36 ii. 7- 



of showers. For when vapor steams up from the earth under the 

 influence of heat and is carried into the upper regions, so soon, 

 as it reaches the cold air that is above the earth, it condenses 

 again into water owing to the refrigeration, and falls back 

 to the earth as rain.'^ These however are matters which may 

 be suitably considered in the Principles of Diseases, so far as 

 natural philosophy has anything to say to them.^* 



It is the brain again, — or, in animals that have no brain, the 

 part analogous to it, — which is the cause of sleep. For either 

 by chilling the blood that streams upwards after food, or by 

 some other similar influences, it produces heaviness in the region 

 in which it lies (which is the reason that drowsy persons hang 

 the head), and causes the heat to escape downwards in company 

 with the blood. It is the accumulation of this in excess in the 

 lower part that produces complete sleep, taking away the power 

 of standing upright from such animals as are able to assume 

 that posture ; and from the rest the power of holding up the 

 head. These however are matters which have been separately 

 considered in the treatises on Sensation and on Sleep.^^ 



That the brain is a compound of earth and water ^^ is shown 

 by what occurs when it is boiled. For, when so treated, it turns 

 hard and solid, inasmuch as the water is evaporated by the heat, 

 and leaves the earthy part behind.^''^ Just the same occurs when 

 pulse and other fruits are boiled. For these also are hardened by 

 the process, because the water which enters into their composition 

 is driven off" and leaves the earth, which is their main constituent, 

 behind. 



Of all animals, man has the largest brain in proportion to 

 his size ; and it is larger in men than in women.^* This is 

 because the region of the heart and of the lung is hotter and 

 richer in blood in man than in any other animal ; and in men 

 than in women. '^ This again explains why man, alone of animals, 

 stands erect. For the heat, overcoming ahy opposite inclination, 

 makes growth take its own line of direction, which is from the 

 centre of the body upwards.^'' It is then as a counterpoise to 

 his excessive heat that in man's brain there is this superabundant 

 fluidity and coldness ; and ,it is again owing to this superabundance 

 that the cranial bone which some call the Bregma ^^ is the last 

 to become solidified ; ^^ so long does evaporation continue to 

 occur through it under the influence of heat.^^ Man is the only 

 653 a. 



