148 



NORMAL HISTOLOGY. 



The Parotid Gland. This, the largest of the salivary glands, lies 

 between the upper part of the ramus of the lower jaw, which it overlaps 

 both within and without, and the external ear. It is invested by a strong 

 fibrous sheath, continuous with the cervical fascia. The gland is subdivided 

 into many lobules by septa of dense fibre-elastic tissue and, hence, possesses 

 considerable toughness. The parotid consists entirely of serous alveoli, 

 although mucus-producing acini may occur in the accessory lobules along 

 the main (Stenson's) duct. The primary lobules are made up of alveoli, 

 lined with pyramidal glandular epithelium whose appearance changes with 

 functional activity. When at rest, the cells are filled with minute secretion- 

 granules; the latter, however, are sensitive to reagents, often undergoing 



Intermediate duct 



Tubular alveolus 



Alveolar lumen 



Interlobular duct 



Connective tissue 



FIG. 188. Section of parotid gland, showing serous alveoli. X 270. 



partial or complete solution. Hence, the reticulated appearance of the cyto- 

 plasm frequently observed in the gland-cells after fixation. The spherical 

 nuclei occupy the middle of the serous cells. 



The duct-system begins at the alveoli as the intermediate tubules, which 

 in the parotid are relatively long and narrow and lined with low or flattened 

 cells continuous with the alveolar epithelium. Secretion- canaliculi extend 

 between the gland-cells part way to the basement membrane. The intra- 

 lobular tubules, of larger diameter (35 M) than either the immediately pre- 

 ceding or succeeding segments of the duct, are lined with a single layer of 

 columnar cells that exhibit differentiation into an inner zone, finely granular 

 and containing the nucleus, and an outer or basal zone, next the basement 

 membrane and displaying a faint longitudinal striation or "rods." The 

 interlobular and interlobar ducts gradually increase in diameter and, for the 

 most part, possess a single layer of columnar cells; in the larger canals, how- 

 ever, this may be reinforced by an additional imperfect row of small cells. 

 The columnar cells extend to near the termination of the main excretory 

 duct, where they give place to the stratified squamous epithelium continued 

 into the canal from the oral mucous membrane,- The wall of the parotid or 

 Stensori 's duct, in addition to the epithelium, consists of fibrous tissue, mixed 

 with many elastic fibres and a few bundles of unstriped muscle. 



