470 GENERAL PHYSIOLOGY 



perty of these substances , that in very small doses or with very 

 brief administration they produce phenomena of excitation, while 

 with increasing action phenomena of depression become more 

 and more noticeable, and apparently are able to lead to a com- 

 plete standstill of life.^ This fact is well known in pharmacology. 

 Morphine in small doses and at the beginning of its action 

 produces always a stage of excitation, in which the patients are 

 restless and excited, are not able to sleep, and are haunted by all 

 sorts of illusions. But if the dose given be greater, and the 

 stage' of excitation appearing at the beginning of its action be 

 passed, deep sleep comes with total absence of motion and 

 sensation. The same result is seen also with other narcotics and 

 with single cells. In ciliate Infusoria the ciliary motion is 

 increased to furious rapidity under the influence of the vapour of 

 ether or chloroform in small quantity or with brief duration. 

 The excitation of the cilia is so great that the organisms shoot 

 through the water like arrows. But if the dose or the duration 

 of the influence of the narcotic become only slightly increased, 

 the motion becomes slower and slower until, finally, complete 

 paralysis results, and the cells remain motionless. The same 

 phenomena have been observed with the many different kinds of 

 anaesthetics, and in all sorts of living substance. 



Another example of the fact that with increasing intensity of 

 the stimulus excitation is first increased and then after a certain 

 point gives place to depression, is afforded by stimulation by 

 heat.^ With increasing temperature up to a certain degree, which 

 is very different for different forms of living substance and for 

 different vital phenomena in the same form, all vital phenomena 

 undergo an augmentation to a maximum. But if this degree be 

 overstepped, excitation decreases rapidly, and gives place to 

 complete paralysis in the form of heat-rigor. The fermentative 

 activity of yeast-cells, the growth and development of ova, and 

 the protoplasmic and ciliary motions of unicellular organisms, 

 afford distinct examples of this. Other varieties of stimuli 

 illustrate the same general principle. 



But this relation of excitation and depression holds good only 

 for those stimuli which consist in an increase of the factors that 

 under normal circumstances act upon the organism as vital con- 

 ditions, as, e.g., increase of the surrounding temperature, or those 

 which consist in an incoming of foreign factors, as, e.g., poison- 

 stimulations. Those stimuli, however, which depend upon the 

 diminution of vital conditions, as, e.g., decrease of the surround- 

 ing temperature, appear in general with increasing intensity to 

 depress vital phenomena without previous excitation. With the 

 present condition of our knowledge a law covering these facts 

 cannot be formulated with certainty, for a cautious critic requires 

 1 Cf. p. 879. ' Cf. p. 396. 



