594 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



absorbing nearly the total amount which may be absorbed by the blood. 

 It, therefore, would appear that oxygen is absorbed by the blood through 

 the mediation of the red coloring matter. The general characteristics of 

 haemoglobin have been already described under the subject of the blood. 

 They, however, deserve more especial attention in their relationship to 

 the respiratory functions of the blood. 



When a tolerably dilute solution of haemoglobin, which possesses 

 the bright-red color of arterial blood, is placed before the spectroscope, 

 it is found that a portion of the red end of the spectrum is absorbed, 

 together with a portion of the blue end, while two absorption bands are 

 found between the lines D and E ; the line toward the red side of the spec- 

 trum is the narrowest, but is the most distinct, and with very dilute 

 solutions is the only one visible. The other toward the blue side is much 

 broader, but less distinct. These absorption bands are characteristic of 

 solutions of haemoglobin. When stronger solutions of haemoglobin are 

 used the bands broaden and become darker, while more and more light is 

 shut out from each end of the spectrum, until finally the two bands 

 become fused together, and only the green and red rays are now visible : 

 with still stronger solutions the red rays alone pass, and finally they 

 also may disappear, thus indicating the cause of the natural color of 

 solutions of haemoglobin in transmitted light (Fig. 256). 



If crystals of haemoglobin are subjected to a vacuum, oxygen is 

 given up and the crystals become darker and more of a purple color. 

 The quantity of oxygen which is given off is a fixed quantity, each 

 gramme of hasmoglobin giving off 1.76 cubic centimeters of oxygen, 

 measured at a pressure of seven hundred and sixty millimeters and at 

 zero temperature. The haemoglobin is then spoken of ' as reduced 

 haemoglobin. It also is soluble in water and forms a purplish solution. 

 When examined under the spectroscope, instead of the two absorption 

 bands, a single, less distinct, but much broader band is found, whose 

 position lies about midwajr between that of the two absorption bands of 

 the unreduced haemoglobin. 



In strong solutions of reduced haemoglobin less of the blue than of 

 the red end of the spectrum is absorbed, and with concentrated solutions 

 the blue rays may still pass, thus accounting for the difference in color 

 between the arterial and venous blood, the former allowing the red and 

 orange-yellow rays to pass, the latter the red and bluish-green rays; 

 hence arterial blood appears scarlet, venous blood purple. 



The oxygen which is removed from venous blood or from oxyhemo- 

 globin by exposure to a vacuum, or by the use of reducing agents, is not 

 the oxygen which enters into the molecular constitution of haemoglobin, 

 but is a definite volume which is capable of entering into loose chemical 

 combination with it. 



