STIMULI AND THEIR ACTIONS 



371 



which it exists, or upon being transferred to fresh water. The 

 various luminous Bacteria, which produce, e.g., the luminosity of 

 dead sea-fish, behave similarly. 



Finally, the living substance of nerves and ganglion-cells can be 

 excited by chemical stimuli. The excitation in the nerve-sub- 

 stance itself is not visible 

 without special methods ; 

 but a clear expression of it 

 in motor nerves is exhibited 

 in the contraction of muscles 

 supplied by them. If, e.g., 

 the sciatic nerve of a frog 

 be stimulated by its central 

 end being dipped into gly- 

 cerine, a concentrated solu- 

 tion of common salt, or a 

 solution of a mineral acid, 

 an alkali, a metallic salt or 

 sugar, contractions of the 

 leg-muscles of the frog take 

 place, and prove that the 

 nerve is excited. Excita- 

 tion by chemical stimuli can 

 be observed in the excised 



, , f> . , FIG. 163. Noctiluca mihans, a marine flagellate- 



nerve also by means ot the infusorian ceil, 



galvanometer through the 



development of electricity, which influences the current derived 

 from the resting nerve. 



1). The Phenomena of Depression 



In contrast to the exciting effects of the chemical stimuli just 

 mentioned are the effects of certain chemical substances, which 

 depress or wholly suppress vital phenomena. These substances 

 are, hence, termed narcotics or anaesthetics. Among them belong es- 

 pecially those that depress all forms of living substance and 

 all vital phenomena : alcohol, ether, chloroform, and chloral 

 hydrate. With these belong the great group of alkaloids, 

 comprising morphine, quinine, veratrine, digitaline, strychnine, 

 curare, etc., some of which act upon a great variety of living cells, 

 while others affect specific cells only, especially those of the 

 central nervous system. 



The depressing effects of narcotics upon the phenomena of 

 metabolism have been studied especially by Claude Bernard (78). 

 This well-known Parisian physiologist showed that metabolism is 

 suppressed by chloroform-narcosis in very different forms of cells. 

 If yeast-cells, which, as is well known, in the course of their 



B B 2 



