PARTS OF ANIMALS, III. vi. 



them, though the bloodless ones can do their own 

 cooling by means of the connate pneuma.^ Now 

 external cooling must be effected either by water or 

 by air. This explains why none of the fishes has 

 a lung. They are water-cooled, and instead of a 

 lung they have gills (see the treatise on Respiration).^ 

 Animals that breathe, on the other hand, are air- 

 cooled, and so they all have a lung. All land-animals 

 breathe ; so do some of the water-animals (e.g. the 

 whale, the dolphin, and all the spouting cetacea). 

 This is not surprising, for many animals are inter- 

 mediate between the two : some that are land- 

 animals and breathe spend most of their time in the 

 water o^ving to the blend ^ in their bodies ; and 

 some of the water-animals partake of the nature of 

 land-animals to such an extent that the limiting 

 condition of life for them Ues in their breath. 



Now the organ of breathing is the lung. It has 

 its source of motion in the heart, and it affords a 

 wide space for the breath to come into because it is 

 large and spongy : when the lung rises up, the 

 breath rushes in, and when it contracts the breath 

 goes out again. The theory ^ that the lung is pro- 

 vided as a cushion for the throbbings of the heart is 

 not correct. This leaping of the heart is practically 

 not found except in man, and that is because man is 

 the only animal that has hope and expectation of 

 the future. Besides, in most animals the heart is 

 a long way off from the lung and lies well above 

 it, and so the lung cannot be of any assistance in 

 absorbing the throbbings of the heart. ^ 



There are many differences in the lung. Some 



* In quadrupeds the lung is above the heart, but not in 

 man, owing to the difference of posture. 



257 



