Feb.. 1896. THE PIGMENTS OF ANIMALS. 95 



As to the function of haemoglobin under the circumstances in 

 which we know it best, that is, in the blood of vertebrates, there is, 

 fortunately, no doubt. In this case, it is the active agent in the 

 conveyance of oxygen from the organs of respiration to the tissues. 



Among vertebrates, haemoglobin has very rarely any part to 

 play in producing colour. Such a function has been suggested for 

 the human species, but the suggestion is quite unimportant for our 

 present purpose, and hardly affects the general conclusion that in 

 vertebrates the function of haemoglobin is purely physiological. Of 

 its origin and primary meaning in the metabolism of the animal we 

 unfortunately know nothing ; on the question of its fate, however, we 

 have at least the beginnings of knowledge. Leaving aside for a 

 moment the question of the pigments derived from haemoglobin, we 

 may note generally that modern physiology teaches that haemoglobin 

 is only a stage in the metabolism of the animal. We now know that 

 it is constantly being broken down, and as constantly re-formed ; that 

 special organs — the liver, the spleen, the supra-renal capsules (?) — are 

 concerned in these processes, and that the products of its decomposition 

 are continually being expelled from the body. These facts certainly 

 seem to suggest that the continued production of a large amount of 

 haemoglobin, which is actively employed in the physiological pro- 

 cesses of the individual, involves physiological disturbances of great 

 importance to the organism. That it is certainly a great strain upon 

 the organism is confirmed by the great prevalence of anaemia among 

 civilised races, whenever the conditions of development are at. all 

 unfavourable. That is to say, the organism under these conditions is 

 unable to produce sufficient haemoglobin to make up for that which is 

 being continually decomposed. We thus see that while haemoglobin 

 is supremely useful in vertebrates, yet its presence is associated with 

 modifications of the organism of fundamental importance. 



So far, in dealing with haemoglobin, we have had to do with 

 certainties as regards function ; but the case is far otherwise when 

 we come to the haemoglobin of the muscles of vertebrates, and to 

 the haemoglobin of invertebrates. Here is no direct experimental 

 evidence, but in spite of this there has been apparent a general 

 tendency to assume that, wherever found, haemoglobin has important 

 respiratory functions, and to account for its irregular distribution in 

 the muscles of vertebrates and in various parts of the body in inverte- 

 brates, on the hypothesis of special activity or special need. In point 

 of fact, the proofs of universal usefulness which this hypothesis 

 necessitates are difficult to obtain. 



We will consider, in the first place, the question of the red muscles 

 of vertebrates. As is well-known, the red and pale muscles differ in 

 histological character and chemical composition ; and where the 

 matter has been investigated, it has been found that the muscles 

 differ also physiologically in their reaction to stimuli. Thus, under 

 a single stimulus, pale muscles contract more rapidly and completely 



