26 REPORT — 1893. 



principle followed in experimental psychology, Pfeffer ' made it his object 

 to determine, not the relative effects of different doses, but the smallest 

 perceptible increase of dose which the organism was able to detect, with 

 this result — that, just as in measurements of the relation between stimulus 

 and reaction in ourselves we find that the sensational value of a stimulus 

 depends, not on its absolute intensity, but on the ratio between that 

 intensity and the previous excitation, so in this simplest of vital reagents 

 the same so-called psycho-physical law manifests itself. It is not, how- 

 ever, with a view to this interesting relation that I have referred to 

 Pfeffer's discovery, but because it serves as a centre around which other 

 phenomena, observed alike in plants and animals, have been grouped. 

 As a general designation of reactions of this kind Pfefifer devised the 

 term Chemotaxis, or, as we in England prefer to call it, Chemiotaxis. 

 Pfeffer's contrivance for chemiotactic testing was borrowed from the 

 pathologists, who have long used it for the purpose of determining the 

 relation between a great variety of chemical compounds or products, and 

 the colourless corpuscles of the blood. I need, I am sure, make no 

 apology for referring to a question which, although purely pathological, 

 is of very great biological interest — the theory of the process by which, 

 not only in man, but also, as Metschnikoff has strikingly shown, in 

 animals far down in the scale of develc»pment, the organism protects 

 itself against such harmful things as, whether particulate or not, are able 

 to penetrate its framework. Since Cohnheim's great discovery in 1867 

 we have known that the central phenomenon of what is termed by 

 pathologists inflammation is what would now be called a chemiotactic 

 one ; for it consists in the gathering together, like that of vultures to a 

 carcase, of those migratory cells which have their home in the blood 

 stream and in the lymphatic system, to any point where the living tissue 

 of the body has been injured or damaged, as if the products of dis- 

 integration which are set free where such damage occurs were attractive 

 to them. 



The fact of chemiotaxis, therefore, as a constituent phenomenon of 

 the process of inflammation, was familiar in pathology long before it was 

 understood. Cohnheim himself attributed it to changes in the channels 

 along which the cells moved, and this explanation was generally accepted, 

 though some writers, at all events, recognised its incompleteness. But 

 no sooner was Pfeffer's discovery known than Leber,^ who for years had 

 been working at the subject from the pathological side, at once saw that 

 the two processes were of similar nature. Then followed a variety of 

 researches of great interest, by which the importance of chemiotaxis in 

 relation to the destruction of disease-producing microphytes was proved, 



' Pfeffer, Untersvch a. d. botan. Institute z. Tubingen, vol. i., part 3, 1884. 

 - Leber, ' Die Anhaufung der Leucocj'ten am Orte des Entziindungsreizes,' &c. 

 Die EntstehuTig der Enstiinduvg, &c., pp. 423-464, Leipzig, 1891. 



