THE RABBIT. 



49 



commonly either ciliated or secretory. The protoplasm of the 

 free surface of each cell of a ciliated epithelium is produced 

 out into one or (more usually) many delicate processes, called 

 cilia, which during life are constantly performing movements 

 of a very definite and regular kind. Each cilium becomes 

 bent and then straightens itself out again : the motion is 

 either simultaneous for all the cilia or takes place in 

 advancing succession, and is so rapid (under ordinary cir- 

 cumstances) that only the motion can be seen, and not the 

 cilia themselves. When the cilia are dying, or under other 

 exceptional conditions, the motion is slow and its exact 

 nature can be observed. The effect is that of a series of 

 oars being pulled as in rowing : if the boat is fixed they 

 will produce a current in the water ; if not they will make 

 the boat move. Similarly, the effect of the activity of ciliated 

 epithelium is normally to sweep fluid in a definite direction 

 along its surface, but an isolated ciliated cell will be whirled 

 round and round by the action of its own cilia. Cilia, as 

 we shall see, are of great importance among the lower 

 animals, though of only subordinate importance in the 

 rabbit. A cilium may be regarded as a very fine pseudo- 

 podium whose structure is such that it can perform only 

 one movement, but can perform that with a rapidity 

 unknown to the ordinary pseudopodium. Ciliated epi- 

 thelium is found in the 

 rabbit principally in the 

 trachea and bronchi. Other 

 cases will be mentioned in 

 later chapters. 



5. Secretory Epithe- 

 lium. While the function 

 of cilir.ted epithelium is 

 the doing of a particular 

 kind of mechanical work, 

 that of secretory epithelium is the production of some 

 definite substance, either as a result of its metabolism, or 

 by the separation of it from the blood-plasma. It is by 

 epithelia of this kind that all the digestive juices are 

 secreted, and water and urea removed from the blood. 



7 4 OOL. 4 



Fig. 



12.- SECRETORY COLUMNAR 

 EPITHELIUM. 



Section perpendicular to free surface 

 Three goblet-cells shown. 



