360 



HISTOLOGY 



The function of resisting the entrance of foreign and unnecessary fluids 

 is performed by two structures, the outer surface of the cell itself through 

 the physiological processes that take place in or near that surface, and 

 also by a series of terminal bars or " schlussleisten " found between the 

 edges of the outer surfaces of the cells (see Fig. 47). These structures are 

 rodlike and double, the two parts being closely applied and cemented to 

 each other. Each half is structurally a part of the lateral cell-wall, and 

 even when the lateral walls are separated one from the other, they re- 

 main connected by the closing-plates until the separating forces become 

 very much stronger. 



Any special structural device developed to perform the third protec- 

 tive function of resisting abrasion is not well shown in this example on 

 account of the presence of the cilia, which render it unnecessary and 

 impossible. Such a device usually appears as a thickened and hardened 

 portion of the distal end of the cell, forming a platelike structure on the 

 end of the cell. It may be seen to advantage in the digestive cells of 



the small intestine of vertebrates and 

 in other places as on the outer epithe- 

 lium of Amphioxus (Fig. 326). 



The next step in the development 

 of the three functions under discus- 

 sion, in a columnar form of epithelium, 

 is the formation by the cells of an 

 external and extracellular layer of 

 some substance that will perform the 

 functions better than the cells could 

 themselves do. Such an organ is the 

 cuticle, which is formed jointly by all 

 the cells as a continuous layer covering 

 the epithelium. In some cuticles, the portion formed by each single 

 cell can be distinguished from that formed by the surrounding cells, but 

 in most it cannot. 



The cuticle is laid down by the cells in layers in most cases, or ap- 

 parently as a single layer in some. It sometimes contains fibers, and in 

 many cases it bears various points, knobs, and other structures on its 

 surface. These structures may be the product of one cell or of many 

 cells. Various openings, usually of small size, serve as a means of exit 

 for the secretions of glands and the cilia and nerve-endings of other cells 

 as well as to permit odors to reach some olfactory cells. 



The cuticle is often strengthened by the addition of mineral salts 

 and the addition of foreign substances to the exterior. It is also renewed 

 in most forms either by small parts at a time (leech), or by a process of 

 entire shedding of the whole structure at once (lobster). In this case 



FIG. 326. Protective epithelial cells from 

 the body surface of Amphioxus. The 

 two kinds of cells represent two kinds of 

 fixation as well, probably, as different 

 physiological conditions. (First pair of 

 cells after SCHNEIDER.) x 1000. 



