PHYSIOLOGICAL 413 



light. In the lungs, where the blood is richly supplied with oxygen 

 from the inspired air, the pigment in the red blood corpuscles forms 

 oxyhaemoglobin ; in the tissues of the body this reaction is reversed, 

 and oxygen is set free to supply the needs of the Hving cells. In 

 a somewhat different way the haemoglobin helps to carry the carbon 

 dioxide from the tissues to the lungs — that is to say, from the 

 place of formation in combustion to the place where it is eliminated 

 as poisonous waste. It may be noted that while the combination 

 of oxygen with haemoglobin is the commonest of our vital occur- 

 rences, there is something extraordinary about it; it is not 

 strictly comparable to any known chemical combination. (See 

 Respiration.) 



There is a continual breaking-down of haemoglobin in the body, 

 and the products, freed from protein and from iron, are dealt with 

 in various ways. Thus they almost certainly give rise to the bili- 

 rubin and biliverdin of the bile of Vertebrates, and some are laid 

 down as tissue-pigments in various backboneless animals, such as 

 leeches and molluscs. 



Allied to haemoglobin, but less effective, are a number of other 

 blood-pigments which have the power of combining temporarily 

 with oxygen. Some of these are derivatives of haemoglobin itself, 

 in which the "haem" nucleus remains intact. In many molluscs 

 and crustaceans there is a very imperfectly known bluish pigment 

 called haemocyanin, in which there is copper instead of iron. The 

 bluish colour is sometimes so pale that the blood appears colourless, 

 though it is certainly not pigmentless. 



A reference may again be made to an interesting series of pig- 

 ments, called Cytochromes, discovered by Keilin in 1925. Cyto- 

 chrome contains the "haem" nucleus, and is therefore related to 

 haemoglobin, but it is much more widely distributed. It occurs in 

 both Vertebrates and Invertebrates; it is abundant in the wing- 

 muscles of insects. It also occurs in yeast and in certain bacteria, 

 and also in flowering plants. It would indeed seem to be almost 

 universal. Its function is discussed along with respiration, but it 

 may be noted here that, like haemoglobin, it has to do with oxygen. 

 It apparently serves not to carry oxygen over a distance, as from 

 lungs to muscles, as haemoglobin does, but to control oxygen within 

 the cell. 



Melanins. — A third set of pigments, with a wide distribution, 

 is the melanin series. Their diverse occurrence may be illustrated 

 by citing as instances the dark skin of the negro or the dark hair 

 of the "Black Celt", the black feathers (apart from their structiural 

 metallic gleam) of the crow tribe, the black choroid which makes 

 the dark chamber of the eye, and the ink-sac of cuttlefishes, which 

 painters have long used for gloomy pictures, their sepia coming 



