92 THE TISSUES. 



The secretory epithelium of the various types of glands rests 

 upon a thin membrane (membrana propria), which has, according to 

 some authors, a connective-tissue origin, while, according to others, 

 it is the product of the glandular cells themselves. In some cases 

 it appears structureless, in others a cellular structure can be distin- 

 guished ; in the latter case the cells are flattened, with very much 

 flattened nuclei, and show irregular outlines. Macroscopically, 

 compound glands present a more or less lobular structure, 

 the separate lobules being held together by connective tissue. 

 In the immediate neighborhood of the gland and its larger lobes, 

 the connective tissue is thickened to form the so-called tunica 

 albuginea or capsule. In this fibrous-tissue sheath are found numer- 

 ous blood-vessels which penetrate between the lobes and lobules of 

 the gland and form a dense capillary network about the- tubules and 

 alveoli immediately beneath the membrana propria. Nerve-fibers 

 are also plentiful. 



Remarks on the Process of Secretion. The gland-cell 

 varies in its microscopic appearance according to its functional con- 

 dition. In the great majority of the glandular epithelial cells the 

 essential constituents of the secretion are stored in the cell in 

 the form of secretory granules, in others in vacuoles which are 

 filled with the secretion. The secretory process varies. In one 

 case the cell remains intact throughout the process (salivary glands) ; 

 in another a portion of each cell is used up in the production of 

 the secretion, only the basal portion containing the nucleus being 

 preserved. When this occurs, the upper part of the cell is recon- 

 structed from the remaining basal portion, and the cell is ready to 

 renew the process (mammary glands). In a third type the whole 

 cell is destroyed, and is replaced by an entirely new cell (sebaceous 

 glands). 



4. NEURO-EPITHELIUM. 



In certain of the organs of special sense (inner ear and taste-buds) 

 the epithelial cells about which the nerves terminate undergo a high 

 degree of specialization. This differentiation is more apparent in the 

 outer portions of these cells, resulting in the formation of one or sev- 

 eral stiff, hair-like processes, which appear especially receptive to 

 stimuli. Such cells are known as neuro-epithelial cells. In the 

 epithelia in which they occur they are surrounded by supporting 

 or sustentacular cells. 



5. MESOTHELIUM AND ENDOTHELIUM. 



The pleural, pericardial, and peritoneal cavities are lined by 

 a single layer of flattened epithelioid cells which develop from the 

 mesothelium lining the primitive body cavity (celom). For this 

 reason, as has been suggested by Minot (90), the term mesothelium 

 may with propriety be applied to this layer in its developed condi- 



