216 HEMOGLOBIN. 



tube. To the above is then added 1 c.c. of concentrated sulphuric acid 

 and the mixture is cooled under water so that its temperature does not 

 exceed 50 60 C. To detect bile-acids in urine with absolute certainty 

 it is essential to separate them from this excretion before applying 

 Pettenkofer's test. This is effected either by precipitation with basic 

 lead acetate or extraction with alcohol 'or chloroform l . 



THE COLOURING MATTERS AND PIGMENTS 

 OF THE ANIMAL BODY. 



HAEMOGLOBIN AND ITS DERIVATIVES. 



1. Haemoglobin 2 . This is the well-known constituent of the red 

 corpuscles to which the dark colour of the blood from an asphyxiated 

 animal is due. It is also present to a less and somewhat variable 

 amount in ordinary venous blood, in presence of correspondingly 

 variable amounts of the compound which it forms with oxygen, 

 namely oxy-haemoglobin. In normal arterial blood it is probably 

 present in mere traces, if at all, since here its affinities for 

 oxygen are completely satisfied to form oxy-haemoglobin. Haemo- 

 globin is chiefly of interest as an oxygen-carrier or respiratory pigment, 

 in virtue of the ease with which it absorbs and unites in loose 

 combination with oxygen when merely exposed to this gas, and again 

 gives it up when brought into relationship with the oxygen-free tissues 

 of the body. The conditions and phenomena of this fixation and 

 liberation of oxygen by haemoglobin have been very fully investigated; 

 the fundamentally important facts in connection with it have already 

 been stated in some detail in an earlier part of this work ( 343 et 

 seq.), so that it is now only necessary to add some further details of 

 haemoglobin of a more purely chemical character. 



Owing to the ease and avidity with which haemoglobin unites with 

 oxygen to form the distinct and stable compound known as oxy-haemo- 

 globin, its investigation is attended with considerable experimental 

 difficulties, hence our knowledge of it as a chemical substance is on the 

 whole less complete than is thait of oxy-haemoglobin. Haemoglobin 

 may be obtained in a crystalline form 3 , but with some considerable 



1 For details see Hoppe-Seyler, Hdbch. d. phys.-path. Chem. Anal. 1883, S. 399 

 and Neubauer u. Vogel, Analyse d. Harm, 1890, S. 146. 



2 The single name haemoglobin is used here to denote what is more frequently 

 arid usually called ' reduced ' haemoglobin, as distinct from oxy-haemoglobiii. The 

 adoption of the name as here used is both simpler and more logical. 



3 First described by Kiihne, Virchow's Arch. Bd. xxxiv. (1865), S. 423. 



