EPITHELIUM 



53 



middle of the cell, as on the left of Fig. 35, b, the bars are cut across and 

 appear as points; but if either the proximal or distal side of the cell is 

 included in the section, they appear as lines, as on the right of the figure. 

 Terminal bars have been found in many epithelia, especially in mucous 

 membranes and glands. They occur in the epididymis (Fig. 41, b) where 

 they appear as thickenings of the cell wall. According to Stohr they are 

 found in the stratified epithelium of the tongue and bladder. 



The intercellular substance in endothelium and mesothelium is ordi- 

 narily inconspicuous, but it may be demonstrated by treating the tissue 

 with a solution of silver nitrate. The resulting precipitate occurs chiefly 

 in the intercellular "cement substance," which then appears as a wavy 

 black line bounding each cell (Fig. 33). It is of importance since various 

 forms of blood corpuscles make their way through it from the vessels into 

 the surrounding tissue. 



In the lower layers of the epidermis and the thick oral epithelium, 

 the intercellular substance is clearly seen, and here it is bridged by spiny 

 processes from the adjacent cells. These inter- 

 cellular bridges occur in endothelium and many 

 forms of epithelium, but they are most readily 

 observed in the deep layers of the thick stratified 

 epithelia (Fig. 43). Within the bridges, fibrils pass 

 from cell to cell. In the intercellular spaces between 

 the spiny processes, nutrient fluid makes its way to 

 the outer layers. Whatever nutriment they receive 

 must be derived from the intercellular fluid or DE S . F THE EPI * 

 through the bodies of the underlying cells, since 

 neither blood vessels nor lymphatic vessels penetrate the epithelium. 

 This is probably true of all epithelia in man, but in the bladder and renal 

 pelvis the blood vessels approach very close and may appear to enter, 

 and in the amphibia, according to Maurer, capillaries may be observed 

 well within the oral epithelium. Nerve fibers extend among the basal 

 cells of the epidermis and other epithelia, and ramify in contact with 

 these cells, but special methods are required to demonstrate them. 



Basal surface. The basal cells of an epithelium sometimes seem to 

 send out processes which blend with the underlying connective tissue. 

 Usually, however, the lower surface is well defined, and the epithelium 

 is bound down by intercellular cement substance. Often, especially in 

 glands, the epithelium rests upon a thin, well-defined basement membrane 

 or membrana propria. This membrane is usually homogeneous and con- 

 tains very few nuclei. Sometimes it is composed of elastic tissue. Certain 

 basement membranes have been considered as derivatives of the epithe- 

 lium, but generally they are clearly of mesenchymal origin. 



