§ 5.53 HORMONES AND THE ENVIRONMENT 259 



Stimulated, by the same neurosecretion from the brain, but the 

 evidence does not seem to be conclusive (Thomsen, 1952). 



The endocrinokinetic hormones stimulating the formation and 

 release of metabolic hormones from endocrine glands in vertebrates 

 are v^ell known and are all derived from the adenohypophysis. 

 Adrenocorticotrophin, ACTH, stimulates the secretion of at least 

 one of the metabolic hormones from the adrenal cortex (§ 4.231). 

 These have no morphogenetic actions ; but glucagon and thyroxine 

 have these, in addition to their effects upon metabolism. The 

 growth-promoting action of glucagon, stimulated by STH, has 

 already been mentioned (§ 4.222). Thyroxine, the secretion of 

 which is stimulated by TSH (§ 4.221), has the most specific 

 morphogenetic actions, such as the control of amphibian meta- 

 morphosis. 



5.53 HORMONES AND THE ENVIRONMENT 



Many of the relatively quick-acting kinetic hormones induce 

 protective responses to the environment, and maintain short-term 

 changes in diurnal and tidal rhythms. Their release is often 

 directly controlled by the nervous system, and serves mainly to 

 relieve the latter of some of its rather slower and less specific 

 functions. The metabolic hormones control the "internal milieu" 

 in the tissues : while some tend to maintain a steady balance, others 

 allow of relatively slow adaptation to environmental changes. 

 Most of these hormone-controlled responses are initiated by the 

 nervous system, some directly, and others only indirectly through 

 the mediation of a second, or endocrinokinetic, hormone. This last 

 type of control of bodily change is well suited to maintaining a 

 particular response over a long period of time. It is appropriate to 

 some metabolic processes such as diapause ; but it seems best fitted 

 to relate the secretion of morphogenetic hormones concerned with 

 growth and reproduction to seasonal changes in the environment. 

 This complex field of hormonal activity must, however, be left to 

 the second part of the book. 



