. Elements of biology, with special reference to their rôle in the lives of animals. Biology; Zoology. 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.
. Elements of biology, with special reference to their rôle in the lives of animals. Biology; Zoology. 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. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Buchanan, James William, 1888-. New York, London, Harper & brothers
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