Aristotle On Youth and Old Age,On Life and Death,On Breathing-Part 27

Respiration takes place when the hot substance which is the seatof the nutritive principle increases. For it, like the rest of thebody, requires nutrition, and more so than the members, for it isthrough it that they are nourished. But when it increases itnecessarily causes the organ to rise. This organ we must to beconstructed like the bellows in a smithy, for both heart and lungsconform pretty well to this shape. Such a structure must be double,for the nutritive principle must be situated in the centre of thenatural force.Thus on increase of bulk expansion results, which necessarily causesthe surrounding parts to rise. Now this can be seen to occur whenpeople respire; they raise their chest because the motive principle ofthe organ described resident within the chest causes an identicalexpansion of this organ. When it dilates the outer air must rush in asinto a bellows, and, being cold, by its chilling influence reducesby extinction the excess of the fire. But, as the increase of bulkcauses the organ to dilate, so diminution causes contraction, and whenit collapses the air which entered must pass out again. When it entersthe air is cold, but on issuing it is warm owing to its contact withthe heat resident in this organ, and this is specially the case inthose animals that possess a full-blooded lung. The numerouscanal-like ducts in the lung, into which it passes, have each ablood-vessel lying alongside, so that the whole lung is thought tobe full of blood. The inward passage of the air is called respiration,the outward expiration, and this double movement goes oncontinuously just so long as the animal lives and keeps this organin continuous motion; it is for this reason that life is bound up withthe passage of the breath outwards and inwards.It is in the same way that the motion of the gills in fishes takesplace. When the hot substance in the blood throughout the membersrises, the gills rise too, and let the water pass through, but when itis chilled and retreats through its channels to the heart, theycontract and eject the water. Continually as the heat in the heartrises, continually on being chilled it returns thither again. Hence,as in respiring animals life and death are bound up withrespiration, so in the other animals class they depend on theadmission of water.