5/5 DESIGN FOR A BRAIN 



When dry food is chewed, a copious supply of saliva is poured 

 into the mouth. Saliva lubricates the food and converts it from 

 a harsh and abrasive texture to one which can be chewed without 

 injury. The secretion therefore keeps the frictional stresses 

 below the destructive level. 



The volume of the circulating blood may be disturbed by 

 haemorrhage. Immediately after a severe haemorrhage a number 

 of changes occur : the capillaries in limbs and muscles undergo 

 constriction, driving the blood from these vessels to the more 

 essential internal organs ; thirst becomes extreme, impelling the 

 subject to obtain extra supplies of fluid ; fluid from the tissues 

 passes into the blood-stream and augments its volume ; and 

 clotting at the wound helps to stem the haemorrhage. A haemor- 

 rhage has a second effect in that, by reducing the number of 

 red corpuscles, it reduces the amount of oxygen which can be 

 carried to the tissues ; the reduction, however, itself stimulates 

 the bone-marrow to an increased production of red corpuscles. 

 All these actions tend to keep the variables ' volume of circulat- 

 ing blood ' and ' oxygen supplied to the tissues ' within normal 

 limits. 



Every fast-moving animal is liable to injury by collision with 

 hard objects. Animals, however, are provided with reflexes that 

 tend to minimise the chance of collision and of mechanical injury. 

 A mechanical stress causes injury — laceration, dislocation, or 

 fracture — only if the stress exceeds some definite value, depend- 

 ing on the stressed tissue — skin, ligament, or bone. So these 

 reflexes act to keep the mechanical stresses within physiological 

 limits. 



Many more examples could be given, but all can be included 

 within the same formula. Some external disturbance tends to 

 drive an essential variable outside its normal limits ; but the 

 commencing change itself activates a mechanism that opposes 

 the external disturbance. By this mechanism the essential 

 variable is maintained within limits much narrower than would 

 occur if the external disturbance were unopposed. The nar- 

 rowing is the objective form of the mechanism's adaptation. 



5/5. The mechanisms of the previous section act mostly within 

 the body, but it should be noted that some of them have acted 

 partly through the environment. Thus, if the body-temperature 



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