522 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1912. 



the nervous system (Biedl, Asher, Elliott) ; whilst, as we have seen, 

 some of the functions of the nervous system are dependent upon 

 hormones. 



PROTECTIVE CHEMICAL MECHANISMS. 



Time will not permit me to refer in any but the briefest manner to 

 the protective mechanisms wliicli the cell aggregate has evolved for 

 its defense against disease, especially disease produced by parasitic 

 microorganisms. These, which with few exceptions are unicellular, 

 are without doubt the most formidable enemies which the multi- 

 cellular Metazoa, to which all the higher animal organisms belong, 

 have to contend against. To such microoi'ganisms are due inter alia 

 all diseases which are liable to become epidemic, such as anthrax and 

 rinderpest in cattle, distemper in dogs and cats, smallpox, scarlet 

 fever, measles, and sleeping sickness in man. The advances of 

 modern medicine have shown that the symptoms of these diseases — 

 the disturbances of nutrition, the temperature, the lassitude or 

 excitement, and other nervous disturbances — are the effects of 

 chemical poisons (toxins) produced by the microorganisms and acting 

 deleteriously upon the tissues of the body. The tissues, on the other 

 hand, endeavor to counteract these effects by producing other 

 chemical substances destructive to the microorganisms or antagonis- 

 tic to their action: these are known as antibodies. Sometimes the 

 protection takes the form of a subtle alteration in the living sub- 

 stance of the cells which renders them for a long time, or even per- 

 manently, insusceptible (immune) to the action of the poison. Some- 

 times certain cells of the body, such as the white corpuscles of the 

 blood, eat the invading microorganisms and destroy them bodily by 

 the action of chemical agents witliin their protoplasm. The result of 

 an illness thus depends upon the result of the struggle between these 

 opposing forces — the microorganisms on the one hand and the cells 

 of the body on the other — both of which fight with chemical weapons. 

 If the cells of the body do not succeed in destroymg the invading 

 organisms, it is certain that the invaders will m the long run destroy 

 them, for in this combat no quarter is given. Fortunately we have 

 been able, by the aid of animal experimentation, to acquire some 

 knowledge of the manner in which we are attacked by microorganisms 

 and of the methods which the cells of our body adopt to repel the 

 attack, and the knowledge is now extensively utilized to assist our 

 defense. For this purpose protective serums or antitoxins, which 

 have been formed in the blood of other animals, are employed to 

 supplement the action of those which our own cells produce. It is 

 not too much to assert that the knowledge of the parasitic origin of 

 so many diseases and of the chemical agents which on the one hand 

 cause, and on the other combat, their symptoms, has transformed 

 medicine from a mere art practiced empirically into a real science 



