252 PATHOLOGY OF THE BLOOD AND CIRCULATION. 



periraentally demonstrated. A colloid liquid injected into 

 the vessels traverses their walls even under feeble pressure. 

 The microscope fails to find any lesion, though it is claimed 

 by some that an increase in the size of the natural stomata 

 between the endothelial cells can be made out. 



Diapedesis : The demonstration of the migration of leuko- 

 cytes threw a flood of light upon a very important question 

 the source of inflammatory cells. The formation of pus-cells 

 from connective-tissue cells had never been seen by any one. 

 It became very evident that at least the great majority were 

 migrated leukocytes, the resemblance of which to the cells 

 crowding all inflammatory tissues had been commented upon 

 by Virchow and other observers many years before. It is 

 quite probable that their number is greatly augmented by their 

 multiplication subsequent to migration. As an explanation 

 of the phenomena of diapedesis two theories mainly are 

 invoked the theory of chemotaxis and the theory of phago- 

 cytosis. 



Chemotaxis : It has been known for a number of years that 

 vegetable plasmodia are attracted by certain substances and 

 repelled by others : if placed on a surface at a little dis- 

 tance from a nutrient material, such as a decoction of dead 

 leaves, the organism moves toward it ; while numerous other 

 materials, such as a solution of salt and sugar, repel it. To 

 these phenomena Stahl gave the name positive and negative 

 trophotropism, for which Pfefler substituted the terms positive 

 and negative chemotaxis. 



A similar chemotactic sensibility has been demonstrated on 

 the part of the white blood-cells toward various substances. 



It has been shown that whereas some materials, such as 

 quinine, alcohol, chloroform, repel the leukocytes, cultures of 

 various micro-organisms and proteid substances which can be 

 separated therefrom attract them. So, it has been suggested, 

 the leukocytes are attracted by substances formed at the site 

 of inflammation. 



Phagocytosis : Metschnikoff, studying the phenomena of in- 

 flammation from the standpoint of a biologist, assigns to the 

 leukocytes the main role. He attempts to elucidate these 

 complex phenomena by producing them, as far as possible, in 

 organisms of the most simple structure. We find that unicel- 



