5/4 DESIGN FOR A BRAIN 



(3) Almost all the behaviour of an animal's vegetative system 

 is due to such mechanisms. 



5/4. As first example may be quoted the mechanisms which 

 tend to maintain within limits the concentration of glucose in 

 the blood. The concentration should not fall below about 06 

 per cent or the tissues will be starved of their chief source of 

 energy; and the concentration should not rise above about 

 0-18 per cent or other undesirable effects will occur. If the 

 blood-glucose falls below about 0-07 per cent the adrenal glands 

 secrete adrenaline, which makes the liver turn its stores of glycogen 

 into glucose ; this passes into the blood and the fall is opposed. 

 In addition, a falling blood-glucose stimulates the appetite so that 

 food is taken, and this, after digestion, provides glucose. On 

 the other hand, if it rises excessively, the secretion of insulin by 

 the pancreas is increased, causing the liver to remove glucose 

 from the blood. The muscles and skin also remove it ; and the 

 kidneys help by excreting glucose into the urine if the concentra- 

 tion in the blood exceeds 0-18 per cent. Here then are five 

 activities all of which have the same final effect. Each one 

 acts so as to restrict the fluctuations which might otherwise occur. 

 Each may justly be described as ' adaptive ', for it acts to preserve 

 the animal's life. 



The temperature of the interior of the warm-blooded animal's 

 body may be disturbed by exertion, or illness, or by exposure 

 to the weather. If the body temperature becomes raised, the 

 skin flushes and more heat passes from the body to the sur- 

 rounding air ; sweating commences, and the evaporation of the 

 water removes heat from the body : and the metabolism of 

 the body is slowed, so that less heat is generated within it. If the 

 body is chilled, these changes are reversed. Shivering may start, 

 and the extra muscular activity provides heat which warms the 

 body. Adrenaline is secreted, raising the muscular tone and the 

 metabolic rate, which again supplies increased heat to the body. 

 The hairs or feathers are moved by small muscles in the skin so 

 that they stand more erect, enclosing more air in the interstices 

 and thus conserving the body's heat. In extreme cold the human 

 being, when almost unconscious, reflexly takes a posture of 

 extreme flexion with the arms pressed firmly against the chest 

 and the legs fully drawn up against the abdomen. The posture 



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