30 EPITHELIUM [CH. III. 



C. stop the movement, whereas moderate heat and dilute alkalis 

 are favourable to the action, and revive the movement after temporary 

 cessation. The exact explanation of ciliary movement is not known ; 

 whatever may be the exact 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 that the cilium is brought downwards ; 

 where 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 more hyaloplasm into the spongioplasm of the cilium will cause it 

 to curve, the flow of the hyaloplasm back into the body of the cell 

 will cause the cilium to straighten. 



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. 



Transitional Epithelium. 



This term has been applied to cells which are neither arranged 

 in a single layer, as is the case with simple epithelium, nor yet in 

 many superimposed strata, as in stratified epithelium; in other 

 words, it is employed when epithelial cells are found in two, three, or 

 four superimposed layers. 



The upper layer may be either columnar, ciliated, or squamous. 

 When the upper layer is columnar or ciliated the second layer con- 

 sists of smaller cells fitted into the inequalities of the cells above 

 them, as in the trachea (fig. 42). 



The epithelium which is met with lining the urinary bladder and 



