ECONOMY OF THE BODY 135 



(5) The blood also contains the phagocytes whose 

 various functions we have already referred to, and by 

 means of these or other kinds of white blood corpuscles it 

 is able to produce within itself subtle counteractives or 

 " anti-bodies " which are able to antagonise poisons. 



The bird has a first-class, four-chambered heart, with 

 a complete separation of venous and arterial blood, just as 

 in mammals. There is but a slight impei-fection in the 

 valvular arrangements between the right auricle (receiving 

 all the impure blood from the body) and the right ventricle 

 (driving the impure blood to the lungs) ; but perhaps too 

 much has been made of this. For it is surely a relatively 

 trivial evolutionary detail — this imperfection in the right 

 auriculo-ventricular valve of the bird's heart. Its greatest 

 interest, perhaps, is that we find collateral echoes of it in 

 the heart of the egg-laying Duckmole — a primitive mammal. 



The origin of the blood-cells in the individual bird is a 

 question too difficult for this book. It seems that the 

 original stock arises in various centres where mesoderm- 

 cells proliferate. These " blood-cradles " or '* haemato- 

 poietic tissues " may occur in various parts of the body ; 

 leucocytes may be produced in the bursa Fabricii ; leucocytes 

 and erythrocytes both arise in the marrow of the bones and 

 in the spleen. But there is a special difficulty in the 

 question where the multitude of red blood corpuscles in 

 bird's blood has its origin. For in many birds the marrow 

 of the bones is used up very early. It is believed by 

 many, though the evidence is insecure, that the worn-out 

 red blood corpuscles are destroyed and fresh white blood 

 corpuscles are produced in the spleen of mammals. This 

 may be so in birds also. 



The spleen has been removed from mammals without 

 appreciable effects on the health. It is possible that in 

 birds it is more essential, and that it is a " cradle " of new 

 red blood corpuscles, as well as a destructor of those that 

 are worn-out and effete. But we do not know. A large 

 number of measurements made by Magnan and De la 

 Ribosiere (191 1) yielded the curious result that large birds 



