50 HISTOLOGY 



lium" as if it were intermediate between the simple and stratified forms. 

 When the bladder contracts the cells are heaped up in several layers, but 

 when distended the number may be reduced even to two. 



If the cell walls are indistinct and the sections are thick or oblique, 

 the number of layers in an epithelium may be very difficult to determine. 

 Thus in a simple epithelium the nuclei may be at different levels (Fig. 

 35, B), and if the section is not vertical it will show several layers, ap- 

 proaching the condition of the tangential section, Fig. 35, A. Fig. 38 

 represents a vertical section of an epithelium with nuclei at three levels, 

 and in two forms (the basal nuclei being round and the others elongated) ; 

 but yet, as interpreted in Fig. 39, it is not stratified. It is of the form 

 known as pseudo-stratified, in which all the cells reach the underlying 

 connective tissue, but only a limited number extend to the free surface. 

 Pseudo-stratified epithelium occurs in the upper part of the respiratory 

 tract, including the trachea and larger bronchi, and in the epididymis. 



PERIPHERAL DIFFERENTIATION OF EPITHELIAL CELLS. 



Free surface. The free surface of epithelial cells is often provided 

 with a thickened top-plate or cuticula. Under high magnification the 

 cuticular border of the columnar cells in the intestine is seen to be vertically 

 striated (Fig. 35, B), and these striations have been interpreted as minute 

 canals through which protoplasmic processes may be sent out beyond 

 the free surface. In some cases, however, the striated cuticula appears 

 to consist merely of short, parallel protoplasmic rods. In certain cells 

 of the kidney, the rods may become somewhat divergent, giving rise to 

 what is known as the "brush border." Longer processes, which are 

 vibratile but not retractile, are called cilia (the Latin term for eye- 

 lashes). They project from the free surface of certain epithelial cells 

 in the trachea and bronchi (Figs. 38 and 39), in the uterus and uterine 

 tube, in the efferent ducts of the testis, and in the nasal part of the pharynx 

 together with the auditory tube and naso-lachrymal duct which open 

 into it. In the living condition the motion of cilia may be observed in 

 various unicellular animals. It may be studied advantageously in 

 fragments from the margin of the gills of a clam, or in epithelium from 

 the roof of the mouth of a frog. The cilia are numerous, and in the snail 

 Heidenhain counted no arising from a single cell. They do not act 

 together, but rapidly succeeding waves, due to the bending of the cilia, 

 pass over the entire surface. By bending sharply downward, each 

 cilium creates a forward current in the overlying fluid, and passes the 

 particles above it to the cilium in front. No sooner does a cilium begin 

 to bend than the next in front takes up the movement and thus the ciliary 

 waves are propagated. In some animals, however, the wave proceeds 



