32 NORMAL HISTOLOGY. 



The cells lining the ultimate divisions of glands are the modified 

 extensions of the epithelial investment of the adjacent mucous 

 membrane, of which they are the direct outgrowths. Glandular 

 epithelium varies in form from columnar (pancreas) to spherical 

 (parotid) and polyhedral (liver). The protoplasm of such cells is 

 generally more or less filled with particles of secretion, upon whose 

 quantity and arrangement the apparent condition of the protoplasm 

 largely depends. Sometimes the latter is almost entirely displaced 

 by fatty matters, as in the sebaceous glands or in the active mammary 

 acini, or, again, is so encroached upon by particles of secretion that a 

 reticulation of the protoplasm is very conspicuous. 



The elements lining parts of certain glands exhibit more or less stria- 

 tion, on account of which peculiarity such cells are known as rod-epi- 

 thelium ; examples of this are seen in the ducts of the salivary glands, 

 and in the irregular and, to a less evident degree, the convoluted 

 portions of the uriniferous tubules of the kidney. 

 The epithelial coverings of those areas towards 

 which the terminations of the nerves of special 

 sense are particularly directed undergo high 



FIG. 27. 



FIG. 26. 



FIG. 28. 



Glandular epithelium : Rod-epithelium : a, b, c, isolated epithelial cells from uriniferous 

 small acinus from a serous tubules of rat (after Heidenhain) ; d, rod-epithelium from submax- 

 racemose gland. illary duct of dog. (After Schiejfer decker.) 



specialization, resulting in the production of perceptive elements, 

 to which, as a group, the name neuro-epithelium has been 

 applied. The rod- and cone-cells of the retina, 

 the hair-cells of Corti's organ and other parts 

 of the membranous labyrinth, the olfactory cells 

 of the nasal fossae, and the taste-cells of the 

 taste-buds, are all familiar examples of such 

 specialized epithelium. In these elements two 

 parts are present an inner, containing the 

 nucleus, and corresponding to the usual proto- 

 plasm of the cells, and an outer, peripherally- 

 directed segment, which is highly specialized, 

 and not infrequently terminates in stiff, rigid, 

 hair-like processes. The outer segment re- 

 ceives the stimuli from external impressions, 

 while the inner, centrally-directed, segment stands 

 in close anatomical relation with the nerve-fibres. 



Isolated neuro-epithe- 

 lium from nose : o, olfac- 

 tory cells ; s, sustentacular 

 elements. 



