26 



EPITHELIUM 



[CH. m. 



whatever may be the precise cause, the movement must depend upon 

 some changes going on in the cell to which the cilia are attached, as 

 when the latter are cut off from the cell the movement ceases, and 

 when severed so that a portion of the cilia are left attached to the 

 cell, the attached and not the severed portions continue the move- 

 ment. It has been suggested by Engelmann that the contractile part 

 of the protoplasm is only on the concave side of a curved cilium, and 

 that when this contracts the cilium is brought downwards ; when 

 relaxation occurs, the cilium rebounds by the elastic recoil of the 

 convex border. 



Schafer has suggested that the flow of hyaloplasm backwards and 

 forwards will explain ciliary as it will amoeboid movement. In an 

 amoeboid cell, the spongioplasm is irregular in arrangement, hence an 

 outflow of hyaloplasm from it can occur in any direction. But in 

 the curved projection called a cilium, the hyaloplasm can obviously 

 flow in only one direction into the cilium and back again. The flow 

 of hyaloplasm into the cilium will raise the pressure there and cause 

 it to straighten ; a movement in the reverse direction will cause the 

 cilium to curve. 



The action of dilute alkalis and acids on cilia is interesting. 

 Dilute acids stop ciliary motion ; and cilia, if allowed to act in salt 

 solution for a time, get more and more languid, and finally cease 

 acting; in popular language they become fatigued. Now we shall 

 find in muscle that fatigue is largely due to the accumulation of the 

 acid products of muscular activity ; remove the sarco-lactic acid and 

 fatigue passes off. It is probable that the same occurs in other 

 contractile tissues; the cilia gradually stop, due to acid products of 

 their activity collecting around them; when these are neutralised 

 with dilute alkali the cilia resume activity. 



Nutrition of Epithelium. 



Epithelium has no blood-vessels; 

 it is nourished by lymph. When the 

 blood is circulating through the thin- 

 walled small blood-vessels in the 

 tissues beneath the epithelium, some 

 of its fluid constituents escape. This 

 fluid is called lymph ; it penetrates to 

 FIG. 37.-ja g ged ceiis from the middle a u p^fa O f the cellular elements of 



tissues and nourishes them. In the 

 thicker varieties of epithelium, the 

 presence of the irregular minute channels between the cells (fig. 37) 

 enables the lymph to soak more readily between the cells than it 

 would otherwise be able to do. Epithelium is also destitute of 



vertical section of the gum of a new- 



born infant. (Klein.) 



