126 ELEMENTARY PHYSIOLOGY 



redect the light more strongly, so as to give a more 

 distinct colourtition to the blood ; while, under the latter, 

 they may reflect less light, and, in that way, allow the 

 blood to appear darker and duller. 



This, however, can only be a small part of the whole 

 matter ; for solutions of haemoglobin or of blood-crystjils 

 (Lesson III.), even when perfectly free from actual blood- 

 corpuscles, change in colour from scarlet to purple, ac- 

 cording as they gain or lose oxygen. It has already been 

 stated (p. 98), that oxygen exists in the blood in loose 

 combination with ha'iuoglobin. And further, a solution 

 of hajmoglobin, when thus loosely combined with oxygen, 

 has a scarlet colour, while a solution of haemoglobin 

 deprived of oxygen has a purplish hue. Hence arterial 

 blood, in which tlie lu^nioglobin is richly provided 

 with oxygen, is naturally scarlet, wliile venous blood, 

 which not only contains an excess of carbonic acid, but 

 whose hiiemoglobin also has lost a great deal of its oxygen, 

 is purple. 



The conditions under which the gases exist in blood are 

 peculiar and important in connection with a point we 

 shall have to discuss later on, namely horn venous blood 

 becomes arterial in the lungs and how arterial blood 

 becomes venous in the tissues. As to the nitrogen, we 

 may say at once that it is a{)i)arently in a state of simple 

 solution, as though the blood were so nmch water. A 

 very snaall part of the i>xygen is similarly simj^ly dissolved 

 in the blood, but practically almost the whole of it is in a 

 state of loose chemical combination loith the haviofilobin of 

 the red coi-jjuscles. The facts which prove this are simple 

 and conclusive. In the absence of red corpuscles plasma 

 and serum only absorb as much oxygen as does an equal 

 quantity of water, namely about one volume per cent. ; 

 but blood, where the red corpuscles are present, may con- 

 tain as much as 20 volumes per cent, of oxygen. Again, 



