14 IRRITABILITY 



of the organism, and a physiological explanation of this important 

 group of activities of the central nervous system. This inhibitory 

 effect of stimulation, brought about by the involvement of the 

 central nervous system in the normal organism, was studied side 

 by side with the depressing effects of stimulation. Claude Ber- 

 nard (1813-1878)^ first discovered that the excitation of all living 

 substance could be depressed or totally suspended through the 

 influence of certain anaesthetics, such as ether or chloroform. 

 By a series of experiments, as simple as they were convincing, 

 the French scientist showed that irritability could be depressed 

 in mimosa leaves, the growth of germinating plant seeds and the 

 ferment action of yeast cells stopped, likewise the disintegration 

 of the carbon dioxide in the cells of the green leaf, as well as the 

 development of the egg cells, and also the movements of the 

 animal organism and the sensations of man. By this means he 

 recognized that not only does all living protoplasm possess irri- 

 tability, but that it can also by means of certain substances be put 

 into the condition of "anaesthesia," a state dependent upon a 

 change of the protoplasm, which he termed "semi-coagulation." 

 Finally, besides the more apparent processes of excitation and 

 those less so, belonging to the group of inhibition and depression, 

 in the last century the knowledge of the subject was greatly in- 

 creased by the addition of another group, which recently in con- 

 sequence of various reasons has met with particular interest. These 

 being effects of stimuli on the direction of movements of motile 

 organisms, it became more and more recognized that these curious 

 manifestations of irritability, which appeared to have such a sur- 

 prising likeness to the mysterious attraction and repulsion in the 

 sphere of electricity and magnetism, occur universally in the vege- 

 table as well as in the animal world. These movements are of 

 the greatest biological importance for the obtaining of food, 

 propagation, protection against disease, etc. Botanists have long 

 known of the geotaxis of the roots and stems of plants, the 

 heliotaxis of their leaves and flowers and of the thigmotaxis 

 of their tendrils. Likewise the phototaxis of freely moving 



1 Claude Bernard: "Lecons sur les phenomenes de la vie communs aux animaux et 

 aux vegetaux." Paris 1878. 



