SECRETING GLANDS 



185 



which are subject to alternate collapse and distention ; hence the 

 membranes are frequently much folded. The deeper portion of 

 the mucous membrane usually 

 contains a more or less well- 

 defined layer of smooth muscle 

 fibres, the muscularis mucosce. 



The mucous membranes, 

 as their name indicates, are 

 nearly all moistened by a 

 mucus containing secretion. 

 The relative amount of mucus 

 which its secretion contains, 

 and consequently the viscidity 

 of the secretion, bears a close 

 physiologic relation to the in- 

 tensity of the mechanical irri- 

 tation to which the membrane 

 is subjected. Thus the mucus 

 secreting goblet cells of the 

 gastro-pneumonic membrane 

 are here and there reinforced 

 by numerous mucus secreting 

 glands of considerable size ; 

 these are especially abundant 

 in the oral cavity, pharynx, 



and esophagus, and in the nose, trachea, and bronchi ; in the 

 urinary system even the goblet cells are absent. 



The basement membranes, upon which the epithelium of the 

 mucous membranes and the secreting glands is supported, are con- 

 nective tissue structures. They are sometimes formed by inter- 

 lacing bundles of delicate white fibres intermingled with numerous 

 elastic fibres. Frequently, however, they consist of reticular tissue. 

 Basement membranes of this nature have been demonstrated by Mall 

 and his pupils in the mucous membranes and glandular tubules of the 

 stomach, intestine, liver, salivary glands, kidney, testis, and thyroid. 

 Occasionally basement membranes are homogeneous or hyaline in 

 structure and present a more or less clear or glassy appearance. 



SECRETING GLANDS. The secreting glands may be quite 

 properly considered as invaginations of the epithelial surfaces of 

 the mucous membranes. They appear as such in the embryo. 

 Their earliest anlage is formed by a solid or funnel-shaped process 



a y 



FIG. 170. DIAGRAM OF A MUCOUS MEMBRANE 

 HAVING SIMPLE TUBULAR GLANDS. 



a, artery ; 6-6, basement membrane ; C, 

 connective tissue ; d, duct of the gland, lined 

 by cuboidal cells ; J, epithelium of the free 

 surface, clear, columnar cells ; (?, lumen of the 

 fundus of the gland, lined by granular, serous 

 secreting, columnar cells surrounded by se- 

 cretory capillaries; V, veins. The arteries 

 are striped, the capillaries and veins, black. 

 Nerves are not represented. 



