 

 bands of the oxyhaemoglobin vanish too. If even only a small 

 quantity of oxygen be present so distinct are the two bands that a 

 solution of completely reduced haemoglobin may be used as a test 

 for the presence of oxygen; if oxygen be present in any fluid 



