22 THE GENERAL CHARACTERS OF CELLS, BY S. STRICKER. 



(a) Every variation in the intensity of the current, whether 

 positive or negative, providing it be sudden, acts as an 

 excitant. 



() A single variation in the intensity of the current induces 

 a series of alternate contractions and relaxations. 



(<?) A single excitation induces changes which may be 

 divided into three stages : That of latent excitation (which 

 with opening-induction shocks is scarcely perceptible, and is 

 hence generally longer the weaker the shock) ; secondly, the 

 stage of increasing energy (which also lasts longer in propor- 

 tion to the feebleness of the shock) ; and, lastly, the stage of 

 diminishing energy (which is so much the more rapid the 

 weaker the shock). 



(d) The closure of a constant current is a stronger stimulus 

 than its opening. 



(e) The direction in which the current traverses ciliated 

 cells appears to have no influence on the amount of irritation 

 exerted. 



(/) The movement may be retarded by the application of a 

 very strong electrical current, or may even be altogether 

 stopped, the cell at the same time being destroyed. The same 

 thing occurs on producing long-continued tetanisation with 

 strong alternate currents. 



d. NERVOUS EXCITATION. On this point only a single 

 direct observation by Kuhne can be adduced ; namely, that the 

 contraction of certain stellate cells in the cornea of the frog 

 may be induced by excitation of the corneal nerves. Briicke* 

 had long before furnished evidence in regard to this by refer- 

 ring to the very well-marked phenomena of contraction that 

 are visible in the pigment cells of the skin of the chameleon, 

 and which can readily be excited reflectorially by irritation of 

 the sensory nerves. 



e. CHEMICAL STIMULI. Amongst these we may first men- 

 tion the influence of water. Amoeboid movements can be 

 excited in the segmentation spheres of the egg of the frog by 



* Denkschriften die Wiener Akad., Band iv., p. 203. 



