440 ZOOLOGY AND MEDICINE. 



In the blood, in the lymph, and in connective tissue are fonnd 

 anatomical elements Avhich have for a long time been known under 

 the name of " leucocytes," or white corpuscles. A well-worn com- 

 parison likens them to the Amoebse, which they resemble, indeed, 

 l)oth in their method of locomotion and in the manner in which they 

 ingest solid j^articles. Several varieties are known, but the dis- 

 tinction between them remains as yet merely a histological curiosity. 

 Now these elements, which resemble the lowest animals in their struc- 

 ture and physiology, are found to play a most important part in the 

 organism. 



The physiological equilibrium that constitutes health is assured 

 only because of their constant vigilance. Distributed to all parts of 

 the body, they keep watch over every point and resist the various per- 

 turbations that may arise at any moment in our organs; they have 

 for their special function the arrest of foreign bodies, of microbes, 

 and parasites in general that invade our economy by the most vari- 

 ous routes. According as these are greater or smaller the leucocytes 

 vary their method of attack ; they either engage singly or work in 

 combination to arrest the advance of the parasitic invaders. If the 

 infectious agent is not an organized l)ody, but consists of chemical 

 substances endowed with toxic qualities, they intervene in another 

 manner and, adapting themselves to the new conditions, elaborate 

 and supply to the humors of the organism sul)stances capable of 

 neutralizing the dangerous effects. 



Is not the theory of phagocytosis, wdiich we owe to the sagacious 

 observations of MetchnikofF, one of the manifestations of these leuco- 

 cytes? Everyone knows the nature of this theory; everyone at least 

 knows the Amoebse that live in stagnant waters. These animalcules 

 present the lowest degree of animality ; their sarcode or plastic sub- 

 stance extrudes prolongations which enable it to inclose solid par- 

 ticles with which it may be in contact ; these are, according to their 

 nature, either digested and assimilated by the Amoeba or, on the 

 contrary, rejected after a certain lapse of time. Nothing is better 

 known than this phenomenon; Dujardin and others have studied it 

 very thoroughly ; they consider it as a manifestation of the simplest 

 act of nutrition. It is so, indeed, but it is also an act of exceptional 

 importance, since it was the point of departure for the cliscoA^ery of 

 phagocytosis, a doctrine which touches upon some of the most 

 obscure problems of physiology. 



Thus it was that a simple fact of zoological observation, well inter- 

 preted by a mind of rare penetration, completely overthrew the hesi- 

 tating and misty conceptions derived from humorism, that doctrine 

 by which medicine endeavored to explain the great fact of the resist- 

 ance of the organism to infections. Phagocytosis offered the key to 

 the problem. It also enables us, or soon will enable us, to compre- 



