38 THE EPITHELIAL TISSUES 



teristic mo^'ement. According to other investigators, cilia have a 

 contractile band along each of two opposite surfaces and the move- 

 ment is effected by the alternate contraction of these bands. In 

 either case the movement appears as a sharp initial bending and a 

 slow recovery. The motion of one row of cilia apparently initiates 

 similar activity in adjacent rows, so that a series of waves pass 

 along the field of cilia. Since the l)eat is in one direction, particles 

 caught on the tips of the cilia are usually propelled in the direction 

 of the beating. Ciliated epithelia lining the respiratory passages 

 tend to mo^'e dust particles and material in the lumen out into the 

 mouth or nasal sinuses. Ciliary action is independent of nerve 

 action, as may be demonstrated by removing the ciliated membrane 

 from the roof of the frog's mouth and studying it in a saline solution 

 hours after the frog itself is dead. Cilia possessing the power of 

 movement are called kinocilia, to distinguish them from static cilia 

 or stereocilia of such cells as those lining the epididymis. 



Another structural featiu-e associated with the free boimdary of 

 some epithelia is a cuticle. The superficial protoplasm of the cell 

 is not modified in any marked manner and is not continuous with 

 this modified liorder which is formed by the cell during development. 

 The cuticle may become impregnated with \'arioiis salts and be firm 

 and hard. The enamel of the teeth is an example of such a special- 

 ization. This type of modification plays a much wider role in the 

 invertebrates where cuticles are more complex and form the skeleton 

 of many forms. 



The boundaries of epithelial cells in contact with adjacent cells 

 are so delicate that they do not always show in preparations. 

 Some believe a cementing substance is present between adjacent 

 cells but it has also been indicated that the adherence of the cells 

 to each other is due to the adhesion of adjacent cell membranes. 

 Proto])lasmic bridges may extend from cell to cell. (Fig. 16.) 

 These ha^'e been interpreted by some as solid strands between 

 vacuoles in the intercellular spaces or as products of technique and 

 therefore artefacts. In silver nitrate technique silver salts are 

 deposited about the cells so that in face \'iew a network of poly- 

 hedral outlines define the cells as described in the case of mesothe- 

 lium. The cells composing an epithelial membrane are ca})able of 

 gliding by one another when the membrane is stretched. 



The basal surface of simple epithelia and the basal siu'face of 

 the deepest layer of the stratified types may possess a cuticle. In 

 addition, most epithelial cells rest upon a distinct basement mem- 



