140 



NORMAL HISTOLOGY. 



upon which rests a single layer of irregularly spherical or polygonal 

 secreting cells ; these latter do not entirely fill the acinus, but leave 

 an intra-cellular cleft, in which the system of tubes for the conveyance 

 of the secretions commences. 



Glands are often divided into serous and mucous, a differentia- 

 tion depending upon the peculiarities of the cells lining the acini as 

 well as upon the character of their secretion. The cells of the serous 

 glands are distinguished by being distinctly granular, generally 

 spherical in form, readily and deeply stained with carmine, and by 

 having conspicuous nuclei situated near the centre of the cells ; the 

 elements of the mucous glands, on the contrary, are distended, 



FIG. 168. 



FIG. 167. 



Serous acini of human pa- 

 rotid gland ; the deeply-stain- 

 ing granular cells are sur- 

 rounded by the basement- 

 membrane. 



Mucous acini of human lingual gland : 

 the secreting cells (a), being loaded with 

 the slightly-staining secretion, appear clear 

 and transparent ; c, c, crescentic masses of 

 granular cells the demi-lunes of Heiden- 

 hain ; b, interacinous connective tissue. 



very clear and transparent, slightly stained with carmine, and have 

 the nuclei displaced to the outer edge of the cells, not infrequently 

 immediately beneath the basement-membrane. In the embryonal 

 pre-functionating condition these two kinds of glands are identical, 

 both as to mode of origin and histological characteristics ; the varia- 

 tions and the conspicuous differences subsequently appearing depend 

 on differences of physiological function and character of secretions, 

 and not on structural differences in the original cells. 



Fluids elaborated by the serous glands are thin and watery, appear- 

 ing within the protoplasm of the secreting cells as minute dark gran- 

 ules ; the general appearance of the cells depends upon the number 

 of these granules stored up within their protoplasm. When a serous 

 gland is in a condition of rest, the cells are loaded with granules, 

 and when stained they appear larger, clearer, and coarsely granular ; 

 after active secretion the cells are exhausted and contain fewer 

 granules, appearing, therefore, smaller, darker, and finely granular. 



