apex of the heart points, seeing that they do not incline their

heads in the same direction as land-animals do. Now from the extremity

of the heart a tube of a sinewy, arterial character runs to the centre

where the gills all join. This then is the largest of those ducts, but

on either side of the heart others also issue and run to the extremity

of each gill, and by means of the ceaseless flow of water through

the gills, effect the cooling which passes to the heart.

In similar fashion as the fish move their gills, respiring animals

with rapid action raise and let fall the chest according as the breath

is admitted or expelled. If air is limited in amount and unchanged

they are suffocated, for either medium, owing to contact with the

blood, rapidly becomes hot. The heat of the blood counteracts the

refrigeration and, when respiring animals can no longer move the

lung aquatic animals their gills, whether owing to discase or old age,

their death ensues.



23



To be born and to die are common to all animals, but there are

specifically diverse ways in which these phenomena occur; of

destruction there are different types, though yet something is

common to them all. There is violent death and again natural death,

and the former occurs when the cause of death is external, the

latter when it is internal, and involved from the beginning in the

constitution of the organ, and not an affection derived from a foreign

source. In the case of plants the name given to this is withering,

in animals senility. Death and decay pertain to all things that are

not imperfectly developed; to the imperfect also they may be

ascribed in nearly the same but not an identical sense. Under the

imperfect I class eggs and seeds of plants as they are before the root

appears.

It is always to some lack of heat that death is due, and in