THE FLUIDS OF THE BODY 69 



reabsorbed by the wall of the alimentary canal is eventually 

 excreted as the pigment of urine. 



Such is the history of the changes which blood-pigment 

 undergoes within the living body. To a certain extent its 

 chemistry can be followed in the laboratory ; but it must be 

 remembered, when we are treating of the chemistry of a sub- 

 stance as complex as haemoglobin, that the products which can 

 be obtained from it in the laboratory are not necessarily those 

 into which it is transformed in the body. In the laboratory 

 oxyhaemoglobin is easily changed into methsemoglobin, a sub- 

 stance of the same percentage composition, but with its oxygen 

 more firmly fixed. Methsemoglobin can be decomposed into 

 a proteid substance and haamatin. Hsematin, when acted on 

 by reducing agents, becomes hsemochromogen. Hsemochromo- 

 gen, when subjected to such a reducing agent as a mixture of 

 tin and hydrochloric acid, gives rise to coloured bodies closely 

 resembling bile-pigments not as they are secreted by the bile, 

 but as they appear in the urine. It is impossible to prove that 

 the changing colours of a bruise indicate a sequence of chemical 

 transformations from haemoglobin to bile-pigment, but it is 

 not improbable that such a description is correct. The test 

 commonly used to ascertain the presence of bile-pigment, 

 i.e., bilirubin, is the play of colours which it exhibits when 

 oxidized by fuming nitric acid. From yellow it turns to green, 

 to blue, and then to purple, more or less reversing the colours 

 of the bruise. It is fairly certain that effused blood undergoes 

 changes along lines which, if not identical with those through 

 which blood passes on its road to bile-pigment, are at any rate 

 very similar. 



Coagulation of Lymph and Blood. Two or three minutes 

 after blood has been shed it begins to clot. In ten minutes 

 the vessel into which it has been received may be inverted 

 without spilling the blood. After a time the jelly, holding all 

 the corpuscles, shrinks from the sides of the jar. It squeezes 

 out a transparent, straw-coloured fluid serum. The clot con- 

 tinues to contract until, in a few hours, about one-half of the 

 weight of the blood is clot, the other half serum. Lymph 

 coagulates like blood, but most specimens clot more slowly, 

 and the product is less firm. 



When the process is watched through the microscope a 



