312 COLOURING MATTERS. 



accurately traced, by microscopical examination, the conversion of 

 isolated coagula in obliterated veins into amorphous and crystalline 

 pigment, and from these morphological investigations it can hardly 

 be doubted, that at all events the melanin of morbid products is 

 formed from the hsernatin. Kolliker* has moreover convinced 

 himself that in the blood- corpuscles enclosed in the enveloping 

 membrane, the haematin affords the matter from which the black 

 pigment in the granular cells is formed. Hence it only remains 

 for the chemist to continue his investigations on this subject, in 

 order to obtain perfectly satisfactory scientific proof of this meta- 

 morphosis. 



Uses. That the use of pigment in the choroid coat is princi- 

 pally to render the eye achromatic, is sufficiently obvious from the 

 principles of physics. We are ignorant of the uses which it serves 

 in the walls of the blood-vessels in the amphibia. 



BiLE-PlGMRNT. 



Chemical Relations. 



Properties. This substance, like so many of the pigments, 

 belongs to that vast group of bodies, whose chemical properties 

 have never been thoroughly investigated ; this is partly dependent 

 on the circumstance that we can only procure it in very small 

 quantity, and partly on its extreme instability, for not only does it 

 occur in the animal organism under various modifications, but it is 

 at once changed by the simplest chemical treatment. The most 

 frequent modification which the primary substance of the bile- 

 pigment in the higher animals appears to present, is the brown 

 pigment, the cholepyrrhin of Berzelius, and the biliphain of 

 Simon. It occurs as a reddish brown, non-crystalline powder, 

 devoid of taste and smell ; it is insoluble in water, very slightly 

 soluble in ether, and more so in alcohol, to which it communicates 

 a distinct yellow tint ; it is more soluble in caustic potash than in 

 caustic ammonia, the alkaline solutions being at first of a clear 

 yellow colour, but on exposure to the air gradually changing to a 

 greenish brown tint. It is on this modification of the bile -pigment 

 that the well-known changes of colour which occur in some of the 

 animal fluids are dependent. The yellow solution of this pigment 

 when gradually treated with nitric acid (and especially, according 

 * Zeitschr. f. iviss. Zoologie. Bd. 1, S. 260-267- 



