396 



HISTOLOGY 



increase in size. When they have attained to the third row, however, 

 it can be seen that a number of vacuoles form in the cytoplasm and these 

 continue to rapidly grow as the cell moves outward, until at the eighth 



or tenth row, or layer, 

 the cell is many times 

 larger on account of 

 the increase in the 

 size of the vacuoles, 

 and the nucleus be- 

 gins to shrivel and to 

 lose its structure. 



The vacuoles are 

 filled with an oil; in 

 the two outer rows 

 the cell structure 

 completely breaks 

 up, leaving only the 

 oil containing some 

 remnants of the nu- 

 cleus and cytoplasm. 

 The lubricant has 

 been formed, but, 

 unlike the mucous 

 cell, the process can- 

 not be repeated by 

 the cell because it 

 has been destroyed 

 in the process. Sev- 

 eral other forms of 

 oil-secreting glands 

 are to be found 

 among the reptiles, 

 birds, and mammals, and some of these have such a secondary use that 

 they are treated of in the next part. 



The lubrication of the eye in such vertebrate animals as live in the 

 air is somewhat complex. Two types of gland tissue are employed in 

 this function besides the few true mucous cells that occur in the con- 

 junctiva. The first of these is a tissue that produces a peculiar oily fluid 

 and is represented, in a simple form, by the glandular epithelial surface 

 found on the conjunctiva of the alligator (Fig. 360). 



The epithelium is a thin, stratified form, and its layers are but two 

 or three in number in a young animal of eighteen inches in length. It 

 is the outer layer that is interesting because it is not composed of dead 



FIG. 359. A vertical section of a portion of the secreting epi- 

 thelium that lines the tubular oil glands of a chicken. Base- 

 ment membrane below, distal surface and lumen above. 



