CHEMICAL COMPOSITION OF BLOOD 53 



As to the form of the crystals, in the vast majority of animals they 

 are rhombic prisms or needles, but in the guinea-pig they are sphenoids 

 belonging to the rhombic system, and in the squirrel six-sided plates of 

 tae hexagonal system (Fig. 14). Careful study of the crystallography of 

 haemoglobin from a large number of animals has established differences 

 and resemblances so constant and so clear-cut that they may be used for 

 the purposes of classification and for the identification of the source of 

 a specimen of blood (Reichert and Brown). 



Reduced haemoglobin can also be caused to crystallize, though with 

 more difficulty than oxyhaemoglobin, since it is more soluble. Crystals 

 of reduced haemoglobin were first prepared from human blood by Hiifner, 

 who allowed it to putrefy in sealed tubes for several weeks. 



When a solution of oxyhsemoglobin of moderate strength is ex- 

 amined with the spectroscope, two well-marked absorption bands 

 are seen, one a little to the right of Fraunhofer's line D, and the other 

 a little to the left of E. A third band exists in the extreme violet 

 between G and H. It cannot be detected with an ordinary spectro- 

 scope, but has been studied by the aid of a fluorescent eyepiece, by 

 projecting the spectrum on a fluorescent screen, and by photograph- 

 ing the spectrum. The addition of a reducing agent, such as 

 ammonium sulphide, causes the bands in the visible spectrum to 

 disappear, and they are replaced by a less sharply denned band, of 

 which the centre is about equidistant from D and E. This is the 

 characteristic band of reduced haemoglobin. The spectrum of 

 ordinary venous blood shows the bands of oxyhaemoglobin. 



Carbonic oxide haemoglobin is a representative of a class of haemo- 

 globin compounds analogous to oxyhaemoglobin, in which the loosely- 

 combined oxygen has been replaced by other gases (carbon monoxide, 

 nitric oxide) in firmer union. Its spectrum shows two bands very like 

 those of oxyhaemoglobin, but a little nearer the violet end. Carbonic 

 oxide haemoglobin is formed in poisoning with coal-gas. Owing to the 

 great stability of the compound, the haemoglobin can no longer be 

 oxidized in the lungs, and death may take place from asphyxia. It 

 is, however, gradually broken up, and therefore artificial respiration 

 may be of use in such cases. Inhalation of oxygen and especially 

 transfusion of blood are also of great value. 



Methcemoglobin is a derivative of oxyhaemoglobin which can be 

 formed from it in various ways e.g., by the addition of ferricyanide of 

 potassium or nitrite of amyl (Gamgee), by electrolysis (in the neigh- 

 bourhood of the anode), or by the action of the oxidizing ferment 

 ' echidnase ' in the poison of the viper (Phisalix) . It very often appears 

 in an oxyhaemoglobin solution which is exposed to the air. It has been 

 found in the urine in cases of haemoglobinuria, in the fluid of ovarian 

 cysts, and in haematoceles. The strongest band in its spectrum is 

 in the red, between C and D, but nearer C, nearly in the same position 

 as the band of acid-haematin. Reducing agents, such as ammonium 

 sulphide, change methaemoglobin first into oxyhaemoglobin and then 

 into reduced haemoglobin. It has by some been regarded as a more 

 highly oxidized haemoglobin than oxyhsemoglobin. Rebutting evidence 

 has, however, been offered to the effect that the same quantity of 

 oxygen is required to saturate both pigments, and this evidence appears 

 to be sound. The difference lies rather in the manner in which the 

 oxygen is united to the haemoglobin in the methaemoglobin molecule 

 than in the quantity of oxygen which it contains. For methaemo- 



