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The explanation of the admission of air and respiration in thoseanimals in which a lung is found, and especially in those in whichit is full of blood, is to be found in the fact that it is of a spongynature and full of tubes, and that it is the most fully charged withblood of all the visceral organs. All animals with a full-blooded lungrequire rapid refrigeration because there is little scope fordeviation from the normal amount of their vital fire; the air alsomust penetrate all through it on account of the large quantity ofblood and heat it contains. But both these operations can be easilyperformed by air, for, being of a subtle nature, it penetrateseverywhere and that rapidly, and so performs its cooling function; butwater has the opposite characteristics.The reason why animals with a full-blooded lung respire most ishence manifest; the more heat there is, the greater is the need forrefrigeration, and at the same time breath can easily pass to thesource of heat in the heart.

22
In order to understand the way in which the heart is connectedwith the lung by means of passages, we must consult both dissectionsand the account in the History of Animals. The universal cause ofthe need which the animal has for refrigeration, is the union of thesoul with fire that takes place in the heart. Respiration is the meansof effecting refrigeration, of which those animals make use thatpossess a lung as well as a heart. But when they, as for example thefishes, which on account of their aquatic nature have no lung, possessthe latter organ without the former, the cooling is effected throughthe gills by means of water. For ocular evidence as to how the heartis situated relatively to the gills we must employ dissections, andfor precise details we must refer to Natural History. As a summarizingstatement, however, and for present purposes, the following is theaccount of the matter.It might appear that the heart has not the same position interrestrial animals and fishes, but the position really isidentical, for the apex of the heart is in the direction in which theyincline their heads. But it is towards the mouth in fishes that theapex of the heart points, seeing that they do not incline theirheads in the same direction as land-animals do. Now from the extremityof the heart a tube of a sinewy, arterial character runs to the centrewhere the gills all join. This then is the largest of those ducts, buton either side of the heart others also issue and run to the extremityof each gill, and by means of the ceaseless flow of water throughthe gills, effect the cooling which passes to the heart.In similar fashion as the fish move their gills, respiring animalswith rapid action raise and let fall the chest according as the breathis admitted or expelled. If air is limited in amount and unchangedthey are suffocated, for either medium, owing to contact with theblood, rapidly becomes hot. The heat of the blood counteracts therefrigeration and, when respiring animals can no longer move thelung aquatic animals their gills, whether owing to discase or old age,their death ensues.