GLANDS. 



1537 



FIG. 1292. 



delicate varicosities. Arnstein l has described a special minute plate-like end-organ 

 as a widely occurring mode of nerve-ending in glands. W. Krause 2 has noted in 

 certain glands a form of end-capsule resembling a simplified Pacinian corpuscle. The 

 sympathetic fibres are distributed especially to the involuntary muscle of the blood- 

 vessels and the ducts, the peristaltic wave within the muscular coat of the latter facili- 

 tating emptying of the secretion. 



Development. Since glands are only extensions of the mucous membrane or 

 integument upon which they open, their development begins as an outgrowth or 

 budding from the epithelium covering such surfaces. In 

 the simple tubular glands the minute cylinders are closely 

 placed and composed of densely packed cells. In the 

 case of the larger compound glands, as the salivary or 

 pancreas, the first anlage consists of a solid cylindrical 

 plug which, penetrating into the mesoblast, soon begins 

 to branch. The ends of the terminal divisions enlarge 

 and eventually become the alveoli. Meanwhile the sur- 

 rounding mesoblast undergoes condensation and forms 

 the interlobular and other septa, as well as the general 

 envelope, or capsule, thereby giving definite form to the 

 general glandular aggregation. The vascular and other 

 structures usually found within the interparenchymatous 

 tissue are secondary and later formations. The develop- 

 ment of the gland involves a double process of active 

 growth, not only the extension of the epithelial pro- 

 cesses, but also a coincident invasion and subdivision of 

 the latter by the mesoblast to form the constituent units 

 of the organ. The lumen of the gland appears first in 

 the main excretory duct, from which it extends into the 

 secondary tubes and, finally, into the alveoli. Growth, 

 separation, and more regular arrangement of the cells 

 composing the epithelial cylinders are the chief factors in producing the lumen. In 

 the early condition of the glands, before the assumption of functional activity, the 

 cells later constituting alveoli of the serous or mucous type are similar and without 

 histological distinction. Upon the establishment of their different roles, however, 

 the characteristics distinguishing the varieties of glands appear, the differences de- 

 pending upon physiological rather than upon inherent anatomical variation. 



1 Anatom. Anzeiger, Bd. x., 1895. 



2 Zeitschrift f. rational. Med., Bd. xxiii., 1865. 



Duct, 

 with 

 lumen 



Alveoli,' 

 still solid 



Section of foetal oral mucous 

 membrane, showing developing 

 tubo-alveolar gland. X 50. 



