CHAP. IL] RESPIRATION. 599 



solution of iron-free hsematin or haematoporphyrin, as it is some- 

 times called, being a dark rich brownish red ; but is no longer 

 capable of combining 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 hcemin crystals. 



When blood is left until it decomposes, the haemoglobin is very 

 apt to become changed into a peculiar body known as methcemo- 

 globin, in the spectrum of which a very conspicuous band is seen 

 in the red between C and D (see Fig. 76, 4). The same change 

 may be brought about by the action of weak acids, such as 

 carbonic acid, by ozone, and by other agents such as nitrites and 

 potassium permanganate. When a stream of carbonic acid is 

 driven through blood or through a solution of haemoglobin the 

 band in the red characteristic of methaemoglobin soon makes 

 its appearance. Methaemoglobin differs but little if at all in 

 elementary composition from haemoglobin; it is maintained that 

 it contains the same quantity of oxygen as oxyhaemoglobin but 

 in a more stable condition, more intimately associated with the 

 molecule. 



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 arterial 

 blood the haemoglobin holds nearly its full complement of oxygen 

 and may be spoken of as nearly wholly oxyhaemoglobin, in venous 

 blood the haemoglobin is to a large but variable extent, reduced ; 

 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 oxy haemoglobin is scarlet. 



The reldtions of the Carbonic Acid in the Blood. 



352. The presence of carbonic acid in the blood appears to 

 be determined by conditions more complex in their nature and at 



