CH. III.] TRANSITIONAL EPITHELIUM. 33 



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 movement. The most probable cause of the movement is 

 that it is part of the inherent power which protoplasm possesses, 

 and that the cilia are but prolongations of the spongioplasrn 

 of the cell. It has been suggested by Engelmann that if this 

 be the case, 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 arrange- 

 ment, 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 accumu- 

 lation 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 



K.P. D 



