CTLIARY ACTION OF PROTOPLASM. 



31 



opposite directions on opposite sides of the same thread. The cur- 

 rents in the thread may be seen to join currents of the peripheral 

 layer which flow here and there, but without the regularity 

 observed in the protoplasm of Nitella. The protoplasmic net- 

 work, 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. 19. At the free surface of 

 the cell (turned towards the cavi- 

 ty of the trachea) the protoplasm FIG. 19. (After Kleini. Three isolated cili- 

 , . -, -i . . , ated cells from the interior of the wind- 



1S produced llltO delicate Vibrato- pjpe of the cat . c the cilia at the free end; 



rv filaments having a, sickle-shape - the nucleus ; P , the protoplasm. (High- 



ly magnified). 



when bent ; these are known as 



cilia (cilium, an eyelash). They are so small and lash so vigor- 



* The similar circulation which may be seen in a nettle-hair has been de- 

 scribed by Huxley, (Lay Sermons, etc., p. 123, American cd.): "The com- 

 mon nettle owes its stinging property to the innumerable stiff and needle-like, 

 though exquisitely delicate, hairs which cover its surface. Each stinging- 

 needle tapers from a broad base to a slender summit, which though rounded at 

 the end is of such microscopic fineness that it readily penetrates and breaks off 

 in the skin. The whole hair consists of a very delicate outer case o f wood, 

 closely applied to the inner surface of which is a layer of semi-fluid matter, full 

 of innumerable granules of extreme minuteness. This semi-fluid lining is 

 protoplasm, which thus constitutes a kind of bag, full of a limpid liquid, and 

 roughly corresponding in form with the interior of the hair which it fills. 

 When viewed with a sufficiently high magnifying power, the protoplasmic 

 layer of the nettle hair is seen to be in a condition of unceasing activity. Local 

 contractions of the whole thickness of its substance pass slowly and gradually 

 from point to point, and give rise to the appearance of progressive waves, just 

 as the bending of successive stalks of corn by a breeze produces the apparent 

 billows of a corn-field." 



