ECONOMY OF THE BODY 137 



to produce more heat. These orders may follow on the 

 heels of one another very quickly, and the muscles may 

 pass into an irregular sort of contraction, familiar in shivering. 

 But the shivering is also associated with a constriction of 

 the blood-vessels in the skin, for the heat-regulating centre 

 also sends messages to the periphery. If the blood-vessels 

 in the skin contract, there is less loss of heat from the skin. 

 If the regulating centre should be warmed by the blood, it 

 sends out orders which bring about muscular relaxation 

 and therefore less production of heat. And it also brings 

 about dilatation of the blood-vessels in the skin which 

 increases the loss of heat. In mammals the warmth of the 

 body is also reduced by sweating, but birds do not sweat. 

 There may be an approach to " internal perspiration " 

 in the diffusion of water- vapour into the air-sacs. 



We see, then, that the temperature of the body can be 

 regulated by increasing or decreasing the production and 

 also the loss of the animal heat. The importance of the 

 animal heat is that it enables the chemical reactions in the 

 body to go on more rapidly and smoothly. 



All birds are warm-blooded as adults, but in the young 

 birds the thermotaxic arrangements have not been com- 

 pleted, and the danger of over-heating or over-cooling is 

 great. The very young bird is in this respect not far from 

 a reptile. 



§ 9. The Regulatory System and the Resulting 

 Correlation 



There are a number of organs in birds that were till 

 recently great puzzles, for physiologists had no clear idea of 

 their use. These are (i) the paired thyroids lying at the 

 base of the neck, arising in development as a pouch from 

 the food-canal, but ceasing to have any connection with 

 anything save blood-vessels ; (2) small yellowish supra- 

 renal bodies lying on the front part of the kidneys, and having 

 a very complex development, partly in connection with the 

 embryonic kidney and partly in connection with the sym- 

 pathetic nervous system ; and (3) a pituitary body, a 



