CILIARY ACTION. 



31 



on its inner face (Fig. 19). From this layer delicate threads of 

 protoplasm reach into and pass through the central cavity, where 

 they often branch and are connected together so as to form a 

 very loose network. The nucleus (w) is embedded either in the 

 peripheral layer or at some point in the network, and the threads 

 of the latter always converge more or less regularly to it. In 

 active cells currents continually flow to and fro throughout the 

 whole mass of protoplasm. In the threads of the network gran- 

 ules are borne rapidly along, gliding now in one direction, now 

 in another ; and although the flow is usually in one direction in 

 any particular thread, no system can be discovered in the com- 

 plicated movements of the whole. In the larger threads the 

 curious spectacle often appears of two rapid currents flowing in 

 opposite directions on opposite sides of the same thread. The 

 currents in the thread may be seen to join currents of the pe- 

 ripheral layer which flow here and there, but without sthe regu- 

 larity observed in the protoplasm of Nitella. The protoplasmic 

 network also, as a whole, undergoes a slow but steady change of 

 form, its delicate strands slowly 

 swaying hither and thither, while 

 the nucleus travels slowly from 

 point to point. 



Finally, we may consider an 

 example of a form of protoplas- 

 mic movement known as ciliary 

 action, which plays an important 

 role in our own lives and those 

 of lower animals and of some 

 plants. The interior of the tra- 

 chea, or windpipe, is lined by 

 cells having the form shown in 

 Fig. 20. At the free surface of 

 the cell (turned towards the cavi- 

 ty of the trachea) the protoplasm 

 is produced into delicate vibra- 

 tory filaments having a sickle- 

 shape when bent ; these are known as cilia (cilium, an eyelash). 

 They are so small and lash so vigorously as to be nearly or quite 

 invisible until the movements are in some way made sluggish. 



FIG. 20. (After Klein.) -Three isolated 

 ciliated cells from the interior of the 

 windpipe of the cat. c, the cilia at the 

 free end; n, the nucleus; p, the proto- 

 plasm. (Highly magnified.) 



