428 GENERAL PHYSIOLOGY 



The gill-filaments of bivalve mussels are covered by a ciliated 

 epithelium, the cilia of which on account of their length are 

 especially well -fitted for the observation of ciliary motion. When 

 Engelmann stimulated these filaments by means of a single, strong 

 induction-shock, the cilia went into rigor, exactly as the cilia of 

 Infusoria and ciliated epithelia go into heat-rigor after strong 

 thermal stimulation. They bent into the form of a hook in the 

 direction of the stroke, their motion ceased and they remained 

 in this position the longer, the stronger the induction-shock had 

 been. 



Kraft made an analogous observation during the long-continued 

 action of the constant current upon ciliated epithelia of vertebrates. 

 At the beginning of the action the ciliary stroke was accelerated, 

 first at the two poles, but then, by a spreading of the excitation in 

 the tissue, in the whole intrapolar portion ; with long duration of 

 the current the acceleration decreased gradually and gave place to 

 3, depression of the activity, amounting to complete standstill in 

 the whole intrapolar portion. Hence it appears that the same 

 relation is present here as in other, e.g., chemical, depressions, viz., 

 the stimulus in question calls forth first a stage of excitation and 

 then with stronger or longer action a depression. But all these 

 relations have been too little investigated to permit a definitive 

 interpretation. 



B. THE DIRECTIVE EFFECTS OF UNILATERAL STIMULATION 



Among the physical phenomena that are employed for the amuse- 

 ment of children in civilised countries, those of magnetism usually 

 have great attraction for the childish mind. The remarkable facts, 

 that the magnetic needle, freely suspended, under all circum- 

 stances directs one end toward the north pole of the earth, that 

 small boats and animals provided with an iron pin and swimming 

 in a basin of water follow the slightest motions of the magnetic 

 needle with unfailing certainty as if conjured by a magician, that 

 iron filings, strewn upon paper, arrange themselves in very 

 characteristic curves over a magnet placed below them all this 

 has greatly fascinated us as children. Magnetic phenomena must 

 have made the same deep impression upon the fervid fancy of the 

 people of the Orient, who in many respects have retained childish 

 qualities even to the present time. In the tales of the Arabian 

 Nights there is a vivid expression of this, which still takes strong- 

 hold upon the childish heart, in the gloomy stories of the magnetic 

 mountain and the fright of the helpless mariners who saw their 

 ship, attracted by the invisible force, crash upon the smooth 

 metallic rocks. 



In the adult the sense of wonder and fascination connected with 

 the magnet is largely lost because of our being accustomed to its 



