458 BIOLOGY IN RELATION TO OTHER NATURAL SCIENCES. 



tional value of a stimulus depends, not on its absolute intensity, but on 

 the ratio between that intensity and the ijrevious excitation, so in this 

 simplest of vital rea<;ents the same so-called psycho-physical law mani- 

 fests itself. It is not however with a view to this interesting relation 

 that I have referred to Pfeft'er's discovery, but because it serves as a 

 center around which other phenomena, observed alike in plants and 

 animals, have been iiiouped. As a general designation of reactions of 

 this kind Pfeft'er devised the term Gheniotcijcis, or, as we in England 

 prefer to call it, Cliemiotaxis. Pfeffer's contrivance for cliennotactic 

 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 colorless corpuscles of the 

 bh)od. 1 need, I am sure, make no apology for referring to a question 

 which, although purely i)athological, is of very great biological inter- 

 est — the theory of the process by which, not only in man, but also, as 

 Metschnikoft" has strikingly shown, in animals far down in the scale of 

 development, the organism protects itself against such harmful things 

 as, whether particulate or not, are able to penetrate its framework. 

 Since ( 'Ohnheim's great discovery in 18G7 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 togetlier, like that of vultui'as to a carcass, of those migra- 

 tory cells which have their home in the blood stream and in the lym- 

 l^hatic system, to any point where the living tissue of the body has been 

 injured or damaged, as if the ])roducts of disintegration which are set 

 free where such damage occurs were attractive to them. 



The fact of cliemiotaxis therefore as a constituent phenomenon of 

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

 was understood. Cohnheim himself attributed it to changes in the 

 channels along which the cells moved, and tliis exi)lanation was gener- 

 ally accepted, though some writers, at all events, recognized its incom- 

 pleteness. But no sooner was Pfeffer's discovery known than Leber,* 

 who for years had been w^orking on 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 impor- 

 tance of cliemiotaxis in relation to the destruction of disease-producing 

 microphytes was proved, that of Buchnert on the chemical excita- 

 bility of leucocytes being among the most important. Much discussion 

 has taken place, as many present are aware, as to the kind of wander- 

 ing cells, or leucocytes, which in the first instance attack nnn-bittc 

 microbes, and how they deal with them. The question is not by any 

 means decided. It has however I venture to think, been conclusively 



* Leber, ''Die Anliiiufimg iter Leucocyteu am Orte des EntziiuiluDgsreizes," etc., 

 Die Entsithuni/ der Knztilndung, etc., p]i. 421^-464. I^eipzig, 189L 



t Bnclincr, " Die clieni. Reizbtvrkeit der Leucocyteu," etc., Berliner llin Woch., 

 1800, No. 17. 



