122 



THE INDIVIDUAL ORGANISM 



in the cerebellum that correlate the multitude of separate details neces- 

 sary for the performance of such a complex action as (for example) hit- 

 ting a tennis ball with a racket. The midbrain is a center for certain visual 

 and auditory reflexes, and both it and the pons serve as highways for 

 nerve impulses. 



The thalamus is of particular interest. It contains the body- temperature 

 regulating center, as well as others apparently affecting water balance, 

 sugar concentration of the blood, fat metabolism, blood pressure, and 

 sleep. It is also the site of a sort of rudimentary consciousness, manifesting 



pineal body 



thalamus 



medulla 

 Fig. 7.10. The brain in median sagittal section. 



itself in crude and uncritical perceptions of warmth, pain, and the like, 

 and in generalized feelings of well-being or the reverse. In this way, it 

 helps to color the background of consciousness and thus to create moods 

 or feelings. The most important function of the thalamus is, however, 

 that it serves as the gateway to the cerebral cortex. There are no direct 

 connections between the cortex and the receptors and effectors. The 

 influence upon the cortex by the receptors and the control by the cortex 

 of the effectors are usually mediated by the thalamus. It is the portal to 

 the third-level of response, which we shall now consider. 



Third-level response — the cerebrum. The cerebrum is the organ 

 of the mind. Its actions are superposed upon the system of automatic 

 machinelike responses described for the first- and second-level responses. 



