( 104 ) 
quantities. The latter fact appears very clearly from the experi- 
ments of ZWAARDEMAKER and LANs |). 
The existence of a threshold has as a second necessary consequence 
the existence of a latent period, for as long as the increase of the 
stimulus takes place with finite velocity, a definite interval of time 
will always elapse, before the stimulus has reached the value requisite 
to exceed the threshold value. 
When in a system with passive resistances a cyclical process takes 
place, this process is quite or partly irreversible. In this case the 
variation of the entropy of the system is greater than zero and the 
value of this variation is a measure of the irreversibility *). 
If we apply this to the isolated reflex-apparatus, the cyclical 
metabolic process must be quite or partly irreversible in consequence 
of the passive resistances in the chemical system. This is in perfect 
concordance with what we know of metabolism. If the reflex- 
apparatus has, therefore, passed through a cyclical process, the 
entropy of the system must have increased..As soon as the thermal 
equilibrium is reestablished in the reflex-apparatus, the temperature 
of the system has increased. Probably a similar consideration applies 
also to the pressure, so that at the end of the process, when 
the mechanical equilibrium has been reestablished in the reflex- 
apparatus, the pressure in the system will have risen. 
When we inguire what influence this will have on the two 
chemical systems which we suppose as being in equilibrium, we 
find that in general increasing temperature favours the system which 
is formed under absorption of heat *). Now it is beyond doubt that 
in the formation of the system of the products of assimilation the 
endothermic reactions are preponderant. Increase of pressure favours 
the system with the smallest volume“), and it is very probable 
that this will be again the system of the products of assimilation. 
If therefore an isolated reflex-apparatus passes through a cyclical 
metobolic process, this is not complete, while the change that sets in 
‘is such, that the equilibrium is displaced to the side of the system of 
the products of assimilation. If on such a changed system a second 
stimulus acts of the same extent, then the displacement of the equili- 
brium and together with it the external measurable effect must be greater. 
1) ZWAARDEMAKER, Separatabdruck. Sitz.ber. IX. Intern. Opth. Congr. Utrecht 1899. 
2) Ciausius lc. p. 223. 
3) Van ’r Horr. |. c. p. 158, 
4) Van T Horr. lj c‚_ps1b6, 
