2. Endocrines and Populations 243 



the response (Sayers and Sayers, 1949). Nevertheless, it should not be 

 assumed that all adaptive responses are qualitatively similar. Some stimuli 

 may elicit quite similar responses in kind and degree, whereas others will 

 be manifested in qualitatively and quantitatively different manners. This 

 statement should be apparent from the preceding accounts of the mecha- 

 nisms involved. All have in common that if uncompensated they will pro- 

 duce widespread physiologic changes which come under the heading of 

 shock (Selye, 1950). However, whether or not the symptoms commonly 

 associated with shock are elicited depends on the severity of the stimulus 

 (Selye, 1950). In short, almost any change in the external environment of 

 the animal, physical damage to the animal, or emotional upset is po- 

 tentially capable of producing profound deleterious effects on the animal, 

 but within reasonable limits the adaptive mechanisms provide it with the 

 flexibility to accommodate most changes or adverse stimuli. It is not until 

 the latter reach rather serious proportions and evoke marked responses that 

 we customarily consider the animal to be subject to "stress" or that the 

 stimuli are labeled "alarming." 



These stimuli act through or are received through a number of receptor 

 pathways in the host. For example, physical damage, including burns, 

 surgery, acute physical trauma of all sorts, will evoke adaptive responses 

 probably by direct neural pathways as well as via unknown chemical 

 mediators, resulting from tissue damage which may stimulate the release 

 of ACTH or other adaptive responses directly. Hemorrhage, by decreasing 

 blood volimie, can effect the release of aldosterone directly. Many stimuli 

 act directly through the central nervous system, such as light, noise, fear, 

 and rage. Heat, cold, and similar environmental factors act through recep- 

 tors in the individual and produce physiologic responses directly. Regardless 

 of the receptor or pathway, all these primary environmental stimuli, if 

 severe enough or chronic enough, have the ability to produce the symptoms 

 of shock through several common pathways. In so doing they produce 

 physiologic changes, either directly or through the central nervous system, 

 which in turn activate a chain of responses leading to defensive reaction 

 against any adverse physiologic shift which may have been produced bj^ 

 the initial stimulus. The variety of stimuli which have been found to elicit 

 adaptive reactions is legion (Selye, 1950). In fact, one could conclude that 

 almost every experience encountered in daily life by any mammal, if severe 

 enough, is capable of producing shock and evoking marked adaptive re- 

 sponses, but for the purpose of the present discussion, the importance of 

 emotional stimuli in evoking these reactions will be emphasized. These 

 may be acute stimuli, and all of us are personally familiar with some of the 

 reactions that they are capable of producing. However, it should be noted 

 that emotional stimuli of a more chronic nature can produce profound 



