64 THE ANIMALS AND MAN 



are surrounded by a liquid, called lymph, which fills the 

 spaces between them. The capillary blood-vessels run 

 through this liquid and may not actually touch the cells 

 themselves at all, or at only a few points. The walls 

 of the capillaries being very thin, however, the substances 

 needed by the cells diffuse from the blood through the walls 

 into the liquid and thence to the cells themselves. On the 

 other hand, substances from the cells carbon dioxide and 

 other waste matters 'diffuse into the liquid and from this to 

 the blood through the capillary walls. In fact each tissue- 

 cell feeds, like certain one-celled animals, by absorption 

 from a liquid medium, but by means of the circulation this 

 liquid has a prepared food constantly brought to it. 



We may ask how the blood carries the oxygen. In the 

 vertebrates part of the blood consists of little bodies called 

 the red corpuscles. The color of these is due to a chemical 

 substance called haemoglobin. This has the capacity of ab- 

 sorbing oxygen at the lungs and of giving it up to the tissues. 



How animals know things and control their motions.- 

 Thus far we have considered the mechanisms animals have 

 for motion and for obtaining oxygen and food. A more 

 difficult but more interesting subject is how motions take 

 place in the animal, how they are guided, how they are 

 stopped; in short, how the whole conduct of the life of the 

 animal is carried on. To understand better what these 

 processes consist of, let us consider as an example the life 

 of a common bird. We know that after hatching from the 

 egg it takes food, learns the notes of the parent bird, learns 

 to fly, learns to fight or to avoid enemies, all these including 

 motions guided by sight, hearing, touch, and smell. On 

 the approach of winter it migrates to the south; in spring 

 it returns, chooses a mate, builds a nest, and rears young 

 to which it teaches in turn the ways of bird life. While 

 the full explanation of these processes is far from being 

 reached, and while we cannot here discuss them at length, 



