DESIGN FOR A BRAIN 5/4 



The amount of carbon dioxide in the blood is important in 

 its effect on the blood's alkalinity. If the amount rises, the rate 

 and depth of respiration are increased, and carbon dioxide is 

 exhaled at an increased rate. If the amount falls, the reaction 

 is reversed. By this means the alkalinity of the blood is kept 

 within limits. 



The retina works best at a certain intensity of illumination. 

 In bright light the nervous system contracts the pupil, and in 

 dim relaxes it. Thus the amount of light entering the eye is 

 maintained within limits. 



If the eye is persistently exposed to bright light, as happens 

 when one goes to the tropics, the pigment-cells in the retina 

 grow forward day by day until they absorb a large portion of the 

 incident light before it reaches the sensitive cells. In this way 

 the illumination on the sensitive cells is kept within limits. 



If exposed to sunshine, the pigment-bearing cells in the skin 

 increase in number, extent, and pigment-content. By this change 

 the degree of illumination of the deeper layers of the skin is kept 

 within limits. 



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 circu- 

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

 limits. 



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



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