244 



ELEMENTS OF BIOLOGY 



active in response to nerve stimuli set up by foods or by the sight 

 of foods; they may also be activated by chemicals, such as ether. 



According to their structure, glands may be unicellular or multi- 

 cellular (Fig. 169). Multicellular glands may be simple tubes with 

 walls composed of gland cells, or compound tubular arrangements, 

 or they may be simple sac-like or alveolar structures. Unicellular 

 glands are quite commonly distributed in the animal body, in the 



B D 



Fig. 169. — A, unicellular gland; B, C, and D, multicellular glands. B, simple 

 tubular. Compare with sudiferous gland, Fig. 94. C, simple bulb; compare with se- 

 baceous gland in Fig. 94; D, compound tubular; compare with Fig. 11. All are 

 diagrammatic. 



integument of many invertebrates, in the walls of the respiratory 

 and digestive systems of vertebrates, and elsewhere. 



According to the type of outlet, glands perform either internal or 

 external secretion. Glands of internal secretion are known as endo- 

 crine glands; their functions have been discussed elsewhere (p. 204). 

 Their products appear to pass into the appropriate blood capillaries 

 by osmosis through the cell walls. The glands of external secretion 

 (exocrine glands) pass their products to collecting ducts that lead 

 to the surface of the body, or to some organ that is in communica- 

 tion with the external; for example, the sweat glands lead to the 

 surface of the skin; the liver and pancreas deliver their products 



