166 MICROBES AND TOXINS 



by the nerves of the plain muscles and the cardiac muscle, and 

 by certain fibres in the nerve centres : it does not stain the 

 motor-endings of striped muBcle with the exception of those ol" 

 the eyes, of the larynx, and of the diaphragm (Ehrlich). 



The majority of the staining substances fixed by the cerebral 

 cortex are also taken up by fatty substances ; now the cortical 

 cells are rich in lipoids such as cholesterin, lecithin, and 

 cerebrin. The substances which stain " in vivo " are soluble 

 in these lipoids, the non-vital stains are not. Narcotics, 

 cerebral poisons, have an activity proportional to their 

 co-efficient of absorption by the lipoids, and both this activity 

 and this co-efficient vary with the temperature. Antipyretics 

 also have, without doubt, a selective action on certain cells. 



The famous experiment of Wassermann and Takaki showed 

 that a similar selective absorption takes place between the 

 cerebral grey matter of mammals and tetanus toxin when these 

 are mixed together in a test-tube ; after a certain time the 

 tissue fixes the toxin and the liquid is no longer toxic. 



The brain of cold-blooded animals, e.g., the lizard and the 

 tortoise, does not fix toxin at all, or very little. The brain of 

 the frog does not fix toxin in vitro (and yet the frog kept in a 

 fairly hot room is sensitive to tetanus toxin). The fixing 

 power of the brain seems to be proportional to its lipoid 

 content, and there is less of these substances in the brain of 

 cold-blooded animals. Brain material treated with ether, 

 which dissolves the fats, loses a great part of its power of 

 fixation : brain which has been boiled no longer fixes at all. 

 Filtration of a suspension of brain material (removing the cell 

 elements) also destroys the fixing property. Cholesterin, 

 lecithin, and cochineal, a fatty material extracted from the 

 cochineal insect, all fix toxin, but when heated to 60° in 

 presence of moisture, or after a previous maceration in an 

 alkaline fluid, the latter is no longer a fixative. 



The fixation of toxin by sensitive cells is a phenomenon of 

 the order of dyeing or in vivo staining — -a phenomenon of 

 molecular adhesion. 



There is not in Wassermann's experiment, as was thought at 



