40 HISTOLOGY. 



These fibres, basal granules, and cilia are joined witb one 

 another in a continuous whole. 



The theories concerning the function of this fibrillar struct- 

 ure are vague and unsatisfactory. According to one view, the 

 nucleus controls the activity of the cilia by means of the fibres 

 extending from it to the surface. This is not tenable, because 

 parts of the cell containing no nucleus still retain their power 

 of ciliary movement for a considerable time. Other authors 

 consider these fibres to be intracellular nerve-endings. Still 

 others ascribe to the basal granules the power of causing the 

 ciliary movement. . It is probable that, in common with the 

 protoplasmic network of every cell, the fibrillar structure pos- 

 sesses the power of contracting. This contraction would take 

 place mainly in the direction of the strongest fibrils, as it does 

 in muscular tissue. And their action upon the cilia, might be 

 compared with the action of the muscles which move hairs in 

 the skin. 



The cuticular border plainly seen in the intestinal epithelium 

 is a product o v f the cells. The striation is, according to the 

 researches of R. Heidenhain, due to the entrance of fine proc- 

 esses of the cell body into the homogeneous cuticle, and a 

 consequent change in the refractive index of different parts of 

 this mass. These processes may be drawn back into the cell, 

 and in such an instance the striation disappears (see Intestines). 



During activity the glandular epithelium shows on its free 

 surface a layer of fine rods, such as is seen in the convoluted 

 tubules of the kidney. This may occur in cylindrical as well 

 as cubical epithelial cells. There may also be often a longitu- 

 dinal striation at the basal end, which extends more or less into 

 the cell body. These two kinds of differentiation will be spoken 

 of more fully in treating of the salivary glands. 



According to the arrangement of the cells in epithelial tissue 

 \\c have : (a) simple epithelium i. e., consisting of only one 

 layer; find (/>) stratified epithelium, consisting of many layers. 

 This division, together with the form of the cells, gives rise to 

 the following classification : 



(a) Simple (one layer) epithelium : 



