THE FROG 69 



termed phagocytosis, and the particular cells engaged in it are often 

 called phagocytes. The red corpuscles owe their colour to a proteid- 

 like substance, containing a certain amount of iron, haemoglobin. 

 This pigment plays an important part in respiration. In the capil- 

 laries of the lungs it enters into loose combination with the oxygen 

 of the air to form an unstable compound, oxyhaemoglobin, of a bright 

 scarlet colour. This blood is conveyed to the heart by the pulmonary 

 veins, and thence by the arteries all over the body. Hence the 

 arteries (except the pulmonary) contain bright red oxygenated blood, 

 and such blood is sometimes termed arterial. In the capillaries of 

 the tissues the oxygen is yielded up and haemoglobin of a darker 

 bluish-red colour is again produced. This is collected by the veins, so 

 that these vessels (again with the exception of the pulmonary veins) 

 contain a non-oxygenated darker blood, often called venous blood. 

 The carbon dioxide produced in the organs and tissues is on the 

 other hand not carried in a combined form ; it simply passes into 

 solution in the blood and lymph, and is passed out again in the 

 capillaries of the lungs. The blood then plays an extremely impor- 

 tant part in respiration, being the agent, by means of which the 

 oxygen is distributed to and the carbon dioxide collected from all 

 parts. This is but one of the functions of the blood, and in a like way 

 it acts as a collector of food in the intestine, and then conveys it all 

 over the body. It will also be remembered that one of the activities 

 of the liver is to transform the nitrogenous waste matter brought 

 to it by the blood into urea, which it returns to that fluid, so that it 

 may be taken to the kidneys, where it is eliminated. In the warm- 

 blooded animals, like the rabbit, the blood is also concerned with the 

 equalisation of temperature. For example, when we are exerting 

 ourselves the blood comes to the surface of the body, where it is 

 slightly cooled down. The distributing power of the blood is also 

 manifested in the transportation of certain active substances, the 

 hormones or internal secretions, which will be dealt with more 

 fully later. 



Before leaving the blood, another very interesting form of 

 activity, intimately connected with it ; calls for attention, and that is 

 the phenomenon termed immunity. It is not possible here to do 

 more than mention one or two of its most obvious points, but it is 

 a subject of great importance in practical medicine and interesting 

 to the zoologist, since it is sometimes the factor determining whether 

 or not an animal can live on a certain area. The meaning of the 

 term is readily made clear by considering what happens when an 

 epidemic of an infectious disease breaks out. Many persons contract 

 the complaint severely, perhaps even fatally, others less severely, 

 and lastly, certain persons do not catch it, however much they may 



