iii. 6. 75 



either air or water. In fishes the agent is water. Fishes there- 

 fore never have a lung, but have gills in its place, as was stated 

 in the treatise on Respiration. But animals that breathe are 

 cooled by air. These therefore are all provided with a lung.^ 



All land animals breathe, and even some water animals, such 

 as the whale,* the dolphin, and all the spouting Cetacea. For 

 many animals lie halfway ^ between terrestrial and aquatic ; some 

 that are terrestrial and that inspire air being nevertheless of such 

 a bodily constitution that they abide for the most time in the 

 water ; and some that are aquatic partaking so largely of the 

 land character, that respiration constitutes for them the main 

 condition of life. 



The organ of respiration is the lung. This derives its motion 

 from the heart ; but it is its own large size and spongy texture 

 that affords amplitude of space for the entrance of the breath. 

 For when the lung rises up the breath streams in, and is again 

 expelled when the lung contracts.^ It has been stated, but 

 incorrectly, that it is to the lung that the beating of the heart 

 is due. That this is not so is shown by the phenomenon of 

 palpitation occurring, so to speak, in man alone ; inasmuch as 

 man alone is influenced by hope and expectation.' Again in 

 most animals the heart is at a distance from the lung and 

 placed above it;^ so that its beating can in no degree be 

 brought about by this latter. 



The lung differs much in different animals. For in some it is 

 of large size and contains blood ; while in others it is smaller and 

 of spongy texture. In the vivipara it is large and rich in blood, 

 because^ of their natural heat ; while in the ovipara it is small 

 and dry but capable of expanding to a vast extent when inflated. 

 Among terrestrial animals, the oviparous quadrupeds, such as 

 lizards, tortoises, and the like, have this kind of lung ; and, 

 among inhabitants of the air, the animals known as birds.'" For 

 in all these the lung is spongy, and like foam. For it is bladdery 

 and collapses from a large bulk to a small one, as does foam when 

 it runs together. In this too lies the explanation of the fact that 

 these animals are little liable to thirst and ^^ drink but sparingly, 

 and that they are able to remain for a considerable time under 

 water,^^ For, inasmuch as they have but little heat,^' the very 

 motion of the lung, airlike and void, suffices by itself to cool 

 them for a considerable period.^* 

 669 b. 



