360 RESPIRATION. 



metallic lustre, having the probable composition of C 32 , H 34 , N 4 , Fe, O- a . 1 It 

 is fairly soluble in dilute acid or alkaline solutions, and then gives charac- 

 teristic spectra (Fig. 98, 1, 2, 5). 



An interesting feature in haematin is that its alkaline solution is capable 

 of being reduced by reducing agents, the spectrum changing at the same 

 time (Fig. 98, 3), and that the reduced solution will, like the haemoglobin, 

 take up oxygen again on being brought into 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 hsematin may be robbed of all its 

 iron. It still retains the feature of possessing color, 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, hsematin forms with hydrochloric 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 methcemoglobin, in the spec- 

 trum of which a very conspicuous band is seen in the red between C and D 

 (see Fig. 98, 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 rnethaenioglobin soon makes its appearance. Methae- 

 nioglobin differs but little, if at all, in elementary composition from haemo- 

 globin ; it is maintained that it contains the same quantity of oxygen as 

 oxy-haemoglobin 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 pressure (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 haemo- 

 globin 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 oxy-haemoglobin, in venous blood the haemoglobin is to a large but 

 variable extent reduced ; and the characteristic colors of venous and arte- 

 rial blood are in the main due to the fact that the color of reduced haemo- 

 globin is purple, while that of oxy-haemoglobin is scarlet. 



The Relations of the Carbonic Acid in the Blood. 



295. The presence of carbonic acid in the blood appears to be deter- 

 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 



1 This formula is the old one of Hoppe-Seyler. 



