the plants.

Some animals pass their life in the water, others in the air, and

therefore these media furnish the source and means of refrigeration,

water in the one case, air in the other. We must proceed-and it will

require further application on our part-to give an account of the

way and manner in which this refrigeration occurs.



7



A few of the previous physical philosophers have spoken of

respiration. The reason, however, why it exists in animals they have

either not declared or, when they have, their statements are not

correct and show a comparative lack of acquaintance with the facts.

Moreover they assert that all animals respire-which is untrue. Hence

these points must first claim our attention, in order that we may

not be thought to make unsubstantiated charges against authors no

longer alive.

First then, it is evident that all animals with lungs breathe, but

in some cases breathing animals have a bloodless and spongy lung,

and then there is less need for respiration. These animals can

remain under water for a time, which relatively to their bodily

strength, is considerable. All oviparous animals, e.g. the frog-tribe,

have a spongy lung. Also hemydes and tortoises can remain for a long

time immersed in water; for their lung, containing little blood, has

not much heat. Hence, when once it is inflated, it itself, by means of

its motion, produces a cooling effect and enables the animal to remain

immersed for a long time. Suffocation, however, always ensues if the

animal is forced to hold its breath for too long a time, for none of

this class take in water in the way fishes do. On the other hand,

animals which have the lung charged with blood have greater need of

respiration on account of the amount of their heat, while none at

all of the others which do not possess lungs breathe.