342 CARBONIC ACID IN THE BLOOD. [BOOK n, 



contact with air or oxygen. This would seem to indicate that the 

 oxygen-holding power of haemoglobin is connected exclusively 

 with its haematin constituent. By the action of strong sulphuric 

 acid haematin may be robbed of all its iron. It still retains the 

 feature of possessing colour, the solution of iron-free haematin 

 being a dark rich brownish red ; but is no longer capable of com- 

 bining loosely with oxygen. This indicates that the iron is in 

 some way associated with the peculiar respiratory functions of 

 haemoglobin; though it is obviously an error to suppose, as was 

 once supposed, that the change from venous to arterial blood 

 consists essentially in a change from a ferrous to a ferric salt. 



Though not crystallizable itself, haematin forms with hydro- 

 chloric acid a compound, occurring in minute rhombic crystals, 

 known as haemin crystals. 



In conclusion, the condition of oxygen in the blood is as 

 follows. Of the whole quantity of oxygen in the blood, only a 

 minute fraction is simply absorbed or dissolved, according to the 

 law of pressures (the Henry-Dalton law). The great mass is in a 

 state of combination with the haemoglobin, the connection being of 

 such a kind that while the haemoglobin readily combines with the 

 oxygen of the air to which it is exposed, dissociation readily occurs 

 at low pressures, or in the presence of indifferent gases, or by the 

 action of substances having a greater affinity for oxygen than has 

 haemoglobin itself. The difference between venous and arterial 

 blood, as far as oxygen is concerned, is that while in the latter 

 there is an insignificant quantity of reduced haemoglobin, in the 

 former there is a great deal; and the characteristic colours of 

 venous and arterial blood are in the main due to the fact that the 

 colour of reduced haemoglobin is purple, while that of oxyhaemo- 

 globin is scarlet. 



The relations of the Carbonic Acid in the Blood. 



The presence of carbonic acid in the blood appears to be detei- 

 mined by conditions more complex in their nature and at present 

 not so well understood as those which determine the presence of 

 oxygen. The carbonic acid is not simply dissolved in the blood; its 

 absorption by blood does not follow the law of pressures. It exists 

 in association with some substance or substances in the blood, and 

 its escape from the blood is a process of dissociation. We cannot 

 however speak of it as being associated, to the same extent as is 

 the oxygen, with the haemoglobin of the red corpuscles. So far 

 from the red corpuscles containing the great mass of the carbonic 

 acid, the quantity of this gas which is present in a volume of serum 



