138 AN INTRODUCTION TO SCIENCE 



has been killed by heat. The inoculations just described 

 illustrate four different methods of securing immunity. 



How it is the antitoxic results of an invasion remain in 

 the system for years after the germs have been defeated is a 

 problem which still awaits solution. It seems impossible 

 that the antitoxins formed at the time of an attack should be 

 stored for long. Rather must \ve suppose that the tissues 

 are in some way trained to produce them as required. For 

 a long time pathologists have looked upon the white blood- 

 corpuscles or leucocytes as the body's medical officers of 

 health. Undoubtedly they have the power of catching and 

 devouring germs or any other foreign particles which may 

 force admittance. Their independent existence, and the 

 situation of their camps, points them out as Nature's police. 

 Their breeding grounds are the tonsils at the entrance to the 

 throat, the submucous tissue of the wind-pipe and bronchi, 

 Peyer's patches in the intestine, the glands of the neck, the 

 armpit, and the groin, which guard the outflows of lymphatic 

 vessels and bar the passage into the blood-stream and the 

 vital organs. When the throat is sore the tonsil enlarges, 

 and the leucocytes can be seen to sweep down from their 

 fortress, to work their way among the cells of the mucous 

 membrane, and even to reach its surface in their eagerness 

 to give battle to any noxious germs which might try to force 

 an entrance into the connective tissue which lies beneath it. 

 They patrol the blood-vessels to the number of about one 

 leucocyte for every three hundred red blood-corpuscles ; 

 now rolling down the blood-stream, now clinging to the 

 vessel-wall and squeezing themselves between its lining 

 cells in search of effused blood or broken-down tissue which 

 would set up mischief if not speedily removed. They are 

 entirely independent of nervous control, are as free to 

 wander within the body as an amceba in a pond. The best 

 of our nutrient juices are at their command. They are fed 

 as no fixed tissue-cells are fed. And in return for this hospi- 

 tality they do, as far as we know, no work, save that of re- 



