On The Parts Of Animals   

abundant and thick. We can now understand why defluxions have their
origin in the head, and occur whenever the parts about the brain have
more than a due proportion of coldness. For when the nutriment steams
upwards through the blood-vessels, its refuse portion is chilled by
the influence of this region, and forms defluxions of phlegm and
serum. We must suppose, to compare small things with great, that the
like happens here as occurs in the production of showers. For when
vapour steams up from the earth and is carried by the heat into the
upper regions, so soon as it reaches the cold air that is above the
earth, it condenses again into water owing to the refrigeration, and
falls back to the earth as rain. These, however, are matters which may
be suitably considered in the Principles of Diseases, so far as
natural philosophy has anything to say to them.
It is the brain again-or, in animals that have no brain, the part
analogous to it-which is the cause of sleep. For either by chilling
the blood that streams upwards after food, or by some other similar
influences, it produces heaviness in the region in which it lies
(which is the reason why drowsy persons hang the head), and causes the
heat to escape downwards in company with the blood. It is the
accumulation of this in excess in the lower region that produces
complete sleep, taking away the power of standing upright from those
animals to whom that posture is natural, and from the rest the power
of holding up the head. These, however, are matters which have been
separately considered in the treatises on Sensation and on Sleep.
That the brain is a compound of earth and water is shown by what
occurs when it is boiled. For, when so treated, it turns hard and
solid, inasmuch as the water is evaporated by the heat, and leaves the
earthy part behind. Just the same occurs when pulse and other fruits
are boiled. For these also are hardened by the process, because the
water which enters into their composition is driven off and leaves the
earth, which is their main constituent, behind.
Of all animals, man has the largest brain in proportion to his size;
and it is larger in men than in women. This is because the region of
the heart and of the lung is hotter and richer in blood in man than in
any other animal; and in men than in women. This again explains why
man, alone of animals, stands erect. For the heat, overcoming any
opposite inclination, makes growth take its own line of direction,
which is from the centre of the body upwards. It is then as a
counterpoise to his excessive heat that in man's brain there is this
superabundant fluidity and coldness; and it is again owing to this
superabundance that the cranial bone, which some call the Bregma, is
the last to become solidified; so long does evaporation continue to
occur through it under the influence of heat. Man is the only
sanguineous animal in which this takes place. Man, again, has more
sutures in his skull than any other animal, and the male more than the
female. The explanation is again to be found in the greater size of
the brain, which demands free ventilation, proportionate to its bulk.
For if the brain be either too fluid or too solid, it will not perform
its office, but in the one case will freeze the blood, and in the
other will not cool it at all; and thus will cause disease, madness,
and death. For the cardiac heat and the centre of life is most
delicate in its sympathies, and is immediately sensitive to the
slightest change or affection of the blood on the outer surface of the
brain.
The fluids which are present in the animal body at the time of birth
have now nearly all been considered. Amongst those that appear only at
a later period are the residua of the food, which include the deposits
of the belly and also those of the bladder. Besides these there is the
semen and the milk, one or the other of which makes its appearance in
appropriate animals. Of these fluids the excremental residua of the
food may be suitably discussed by themselves, when we come to examine
and consider the subject of nutrition. Then will be the time to
explain in what animals they are found, and what are the reasons for
their presence. Similarly all questions concerning the semen and the