436 BLOOD AND LYMPH. 



avoid a deterioration in color value. Methemoglobin may be 

 obtained rapidly by the action of various reagents on the blood, 

 some of them oxidizing substances, such as permanganate of potash 

 or ferricyanid of potash, some of them reducing substances. In- 

 deed, it is known that the change may occur within the blood-vessels 

 by the action of such bodies as the nitrites, antifebrin, acetanilid, 

 etc. According to most observers methemoglobin contains the 

 same amount of oxygen as hemoglobin ; it is combined differently, 

 however, forming a more stable compound, which can not be dis- 

 sociated by the action of a vacuum. On this account, therefore, 

 methemoglobin is not capable of acting as a respiratory pigment, 

 and to the extent that it is formed in the blood this tissue suffers a 

 loss of its functional value as a carrier of oxygen. By the stronger 

 action of reducing solutions such as ammonium sulphid the 

 oxygen may be removed from the methemoglobin and reduced 

 hemoglobin be obtained. Methemoglobin crystallizes in needles, 

 and its solutions give an absorption spectrum which varies ac- 

 cording as the solution is neutral or has an alkaline reaction. In 

 neutral solutions the characteristic band is one in the orange, as 

 indicated in Fig. 183. In alkaline solution the absorption spectrum 

 has three bands, two of which are nearly identical with those of 

 oxyhemoglobin. 



Hematin (CssHss^FeOs) is obtained when hemoglobin is 

 decomposed by the action of acids or alkalies in the presence of 

 oxygen. It may occur in the feces if the diet contains hemoglobin 

 or hematin, or in case of hemorrhage in the stomach or small intes- 

 tine, since both the pancreatic and the gastric secretion break up 

 hemoglobin, with the formation of hematin. It is an amorphous 

 substance, of a dark-brown color, easily soluble in alkalies or in 

 acid alcoholic solutions. These solutions give a characteristic 

 absorption spectrum which is represented in Fig. 183. The 

 chemical structure of hematin has not been established satisfac- 

 torily, but it is probable that the molecule contains four pyrrol 



groups : 



CH = CHs 

 I >NH. 



CH = CH/ 



Two of the amino-acid derivatives of proteins, tryptophan and 

 prolin (see p. 795), contain this same grouping, and it is possible, 

 therefore, that these amino-acids may serve as material from which 

 the body constructs its hematin. 



Hemin (CssH^C^^FeCl) is a derivative of hematin, obtained 

 by the action of hydrochloric acid or chlorids, in which an hy- 

 droxyl group is replaced by chlorin. The compound is obtained 

 in the form of crystals, which under the microscope appear usually 

 as small, rhombic plates of a dark-brown color. These crystals 



