INTERNAL SECRETIONS OR HORMONES 41 1 



has no effect on the metabolism is in the presence of the islets 

 of Langerhans, it has been suggested that they are related to 

 this function of the organ. Some recent observations, however, 

 tend to show that these islets are not permanent structures, 

 but that they are formed from and revert to the ordinary 

 secreting tissue. 



8. Duodenum. As already pointed out the duodenum yields 

 a hormone (secretiri), which acts directly upon the pancreas to 

 stimulate its secretion. 



Toxic Action and Immunity 



It is not definitely known how each of these internal secre- 

 tions performs its special action, but light seems to be thrown 

 upon the question by the study of the mode of action of various 

 toxic substances, and the mode of production of a condition 

 of immunity against them. As will be presently shown, a 

 process of the same nature as the production of internal 

 secretions is involved. 



Snake and Diphtheria Toxins. The question may be most 

 simply approached by considering first the probable mode of 

 action of the toxin or poison of snake venom, or of that pro- 

 duced by the diphtheria bacillus, and the way in which protection 

 against these is established by the development of antitoxins. 



By injecting under the skin of the horse increasing doses 

 of such toxins the animal becomes quite resistant to the 

 poison. A certain quantity of its serum can then neutralise a 

 definite quantity of the toxin, so that if the mixture of serum 

 and toxin be injected the animal is uninjured. Something has 

 been formed in the horse which seizes on the molecules of the 

 toxin and makes them harmless, just as when soda is added 

 to sulphuric acid it forms a neutral salt. 



The two molecules have a definite chemical affinity for one 

 another, so that the toxin is no longer free to seize upon the 

 protoplasm of the animal's body. To explain this Ehrlich 

 has suggested that the protoplasm molecule, like the protein 

 molecule (p. 7), is to be considered as made up of a central 

 core with a number of side-chains, hands, or receptors, which 

 play an important part in taking up nourishment of different 

 kinds, for each variety of which special side-chains have a 



