136 THE BIOLOGY OF BIRDS 



have a relatively smaller and lighter spleen, and smaller 

 birds a relatively larger and heavier one. 



§ 8. Warm-bloodedness 



This is an awkward word for a very important quality, 

 namely, that of keeping up a practically constant body- 

 temperature. It means that there are arrangements for 

 keeping a balance between loss of heat and production of 

 heat, and the only warm-blooded animals are birds and 

 mammals. They are technically called stenothermal (of 

 constant temperature) in contrast to reptiles, amphibians, 

 and fishes which are poikilothermal (of changeable tempera- 

 ture) — in plain English, " cold-blooded." But this is just 

 a case where the " plain EngHsh " is not very useful. If 

 we say that a snake is a " cold-blooded " animal, this surely 

 suggests that it must have a low temperature ; but that 

 cannot be true of a snake basking in the sun in the Sahara. 

 " Cold-blooded " means tending to approximate to the 

 temperature of the surroundings ; ** warm-blooded " means 

 keeping the same body-temperature, day and night, year in 

 year out. We repeat that birds and mammals are the only 

 warm-blooded animals ; and the fact that birds have a 

 body-temperature 2°-i4° F. higher than that of mammals 

 does not make them any more warm-blooded. 



There are many oxidations going on in the body which 

 result in the production of heat, but the chief source is in 

 connection with the contraction of the muscles — wliich goes 

 on to some extent even when the animal is at rest or asleep. 

 Every one knows that the body becomes cold more rapidly 

 during sleep and that is mainly because most of the muscles 

 are at rest. The regulation of the heat-production and the 

 heat-loss is due to the activity of a nerve-centre in the 

 corpus striatum of the cerebral hemispheres. It is called 

 the thermotaxic centre, and it is sensitive to the temperature 

 of the blood and probably to messages from the skin. 

 When the temperature is lower than usual, the centre sends 

 out orders to the muscles, commanding tlicni, as it were, 



