612 VETERINARY PHYSIOLOGY 



is shown as gradually increasing in importance; where the 

 primary chemical regulators are indicated as playing an im- 

 portant part towards the end of foetal and the beginning of 

 extra-uterine life ; and where the part played by the neuro- 



Embryonl'c Life . Post-embryonic Life 



Fie. 232.— To show the Relative Parts played during Embryonic and Post- 

 embryonic Life in the Regulation of Metabolism by {a) hereditary 

 inertia, (h) the primary chemical regulators, (c) the nervous system, 

 and (d) the neuro-chemical regulators. 



chemical regulators is shown as advancing with the increased 

 importance of the nervous system. 



Modifications of Metabolism for Protection 

 against Toxic Agents. 



The study of the production and modes of action of these 

 internal secretions leads to the consideration of the protection 

 of the organism against the action of various poisons of animal 

 or bacterial origin. 



This will he dealt with very briefly, since it must he 

 studied fully in connection with Pathology. 



The question may be most simply approached by consider- 

 ing first the probable mode of action of the toxin or poison of 

 snake venom and of that produced by the diphtheria bacillus, 

 and the way in which protection against these is established 

 by the development of antitoxins. 



1. Snake Venom and Diphtheria Toxin. — By injecting, under 

 the skin of the horse, increasing doses of such toxins the animal 

 is made quite resistant to the poison. A certain quantity of 

 its serum can then neutralise a definite quantity of the toxin, 

 so that, when the mixture of serum and toxin is injected into 

 another animal, the latter is uninjured. Something has been 



