790 COMPARATIVE PHYSIOLOGY. 



an atmosphere of pure oxygen. It is a fact of common observation, 

 that portions of animals' bodies which are shaded from the light tend to 

 be pale in colour. 



Most of the pigments fall into chemical groups ; of these, the best 

 defined and perhaps most widely spread is the Lipochrome group. 

 The Lipochromes are characterised (i) by their colour, which varies 

 from yellow through orange to red ; (2) by giving in the dry state a blue 

 coloration with strong H. 2 SO 4 ; (3) by their ready decomposition when 

 exposed to light, when they lose their colour and yield cholesterin ; 

 (4) by the fact that they consist only of carbon, hydrogen, and oxygen. 

 Among animals they are perhaps most abundant, and show ihe 

 greatest variety of tint in the Crustacea, but they are also common in 

 Echinoderms, Birds, Fishes, and many other classes, occurring both in 

 the integument and the internal organs. 



COMPARATIVE PATHOLOGY. 



Within recent years pathologists have begun to study 

 diseased conditions comparatively an obviously rational 

 method which promises to lead to very important results, 

 both practical and theoretical. For man has no monopoly 

 of disease, and some of the processes by which unhealthy 

 conditions are dealt with by the organism are more readily 

 studied in lower animals than in him. Of this we shall give 

 one illustration. In 1862, Haeckel observed that grains of 

 indigo injected into the mollusc Thetys were surrounded by 

 the amoeboid blood corpuscles. Other observers followed 

 the hint which this suggestive fact supplied, and Metch- 

 nikoff, above all others, has shown the important role which 

 these amoeboid cells fill in waging war against intruding 

 germs and parasites, in surrounding irritant particles, in 

 repairing injuries, and the like. In fact, Metchnikoff has 

 worked out the evolution of the phagocyte, as he terms the 

 amoeboid cell, whose function it is to discharge the role 

 above indicated. It is this evolution, as stated in Metch- 

 nikoff's lectures on the comparative pathology of inflamma- 

 tion (Trans., London, 1893), which we shall take in 

 illustration of comparative pathology. 



The simplest conditions are, of course, illustrated by the 

 Protozoa. These enjoy comparative immunity from the 

 injurious effects of wounds and from infectious disease. 

 For injuries are very rapidly repaired ; a fragment, if 

 nucleated, can usually regrow the whole ; infecting organisms 

 are in most cases digested, and irritant particles are got rid 



