66 PRACTICAL HISTOLOGY. 



gradually in contact with the slowly-moving cilia. 

 The action is immediate ; the cilia revive and vigor- 

 ously lash the liquid into which they project, but 

 the effort is soon exhausted, for the alkaline liquid 

 penetrating the cells destroys their vitality, and the 

 motion of their cilia stops beyond recovery. The 

 stimulant action is not, however, peculiar to weak 

 alkalies, for it is exhibited also by acids and by 

 many other substances which, applied concentrated, 

 would instantly destroy the tissue, but when much 

 diluted tend to revive and for a time maintain accele- 

 rated the ciliary motion. 



Preparation 11. Action of Carbonic Acid 

 Gas. This reagent is to be applied to a cilia pre- 

 paration in the gas-chamber in the manner directed 

 for the investigation of its action upon the newt's 

 blood-corpuscles (p. 48). Everything being ready, 

 choose a part of the preparation where the cilia are 

 not acting very vigorously, and whilst still watching 

 allow the gas to pass over the preparation. Its im- 

 mediate action is seen to be that of a weak acid 

 that is to say, the rate of movement, if not already 

 at its fastest, becomes accelerated but as soon as 

 the oxygen of the air in the chamber is entirely dis- 

 placed by the continued stream of carbonic acid the 

 motion ceases altogether. As soon as this result is 

 obtained cut off the stream of C0 2 and reverse the 

 experiment by blowing air in at the side-tube, and 

 thus displacing the carbonic acid from the chamber. 

 The motion will almost instantly recommence. This 

 shows that it was the absence of oxygen and not the 

 presence of C0 2 which produced the stoppage of 

 movement ; for there is, of course, an appreciable 

 quantity of carbonic acid in the air which is thus 

 blown from the mouth into the chamber. 



Preparation 12. Chloroform. The gas-cham- 

 ber is again used for this reagent, the apparatus 

 being arranged in the way previously recommended 

 (p. 39). Choosing a part of the preparation where 

 the ciliary motion is vigorous, gently blow a stream 



