MULTICELLULAR ANIMALS 



243 



muscles on the back of the arm. It is perfectly clear that such an 

 arrangement of antagonistic muscles is the only practical one, for 

 in the absence of an antagonistic muscle a joint once flexed could 

 not be straightened again. 



Glands. Glands respond to nerve impulses by forming and ex- 

 pelling secretions. In a broad sense all living cells secrete substances, 

 but here we are confining our attention to those concerned in the 

 activities of specialized glands. Glandular secretions cannot well be 

 arranged in related groups, for they are most diverse, ranging from 



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Fig. 168. — Fully charged, partially discharued, and almost completely discharged 

 secretory cells in the pancreas of a toad. (After Child: Senescence and Rejuvenescence, 

 published by the University of Chicago Press.) 



such simple substances as hydrochloric acid to exceedingly complex 

 organic molecules. It is not )'et understood how a nerve impulse 

 sets in motion the physical and chemical processes of glandular 

 action. Secretory activity represents the expenditure of energy, for 

 active glands absorb more oxygen than do resting glands. It is 

 also known that electrical factors are involved, for an active gland 

 is electro-negative to adjacent tissues. The secretions are apparently 

 formed in connection with certain plastids in the cytoplasm; fur- 

 thermore, resting glands often contain stored-up materials that are 

 seen to disappear when the gland becomes active (Fig. 168). Some 

 glands secrete constantly; others are active only during stimulation. 

 The activity of the gland may be a response to a nerve impulse or 

 to some chemical activator. For example, the salivary glands become 



