228 /• /• Christian 



It is well known that epinephrine produces a marked rise in calorigenesis 

 and oxygen consumption in intact animals (Hartman and Brownell, 1949; 

 Gaddum and Holzbauer, 1957). This calorigenic activity of epinephrine 

 and norepinephrine is important in adaptation to cold (Hsieh ct al., 1957a, 

 b) and is potentiated strikingly by thyroxine (Swanson, 1956, 1957) and 

 further intensified by the action of growth hormone, apparently by the 

 effect of the latter in increasing thyroid function (Evans ct al., 1958). 

 Evidently both epinephrine and thyroxine (and an intact pituitary-thyroid 

 system) are essential for survival in cold exposure and for adaptation to 

 cold, as, in the absence of thyroxine, epinephrine does not exert its calori- 

 genic action in rats (Swanson, 1956). It was found that oxygen consump- 

 tion in thyroidectomized rats increased in proportion to the dose of epi- 

 nephrine when thyroxine was supplied at a fixed standard dose (Swanson, 

 1957). On the other hand, oxygen consumption varied linearly with the 

 log-dose of thyroxine when the animals were kept on a standard dose of 

 epinephrine (Swanson, 1956). Epinephrine apparently is essential for in- 

 creased calorigenesis, but requires thyroxine for its activity. Swanson 

 (1957) has expressed the opinion that since reactivity to epinephrine is di- 

 rectly and intimately dependent on the level of thyroxine, the main role of 

 the increased secretion of thyroxine in acclimitization to cold may be to 

 potentiate the calorigenic activity of epinephrine. In any event, both 

 epinephrine from the adrenal medulla and an intact properly functioning 

 pituitary-thyroid system are essential for increased calorigenesis and adap- 

 tation to cold. The necessity of the adrenal medulla and an intact thyroid 

 is shown by the fact that the calorigenic response to cold is abolished either 

 by adrenal demedullation (Morin, 1946a, b) or by thyroidectomy (Swan- 

 son, 1957) . 



It should be obvious from the foregoing discussion that the adrenal 

 medulla and sympathetic nervous system and their hormones, epinephrine 

 and norepinephrine, are vital components of a variety of adaptive mecha- 

 nisms and, if anything, their importance has tended to be underestimated. 

 It appears from the available evidence that the sympathico-adrenal hor- 

 mones may play a key role in the physiologic responses to sociopsychologic 

 factors associated with changes in population density, and therefore deserve 

 more investigation. 



B. The Thyroid Gland 



1. Introduction 



The thyroid gland and its hormones, primarily thyroxine and to a lesser 

 extent triiodothyronine, are important components of the internal mecha- 

 nisms which provide the organism with sufficient physiologic flexibility to 



