§4.31 CHARACTERISTICS OF KINETIC HORMONES 153 



among the hormones which can control the activities of both 

 muscles and exocrine glands. Among invertebrates there are several 

 examples of hormones controlling muscles, but as yet none have 

 been found to control secretion of any exocrine glands in these 

 animals, so that there is no overlap; its investigation might be 

 interesting. 



The control of the secretion of endocrine glands is usually 

 separate from that of other glands; but even here there is an 

 exception in that prolactin can stimulate the exocrine mammary 

 glands as well as the endocrine corpora lutea of the ovary. 



Apart from adrenaline, the hormones controlling pigmentary 

 effectors do not overlap with the foregoing at all, unless the 

 hormone that stimulates dispersion of pigment in the stick insect, 

 Caraustus, should prove to be the same as that which stimulates 

 locomotor activity in Periplaneta^ since both come from the 

 suboesophageal ganglia. There is also a difference in the number of 

 hormones involved; the control of pigmentary effectors, and 

 particularly of chromatophores with movable pigment, is nearly 

 always achieved by the interaction of a pair of antagonistic hor- 

 mones (§ 3.24), but the same can rarely be said of muscles or 

 glands. The only unequivocal case of a pair of hormones is gastrin 

 and enterogastrone, which have opposing actions on both the 

 peristaltic muscles and the acid-secreting cells of the mammalian 

 stomach (§§3.112 and 4.11). Even the apparently opposed actions 

 of progesterone and oxytocin on uterine muscle may not be direct 

 inhibition and stimulation, but rather a case of the presence of 

 progesterone rendering the muscle insensitive to oxytocin 

 throughout gestation. Nearly all the other kinetic hormones, acting 

 upon muscles, myoepithelial cells and glands, serve to stimulate 

 these effectors, for which there are no known inhibitors. A general 

 inhibition of locomotion seems to be the action of an eyestalk 

 hormone in some Crustacea, and for these no stimulator is known 

 (§3.12). Elsewhere different effectors react differently to the same 

 hormone, some being stimulated and others inhibited. This occurs 

 with cholecystokinin, which causes contraction of the main muscles 

 of the gall bladder but relaxation of the sphincter muscles, and with 

 adrenaline, which contracts the intestinal sphincters and inhibits 

 the peristalsis of the longitudinal muscles. In the frog, adrenaline 



