Aristotle On Youth and Old Age,On Life and Death,On Breathing-Part 17

Every animal in order to exist requires nutriment, in order toprevent itself from dying, refrigeration; and so Nature employs thesame organ for both purposes. For, as in some cases the tongueserves both for discerning tastes and for speech, so in animals withlungs the mouth is employed both in working up the food and in thepassage of the breath outwards and inwards. In lungless andnon-respiring animals it is employed in working up the food, whilein those of them that require refrigeration it is the gills that arecreated for this purpose.We shall state further on how it is that these organs have thefaculty of producing refrigeration. But to prevent their food fromimpeding these operations there is a similar contrivance in therespiring animals and in those that admit water. At the moment ofrespiration they do not take in food, for otherwise suffocationresults owing to the food, whether liquid or dry, slipping inthrough the windpipe and lying on the lung. The windpipe is situatedbefore the oesophagus, through which food passes into what is calledthe stomach, but in quadrupeds which are sanguineous there is, as itwere, a lid over the windpipe-the epiglottis. In birds and oviparousquadrupeds this covering is absent, but its office is discharged bya contraction of the windpipe. The latter class contract thewindpipe when swallowing their food; the former close down theepiglottis. When the food has passed, the epiglottis is in the onecase raised, and in the other the windpipe is expanded, and the airenters to effect refrigeration. In animals with gills the water isfirst discharged through them and then the food passes in throughthe mouth; they have no windpipe and hence can take no harm fromliquid lodging in this organ, only from its entering the stomach.For these reasons the expulsion of water and the seizing of their foodis rapid, and their teeth are sharp and in almost all cases arrangedin a saw-like fashion, for they are debarred from chewing their food.