42 IRRITABILITY 



demonstrated 1 by means of the infusoria Colpidium colpoda, 

 which can be grown without difficulty in a hay infusion. A 

 number of individuals in a drop of fluid may be placed in a 

 warm stage and observed under the microscope; one then sees 

 that at room temperature they swim about by moving their ciliary 

 processes at a definite rate. Now if the temperature is raised to 

 about 35 C, the ciliary movement becomes enormously increased. 

 The infusoria swim madly through the field of vision. They are 

 in a state of violent excitement. The increase has, therefore, 

 acted as a strong, exciting stimulus. But if one allows the tem- 

 perature to further increase only a few degrees the ciliary move- 

 ments are suddenly greatly retarded. The infusoria now swim 

 sluggishly through the field of vision and finally remain station- 

 ary. In this case the increase in the temperature has had a depress- 

 ing effect. If the infusoria are not quickly removed, the depression 

 is followed by death. Should the increase in temperature be 

 regarded in the first instance as a stimulus, and not as such in the 

 second, in which the temperature rises only a few degrees higher ? 

 Here the change in the vital conditions concerned is in both 

 instances positive. In all cases of overstimulation we are con- 

 fronted by the same question. Nevertheless it is not at all neces- 

 sary to refer to such strong or even life-endangering stimuli for 

 the observation of these conditions. In this connection I would 

 like to cite an even more striking instance and which is of special 

 interest for the understanding of the phenomena in nerve centers. 

 If the posterior spinal roots of a Rana temporara are severed, and 

 the eighth root stimulated with a faradic current, whilst the mus- 

 culus Gastrocnemius of the same side is connected with a writing 

 lever, one obtains, as Vesz'f has found, at the moment of the 

 beginning of stimulation a contraction of the muscle. The faradic 

 stimulus has, therefore, produced an excitation reflexly. If instead 

 of the eighth the ninth posterior root is stimulated, the result 

 obtained is also an excitation of the muscle. In this case, how- 

 ever, the excitation in the form of a tetanic contraction lasts for 



1 Max Verworn: "Physiologisches Prakticum fur Medizinen." Jena 1907. 



2 Julius Veszi: "Der einfachste Reflexbogen im Riickenmark." In Zeitschrift f. 

 allgemeine Physiologic Bd. XI, 1910. 



