Aquatic [animals] do not have a heart, or lungs.[1] Those which have lungs cool their inner warmth by the trachea as a wind-pipe, when the air enters through it. But fish cool their inner warmth through the gills, drawing them in and receiving the water, so that they do not need lungs. And in general aquatic [animals] do not at all need moisture to enter from outside in order to cool them; for the water which surrounds them accomplishes the cooling. The heart primarily needs cooling, for the heart is the well-spring of natural heat for animals, and because of [the heart], the lungs also [need cooling]. So the heart is cooled through breathing in, and through breathing out the smoky refuse is voided. For the most part we are not able to talk without breathing, because the breath in the chest is expended. So we need first to breathe something else in for production of the voice. But it is impossible to breathe in and to talk at the same time, since it is also not possible to breathe in and breathe out [at the same time]; for they are opposite. For this reason when the tragic singers are going to extend their songs, they first breathe in a deep breath, and thus they begin to vocalize, so that it may be sufficient for their breathing out.[2]
The heart of virtue is mercy.[3]