STIMULI AND THEIR ACTIONS 389 



experiments, confirmed in essential the observations of Horvath 

 and Reinke by showing that regular vibrations are able to produce 

 not only an inhibition of growth, but under certain conditions 

 even complete death and granular disintegration of the 

 protoplasm. 



Further, Engelmann (79, 1) made the observation that the 

 motion of Diatomece and Oscillarice ceases upon shaking. But 

 here the question is undecided whether the standstill is to be 

 interpreted as a phenomenon of depression or the expression of 

 tetanic excitation, like the cessation of protoplasmic motion in 

 tetanised Amoebae. 



Finally, in the pressure-paralysis of nerves we have, at all events, 

 a real phenomenon of depression which is to be classed with those 

 produced by narcotics. This pressure-paralysis, which appears 

 when a nerve is compressed for a time but not too strongly, is 

 generally known as the " feeling of going to sleep " of the limbs. 

 Besides the subjective phenomena, the " going to sleep " expresses 

 itself in a diminution or complete interruption of the power of 

 conductivity of the compressed nerve, so that for some 

 time the muscles supplied by the latter cannot be stimulated 

 through it to contraction. A short time after the cessation 

 of the pressure the power of conduction is again established. 



This comprises approximately all the facts known regarding 

 depressing effects of mechanical stimuli. 



3. The Actions of Thermal Stimuli 



The employment of thermal stimulation allows far fewer 

 variations to be made than that of mechanical or even chemical 

 stimulation, for only a rise or a fall of temperature can 

 act as a stimulus upon living substance. In accordance with 

 the nature of the thermal stimulus rapid rhythmical variations of 

 temperature cannot be produced, since heat requires a long 

 time to be communicated to a body or to disappear from it. 

 Hence it is impossible to produce a thermal tetanus corresponding 

 to mechanical tetanus. Thermal stimulation is thus very simple, 

 and its effects are likewise simply shown. 



a. The Phenomena of Excitation 



Starting from the average temperature at which a cell normally 

 exists, which, therefore, represents the optimum of the vital 

 condition, it is found to be a general law that up to a certain point 

 excitation increases with increasing temperature. This holds good 

 for very different vital phenomena and for very different forms of 

 living substance. 



