LUBRICATING TISSUES 



397 



sec. c. \ 



str. 



b.m.'; 



FIG. 360. Part of the epithelium 

 that lines the conjunctiva of an 

 alligator, b.m., basement mem- 

 brane; str.ep., stratified epi- 

 thelium; sec.c., distal layer of 

 secreting cells. X 880. 



cells as in the surface layer of most stratified epithelia, but of a layer of 



columnar secreting cells of great activity. 

 These columnar cells give the epithelium 



its name "pseudostratified" and, besides 



their long bodies packed with the secretion 



product, they are distinguished by having a 



slightly smaller and more chromatic nucleus 



which lies in the proximal end of the cell. 



The secretion of these cells is slowly dis- 

 charged from the distal surface, and used as 



a lubricant for the eyelid. 



It will be observed upon examining this 



epithelium that the secreting cells, unlike 



those of nearly every other kind of gland, do 



not lie in close contact with a blood supply. 



They must, therefore, receive their food sup- 

 ply through the efforts of the cells that lie 



between them and the blood. This involves 



unnecessary labor and is rarely seen, the proximal surface of all such 



cells usually lying directly against the thin wall of an arterial blood 



supply. 



Another form of this kind of lubricating tissue is found in the tear 



gland of the mouse. The 

 secreting epithelium is sim- 

 ple in this case and has 

 been invaginated into a 

 large, compound, saccular 

 gland, a portion of whose 

 secreting epithelium is rep- 

 resented by Figure 361. 



At first sight these cells 

 seem to correspond with 

 those of the sebaceous tissue 

 seen in the chicken's rump 

 glands, both as to character 

 of secretion and method of 

 producing it by the sacrifice 

 of a succession of cells. The 

 latter is easily disproved by 

 noting that the cells of the 

 tear gland form a single 



layer, and that there is no evident means of renewal. The idea that 



the method of production is much the same, however, holds true in 



FIG. 361. Part of a section through an acinus of the 

 tear gland of a mouse. The cells show a vacuolated 

 cytoplasm which discharges its secretion into the 

 lumen. X 800. 



