canal-like ducts in the lung, into which it passes, have each a

blood-vessel lying alongside, so that the whole lung is thought to

be full of blood. The inward passage of the air is called respiration,

the outward expiration, and this double movement goes on

continuously just so long as the animal lives and keeps this organ

in continuous motion; it is for this reason that life is bound up with

the passage of the breath outwards and inwards.

It is in the same way that the motion of the gills in fishes takes

place. When the hot substance in the blood throughout the members

rises, the gills rise too, and let the water pass through, but when it

is chilled and retreats through its channels to the heart, they

contract and eject the water. Continually as the heat in the heart

rises, continually on being chilled it returns thither again. Hence,

as in respiring animals life and death are bound up with

respiration, so in the other animals class they depend on the

admission of water.

Our discussion of life and death and kindred topics is now

practically complete. But health and discase also claim the

attention of the scientist, and not mercly of the physician, in so far

as an account of their causes is concerned. The extent to which

these two differ and investigate diverse provinces must not escape us,

since facts show that their inquiries are, to a certain extent, at

least conterminous. For physicians of culture and refinement make some

mention of natural science, and claim to derive their principles

from it, while the most accomplished investigators into nature

generally push their studies so far as to conclude with an account

of medical principles.


-THE END-