Responses of Higher Animals: The Effectors - 441 



into ducts, and usually the active secretory 

 cells are localized in bulbous chambers, 

 called acini, at the blind ends of the finer 

 ducts. Raw materials used by the gland cells 

 in forming their special secretion are brought 

 to the gland by the blood, which circulates 

 through the capillaries, in close association 

 with each acinus (Fig. 24-9). 



Certain glands, such as the sweat and tear 

 glands, synthesize no special substances, but 

 merely extract the components of their juices 

 from the blood. Such glands expend energy 

 in selecting and concentrating their products, 

 since tears and sweat, compared to blood, 

 contain more salts and scarcely any organic 

 substances, except for traces of urea. But most 

 glands also perform the synthesis of special- 

 ized secretory products, for example, the 

 enzymes of the digestive glands and the hor- 

 mones of the endocrine glands. 



Some glands, especially in the endocrine 

 system, appear to secrete continuously, al- 

 though this activity is subject to change by 

 excitations from the nervous system, or by 

 the action of hormones. But many glands 

 discharge their products only in response to 

 periodic stimulation, in which case each ex- 

 citation is accompanied by a well-defined 

 action potential. 



Precisely how a gland cell discharges its 

 secretion into its duct is not well understood. 

 Secretory granules (p. 31), when present, 



RESTING 



•* J- 4> 



EXCITED 



Fig. 24-10. How secretion may occur. An increase of 

 osmotic solute, together with increased permeability 

 (or complete breakdown) of the membrane on the duct 

 side of the cell, causes a flow of water from the lymph 

 space through the cell, carrying its secretion out into 

 the duct. 



may disintegrate just prior to their discharge 

 (Fig. 24-10), or such granules may be swept 

 bodily into the duct through the end of the 

 cell bordering on the lumen. Probably the 

 membrane at this end of the cell momentarily 

 disintegrates, either partially or totally, al- 

 lowing for the escape of synthesized products 

 that do not ordinarily penetrate the mem- 

 brane. Glandular activity requires an ex- 

 penditure of energy, and the rate of respira- 

 tion in some of the glands is higher than in 

 any of the other tissues of the body. 



ELECTRIC ORGANS 



Galvanoeffectors are used as weapons of 

 attack and defense by several kinds of fish, 

 including the "electric eel" and the "stinging 

 ray" (Fig. 24-11). Typically the electric organ 

 is built up of disc-shaped cells that are 

 stacked like coins in elongate columns. Each 

 of the effector cells responds to excitation 



Fig. 24-11. Electric or stinging ray, Torpedo occ;- 

 dentalis. (Courtesy of the American Museum of Natu- 

 ral History, New York.) 



