28 THE ESSENTIALS OF HISTOLOGY 



end of the tube, and make a deep notch in the ring opposite the tube. Place 

 a small drop of water within the ring (fig. 28). 



Put a bar from the gill upon a cover-glass in the least possible quantity of 

 sea- water ; invert the cover-glass over the putty ring, and press it gently 

 down. The preparation hangs in a moist chamber within which it can be 

 studied through the cover-glass, and into which gases or vapours can be passed 

 and their effects observed. 



Pass CO., through the chamber, and after observing the effect replace it by 

 air (see fig. 29). Repeat with chloroform vapour instead of CO 2 . 



The Movement of Cilia. When in motion a cilium is bent quickly 

 over in one direction with a lashing whip-like movement, immediately 

 recovering itself. When vigorous the action is so rapid, and the 

 rhythm so frequent, that it is impossible to follow the motion with the 

 eye. All the cilia upon a ciliated surface are not in action at the same 



FIG. 29. METHOD OF SUBJECTING A PREPARATION TO A STREAM OF CARBONIC 



ANHYDRIDE. 



6, bottle containing marble and hydroch'oric acid ; b' wash -bottle, connected by indiarubber 

 tube, t, with the moist chamber, s. 



instant, but the movement travels in waves over the surface. If a cell 

 is detached from the general surface, its cilia continue to act for a 

 while, but at once cease if they are detached from the cell. 



The rhythm is slowed by cold, quickened by warmth, but heat 

 beyond a certain point kills the cells. The movement will continue 

 for some time in water deprived of oxygen. Both C0 2 gas and chloro- 

 form vapour arrest the action, but it recommences on restoring air. 

 Dilute alkaline solutions quicken the activity of cilia, or may even 

 restore it shortly after it has ceased. 



