ADAPTATION AS STABILITY 5/4 



is clearly one which exposes to the air a minimum of surface. 

 In all these ways, the body acts so as to maintain its temperature 

 within limits. 



The water content of the blood is disturbed by the intake of 

 water at drinking and eating, by the output during excretion 

 and secretion, and by sweating. When the water content is 

 lowered, sweating, salivation, and the excretion of urine are all 

 diminished ; thirst is increased, leading to an increased intake, 

 and the tissues of the body pass some of their water into the 

 blood-stream. When the water content is excessive, all these 

 activities are reversed. By these means the body tends to 

 maintain the water-content of the blood within limits. 



The pressure of the blood in the aorta may be disturbed by 

 haemorrhage or by exertion. When the pressure falls, centres 

 in the brain and spinal cord make the heart beat faster, increasing 

 the quantity of blood forced into the aorta ; they make the small 

 arteries contract, impeding the flow of blood out of it. If the 

 pressure is too high, these actions are reversed. By these and 

 other mechanisms the blood pressure in the aorta is maintained 

 within limits. 



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. 



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