548 Chapter XVII 



the spleen, lymphatic glands, and bone marrow. Another group of 

 white corpuscles in the Vertebrata is made up of small amoeboid 

 cells which are distinguished by having a nucleus which, although 

 single, is divided into several lobes. These are the microphages 

 [572] whose chief peculiarity, the multi-lobed form of the nucleus, must 

 be regarded as an adaptation for the purpose of passing as rapidly as 

 possible through the walls of capillaries and small veins. 



The diapedesis of the white corpuscles, their migration through 

 the vessel wall into the cavities and tissues, is one of the principal 

 means of defence possessed by an animal. As soon as the infective 

 agents have penetrated into the body, a whole army of white corpuscles 

 proceed towards the menaced spot, there entering into a struggle 

 with the micro-organisms. Aided by the special form of their nucleus 

 the microphages are the first to pass through the walls of the vessels. 

 Each of the several small lobes, into which the nucleus and its 

 protoplasm is divided, passes readily through the minute orifices 

 between the endothelial cells of the vessels. The macrophages 

 follow the microphages and become mixed in greater or less numbers 

 with the exudations. But it is not micro-organisms only which set 

 up this inflammatory reaction accompanied by the emigration and 

 the accumulation of leucocytes. The introduction of inert bodies 

 and of aseptic fluids brings about the same result. The phagocytes 

 are, as a matter of fact, endowed with a special susceptibility, which 

 enables them to perceive exceedingly small changes in the chemical 

 or physical composition of the medium that surrounds them. 



The leucocytes, having arrived at the spot where the intruders 

 are found, seize them after the manner of the Amoebae and within 

 their bodies subject them to intracellular digestion. This digestion 

 takes place in the vacuoles in which usually is a weakly acid fluid 

 which contains digestive ferments; of these a very considerable 

 number are now recognised. 



Just as the Amoebae and the Infusoria make a choice from amongst 

 the small organisms that surround them, so the leucocytes choose 

 bodies which are best suited to their use. The macrophages seize 

 by preference animal cells such as the blood corpuscles, the sperma- 

 tozoa, and other elements which are derived from animals. Among the 

 infective micro-organisms the macrophages have a , predilection for 

 those that set up chronic diseases such as leprosy, tuberculosis, and 

 actinomycosis and also for those which are of animal nature. Into 

 this last category come the amoeboid parasities of malaria, Texas 



