2. Endocrines and Populations 261 



discs (nucleus pulposus) which has been found in voles {Microtus orcaden- 

 sis) subjected to emotional stress (Chitty et al, 1956) . The enlargement of 

 the intervertebral discs could result from altered fluid and electrolyte 

 balances brought about by increased activity of adrenal corticoids, or 

 might follow alterations in the ground substance by cortical hormones; 

 however, these explanations are conjectural as the mechanism is as yet 

 unknown. 



Physiologic adaptation may be accompanied by a variety of shifts in the 

 composition of the blood with changes in hematocrit, electrolyte concentra- 

 tions, and various metabolites. Blood lipids and sugar may also shift mark- 

 edly, as we have mentioned earlier. However, these factors are discussed 

 in considerable detail elsewhere (e.g., Selye, 1950; Hartman and Brownell, 

 1949; Jones, 1957) and will not be considered further in this discussion. 



Part 2. Physiologic Adaptation and 

 Mammalian Populations 



I. Introduction 



The foregoing account dealt largely with the basic endocrine and other 

 physiologic adaptive mechanisms that serve to maintain physiologic homeo- 

 stasis in the face of a variety of stimuli tending to alter the internal environ- 

 ment and to equip the animal to meet the demands of emergency situations. 

 In general, any stimulus which imposes physiological demands on an organ- 

 ism beyond those ordinarily met in undisturbed idyllic daily life calls into 

 play a series of feedback mechanisms that regulate the secretion of hor- 

 mones responsible for the maintenance of a relatively constant internal 

 environment. These mechanisms act upon the distribution of the internal 

 environment via the circulatory channels, the composition of the internal 

 environment with respect to fluids, electrolytes, glucose, fats, and a variety 

 of other metabolites and metabolic products, and on the supply of readily 

 available nutrients and oxygen, especially for the skeletal muscle and ner- 

 vous system. We have seen that these effects are not achieved withovit 

 sacrificing functions less immediately vital to the individual, such as re- 

 production, growth, and resistance to infectious disease and parasitism. 

 We also have pointed out the error in thinking that the responses to all 

 adverse stimuli are the same and that all necessarily are associated with 

 increased secretion of adrenal carbohydrate-active corticoids. 



