Chap. VIII] SPECIAL MEMBRANES AND GLANDS 135 



The simplest form of a gland may consist of just one cell, such 

 as the goblet cells/ or may be a mere depression on the surface of 

 a membrane, or a complex organ like the liver. No matter 

 what the size or shape may be, all glands have three essential 

 characteristics: (1) epithelial cells which are the active secreting 

 agents, (2) a liberal blood supply from which the material for the 

 secretion is drawn, (3) they are under the direct control of the ner- 

 vous system and secretion is their response to stimulation, just as 

 contraction is the response of a muscle. The usual arrangement is 

 for the cells to cover the free surface of a basement membrane, 

 a dense network of capillaries to be spread upon its under surface, 

 and nerve fibrils to form a network in contact with the cells. 



Fig. 94. — Dl^gram showing De\t;lopment of Glands: A, a mere dimple in 

 the surface ; B, enlargement by division ; C, enlargement by dilatation ; D, a combi- 

 nation of B and C ; E, a. raeemosp gland ; F, development of method oi E ; G, & 

 single tube intricately coiled. (Gerrish.) 



In order to economize space and to provide a more extensive 

 secreting surface, the membrane is generally increased by dipping 

 down and forming variously shaped recesses. 



Classification. — The secreting glands are of three kinds : — 



1. Simple. 



2. Compound. 



3. Ductless. 



L Simple glands. — The simple glands are generally tubular or 

 saccular cavities, which open upon the surface by a single duct. 

 They are named simple tubular, or saccular glands. Sometimes 

 the tube is so long that it coils upon itself, as in the sweat glands 

 of the skin. These are named simple convoluted tubular glands. 



2. Compound glands. — In the compound glands the cavities 

 are subdivided into smaller tubular or saccular cavities, opening 



> See page 267. 



