ELEMENTS OF COMPOSITION. 



53 



hitherto been made, must be received with reserve, for the substance is 

 only to be obtained pure with the greatest difficulty. 



Melanin, which with hsematin is the only pigment in the body to 

 which a certain amount of histogenic significance cannot be denied, 

 appears, as a rule, forming the contents of polygonal or stellate cells. It 

 is met with in greatest abundance in the interior of the eye. The large 

 amount also in which it is met with in some of the lower vertebrate animals, 

 as for instance in the frog, is remarkable. 



As a pathological product, it (or something nearly allied to it) is frequently 

 met with in great abundance in different organs, tumours, &c. 



The source of melanin is usually, and probably correctly, supposed to 

 be the colouring matter of the blood. This view is borne out by the 

 nature of pathological black pigments, whose origin from hsematin may 

 in many cases be accurately traced. 



We must be on our guard, however, not to confound the ordinary black 

 pigment found in the human lungs with melanin. This consists of 

 particles of carbon, charcoal, dust, or lamp-black, suspended in the air 

 which is inspired. It is not met with in the lungs of infants or of wild 

 mammalian animals. 



37. 

 Biliary Figments. 



Until very recently but little has been known of the colouring matter 

 of the bile. It is characterised by its reactions with nitric acid. The 

 latter, if it contain nitrous acid, or if concentrated sulphuric acid be 

 added to it, produces in bile a peculiar play of colours, green, blue, violet, 

 red, yellow, following rapidly one upon the other. 



Two kinds of pigment may be usually distinguished in bile : a brown, 

 known as cholepyrrhin or bitiphcein, and a green or biliverdin. 



According to Staedeler's recent investigations, a whole series of probably 

 characteristic pigments may be obtained from bile, although it is still a 

 question whether they all exist in the latter when perfectly fresh. 



BILIRUBIN, C 16 H 18 N 2 3 (or C 9 H 9 ISr0 2 ?) 



A red substance allied to haamatin and haematoidin (but not identical 

 with the latter), which may be obtained from its solutions in chloro- 

 form, sulphide of carbon, and benzol, in beautiful 

 ruby -red crystals. These (fig. 36), when crystal- 

 lized from sulphide of carbon, appear in clino- 

 rhombic prisms, with a basal surface, whose 

 foremost angle is very sharply curved, and prism 

 surface convex, so that a view of the basal 

 surface presents an elliptical figure. Lying upon 

 their convex surface these crystals have a rhombic 

 form. 



Bilirubin is insoluble in water, and nearly so 

 in ether. It is soluble, on the other hand, in 

 alkalies arid in chloroform, communicating to 

 the latter a pure yellow or orange-red colour; 

 also in sulphide of carbon, which is tinged 

 golden yellow by it. It possesses, further, the 

 properties of a weak acid, and shows the play of 

 colours just mentioned with nitric acid containing nitrous acid in the most 



bon 



