530 THE WONDER OF LIFE 



in the sea-swift (Collocallia) it forms, when it solidifies 

 against the rock, the well-known ' edible bird's nest '; 

 in the ant-eater it moistens the worm-like insect- catching 

 tongue ; and in the great majority of cases from snail to 

 man it contains a diastatic ferment which changes the solid 

 starch of the food into fluid and diffusible sugar. We have 

 given only a few instances of the extraordinary gamut of 

 function exhibited by glands which might all be called 

 ' salivary '. 



Some Functional Adaptations. One of the most 

 important of functional adaptations is that by which 

 birds and mammals (the so-called warm-blooded animals) 

 are able to keep the temperature of the body approxi- 

 mately constant. A healthy man may ' feel very warm ' 

 or ' feel very cold ', but his temperature varies very little 

 from the normal 37 C. or 98 -4 F., year in, year out, or 

 from the Poles to Equator. A bird may fly in a very short 

 time, perhaps in a couple of days, from North Africa to 

 within the Arctic circle, but there is no reason to believe 

 that its body-temperature will change at all. This keep- 

 ing of a constant temperature is restricted to Birds and 

 Mammals, which are therefore called homoiothermal or 

 stenothermal. 



The problem is to regulate the production of heat to the 

 loss of heat, and it is solved in Birds and Mammals by a 

 special adaptation of the nervous system. Most of the 

 heat that is lost from the body is lost from the skin ; as 

 the skin gets cold nervous messages travel inwards to the 

 central nervous system, and reflex answers come out com- 

 manding the skin blood-vessels to contract and command- 

 ing the muscles to produce more heat. The contracting 

 of the skin blood-vessels lessens the flow in the skin, and 



