PARTS OF ANIMALS, II. vii. 



which surrounds the brain. And in order to prevent 

 injury being done through heat, the blood-vessels 

 surrounding it are not few and large but small and 

 multitudinous ; and the blood is not muddy and 

 thick but thin and clear. This also explains why 

 fluxes begin in the head ; they occur when the 

 parts around the brain are colder than the rightly- 

 proportioned blend." What happens is that, as the 

 nourishment exhales upwards through the blood- 

 vessels, the residue from it becomes cooled owing 

 to the specific nature of the brain, and produces 

 fluxes of phlegm and serum. And we should be justi- 

 fied in maintaining that this process resembles, on 

 a small scale, the one which produces rain-showers. 

 Damp vapour exhales up from the earth and is carried 

 into the upper regions by the heat ; and when it 

 reaches the cold air up aloft, it condenses back again 

 into water owing to the cold, and pours down to- 

 wards the earth. However, so far as Natural Philo- 

 sophy is concerned with these matters, the proper 

 place to speak of them is in the Origins of Diseases.^ 

 Furthermore, it is the brain (or, if there is no brain, 

 its counterpart) which produces sleep in animals. 

 It cools the onflow of blood which comes from the 

 food (or else is due to other causes of the same sort), 

 and weighs down the part where it is (that is why 

 when a person is sleepy his head is weighed do>\Ti), 

 and causes the hot substance to escape below to- 

 gether with the blood. Hence, the blood accumu- 

 lates unduly in the lower region of the body and 

 produces sleep ; at the same time it takes away 

 from those animals whose nature is to stand upright 

 the power to do so, and the others it prevents from 



* See p. 38. " No such treatise exists. 



153 



