824 PHYSIOLOGY 



absorption spectrum of haemoglobin or reduced haemoglobin. The spectrum 

 of carboxyhaemoglobin is very similar to that of oxy haemoglobin, the bands, 

 however, being shifted slightly towards the red end. This solution is of 

 a brighter red than oxyhaemoglobin. Its tint is best observed on diluting 

 the blood to a large extent, when oxyhaemoglobin acquires a yellowish tint, 

 while the pink colour of CO-haemoglobin is retained so long as any colour is 

 visible. The fact that CO-haemoglobin is not altered by reducing agents 

 can be shown by adding ammonium sulphide to CO-haemoglobin and examin- 

 ing with the spectroscope, when no change is observed. 



All these derivatives of haemoglobin, besides their absorption bands in the visible 

 spectrum, have characteristic absorption of light in the ultra-violet spectrum, as has 

 been shown by Gamgee. In the case of oxyhaemoglobin this absorption causes a band 

 (Soret's band) which occupies the greater part of the spectral region between Fraun- 

 hofer's lines G and H. In reduced haemoglobin this band is displaced towards the 

 visible part of the spectrum. 



Another compound of haemoglobin with oxygen is methcemoglobin. This 

 substance, although not of normal occurrence in the body, is found in urine 

 and in blood whenever there is a sudden breaking down of red blood- 

 corpuscles with the setting free of haemoglobin in the blood-plasma. It may 

 be prepared by the addition of a ferricyanide, permanganate, or nitrite, or 

 other oxidising or reducing agents to the laked blood or to solutions of oxy- 

 haemoglobin. It is a chocolate-brown substance, crystallisable, and gives 

 a distinct absorption band in the red between Fraunhofer's lines C and D. It 

 is unaltered by exposure to a vacuum. On treatment with reducing agents, 

 however, such as Stokes's fluid, the methaemoglobin is converted into 

 haemoglobin, from which by shaking with air oxyhaemoglobin can be re- 

 formed. The fact that methaemoglobin cannot be reduced by exposure 

 to a vacuum indicates that it is a compound of oxygen with haemoglobin in 

 which the oxygen is in a different state of combination. According to Buck- 

 master methaemoglobin only contains half the oxygen contained by oxyhae- 

 moglobin, so that the composition of the two bodies might be represented. 



S 



Hb^ I (oxyhsemoglobin) and Hb = O (methaemoglobin). 



\0 



The change from oxyhaemoglobin to methaemoglobin is not affected, however, 

 by a simple shifting of the oxygen groups, but must be assumed to involve 

 two distinct events. The whole of the oxygen in loose combination with 

 haemoglobin is given off, and the oxygen in the methaemoglobin molecule is 

 derived from the oxidising agent added, so that ferricyanide of potash, for 

 instance, is converted into ferro- cyanide.* Since the whole of the oxygen 

 in the oxyhaemoglobin is given off on the addition of potassium ferricyanide, 

 we may use this fact in order to determine the total amount of oxygen in 

 combination in the blood (v. p. 859). 



* When the change is effected by reducing agents we must assume that the oxygen 

 of the water or air is the source of that required for the oxidation of the reduced haemo- 

 globin to methaemoglobin. 



