224 THE SENSES OF ANIMALS 



changes in the blood that act directly upon specihc centres 

 in the brain. Although there are no hunger or thirst end- 

 organs we know when we are hungry or thirsty and react 

 accordingly — our proprioceptive nerve-endings tell us when 

 the stomach is empty, but the amount of sugar in the blood 

 also informs us when we need more nourishment. Hence a 

 few lumps of sugar or a good sweet "cuppa" pick up your 

 flagging spirits when you feel exhausted by physical exertion. 

 On the other hand the pangs of hunger are a very different 

 thing: only those who have suffered from real starvation 

 know the excruciating pain which is probably caused by the 

 over-strong contractions of the muscles in the wall of the 

 intestine when it is empty of food. Some American Indian 

 tribes are said to have been in the habit of eating clay to 

 "stay the pangs of hunger" by filling the intestine and giving 

 the muscular walls something to grip upon although the 

 material contained little to afford nourishment. Many sorts 

 of seal fast for a month or more when they come ashore for 

 breeding each year, and their stomachs, although empty of 

 food, then commonly contain a considerable quantity of 

 shingle or pebbles. This was so well known to the old fur-seal 

 hunters on the coasts of South Africa that they called a seal's 

 stomach its "ballast bag". The seals probably swallow the 

 shingle while they are on the beaches, though no one has 

 ever seen them doing it, and no one knows whether they do 

 it deliberately or by accident in the turmoil of life in the 

 crowded rookeries. It has been suggested that seals fill their 

 stomachs with indigestible matter merely to stay the pangs 

 of hunger while they are fasting, but no evidence to prove 

 that this theory represents the truth has been forthcoming. 

 Just as we are made aware of hunger and thirst, we are 

 aware of the necessity for excretion through our proprio- 

 ceptive senses, which also give us information even more 

 subtle. Why, after a period of rest and relaxation do we 

 yawn and stretch? During repose the blood tends to dilate 

 the comparatively soft-walled veins, and to stagnate in them 

 instead of hurrying back to the heart to be impelled again 

 through the arteries. In stretching and yawning we squeeze 

 the veins by the action of the muscles and by deeply filling 

 the lungs and so send the sluggish blood racing back into 



