20 THE BODY AT WORK 



immune animal, is the acquisition by a mammal of the power 

 of tolerating the injection of the blood of an eel. Eel's blood 

 contains a toxin which destroys the red blood- corpuscles of 

 a mammal. The dissolution of the blood-corpuscles may be 

 watched with the microscope. If successively increasing doses 

 of serum of eel's blood be injected into the body of a rabbit, 

 the rabbit acquires the power of resisting the toxin. Further 

 than this, the serum of the immune rabbit injected into a 

 rabbit which has not been prepared confers immunity upon 

 the latter. If the blood of the prepared animal be mixed 

 with the blood of an unprepared rabbit and with eel's serum, 

 and the mixture examined under the microscope, it will be 

 seen that red blood-corpuscles are no longer dissolved. The 

 immune serum is able to save the blood-corpuscles of the un- 

 prepared blood from destruction. During its course of pre- 

 paration the rabbit developed an antitoxin. 



If germs of diphtheria are injected into the blood of a horse, 

 the first injections give rise to marked febrile symptoms. 

 After a number of injections the horse becomes completely 

 tolerant of the virus. Not only does its blood develop suffi- 

 cient antitoxin to protect it against the toxin of diphtheria, 

 however large may be the quantity injected into its system, 

 but the serum of the prepared horse, when injected beneath 

 the skin of a child suffering from diphtheria, carries with it 

 sufficient antitoxin to destroy the toxin which has gained 

 admission to the child's blood. 



Many more instances might be cited of this capacity of 

 developing " antibodies " of protoplasm. The leucocytes of the 

 blood are incessantly adapting their chemistry to the needs of 

 the economy. All the tissues, it may be supposed, possess the 

 power of developing resistant ferments ; but the leucocytes 

 (Fig. 4) are the undifferentiated cells, the maids-of -all-work. 

 They have not specialized as makers of ptyalin or makers of 

 pepsin. They are not completely given up to lifting weights, 

 like muscles, or carrying messages, like nerves. 



Bacteria are the world's scavengers. To them ultimately 

 belongs the task of reducing organic matter to the salts which 

 plants reorganize. The cycle of life would be broken if bac- 

 teria were suppressed. No sooner has an animal fallen than 

 these little agents commence their beneficent task of resolving 



