By Galen

Translated by Arthur John Brock

1. Since feeling and voluntary motion are peculiar to animals, whilst
growth and nutrition are common to plants as well, we may look on
the former as effects of the soul and the latter as effects of the
nature. And if there be anyone who allows a share in soul to plants
as well, and separates the two kinds of soul, naming the kind in question
vegetative, and the other sensory, this person is not saying anything
else, although his language is somewhat unusual. We, however, for
our part, are convinced that the chief merit of language is clearness,
and we know that nothing detracts so much from this as do unfamiliar
terms; accordingly we employ those terms which the bulk of people
are accustomed to use, and we say that animals are governed at once
by their soul and by their nature, and plants by their nature alone,
and that growth and nutrition are the effects of nature, not of soul.

2. Thus we shall enquire, in the course of this treatise, from what
faculties these effects themselves, as well as any other effects of
nature which there may be, take their origin. 

First, however, we must distinguish and explain clearly the various
terms which we are going to use in this treatise, and to what things
we apply them; and this will prove to be not merely an explanation
of terms but at the same time a demonstration of the effects of nature.

When, therefore, such and such a body undergoes no change from its
existing state, we say that it is at rest; but, not withstanding,
if it departs from this in any respect we then say that in this respect
it undergoes motion. Accordingly, when it departs in various ways
from its preexisting state, it will be said to undergo various kinds
of motion. Thus, if that which is white becomes black, or what is
black becomes white, it undergoes motion in respect to colour; or
if what was previously sweet now becomes bitter, or, conversely, from
being bitter now becomes sweet, it will be said to undergo motion
in respect to flavour; to both of these instances, as well as to those
previously mentioned, we shall apply the term qualitative motion.
And further, it is not only things which are altered in regard to
colour and flavour which, we say, undergo motion; when a warm thing
becomes cold, and a cold warm, here too we speak of its undergoing
motion; similarly also when anything moist becomes dry, or dry moist.
Now, the common term which we apply to all these cases is alteration.

This is one kind of motion. But there is another kind which occurs
in bodies which change their position, or as we say, pass from one
place to another; the name of this is transference. 

These two kinds of motion, then, are simple and primary, while compounded
from them we have growth and decay, as when a small thing becomes
bigger, or a big thing smaller, each retaining at the same time its
particular form. And two other kinds of motion are genesis and destruction,
genesis being a coming into existence, and destruction being the opposite.

Now, common to all kinds of motion is change from the preexisting
state, while common to all conditions of rest is retention of the
preexisting state. The Sophists, however, while allowing that bread
in turning into blood becomes changed as regards sight, taste, and
touch, will not agree that this change occurs in reality. Thus some
of them hold that all such phenomena are tricks and illusions of our
senses; the senses, they say, are affected now in one way, now in
another, whereas the underlying substance does not admit of any of
these changes to which the names are given. Others (such as Anaxagoras)
will have it that the qualities do exist in it, but that they are
unchangeable and immutable from eternity to eternity, and that these
apparent alterations are brought about by separation and combination.

Now, if I were to go out of my way to confute these people, my subsidiary
task would be greater than my main one. Thus, if they do not know
all that has been written, "On Complete Alteration of Substance" by
Aristotle, and after him by Chrysippus, I must beg of them to make
themselves familiar with these men's writings. If, however, they know
these, and yet willingly prefer the worse views to the better, they
will doubtless consider my arguments foolish also. I have shown elsewhere
that these opinions were shared by Hippocrates, who lived much earlier
than Aristotle. In fact, all those known to us who have been both
physicians and philosophers Hippocrates was the first who took in
hand to demonstrate that there are, in all, four mutually interacting
qualities, and that to the operation of these is due the genesis and
destruction of all things that come into and pass out of being. Nay,
more; Hippocrates was also the first to recognise that all these qualities
undergo an intimate mingling with one another; and at least the beginnings
of the proofs to which Aristotle later set his hand are to be found
first in the writings of Hippocrates. 

As to whether we are to suppose that the substances as well as their
qualities undergo this intimate mingling, as Zeno of Citium afterwards
declared, I do not think it necessary to go further into this question
in the present treatise; for immediate purposes we only need to recognize
the complete alteration of substance. In this way, nobody will suppose
that bread represents a kind of meeting-place for bone, flesh, nerve,
and all the other parts, and that each of these subsequently becomes
separated in the body and goes to join its own kind; before any separation
takes place, the whole of the bread obviously becomes blood; (at any
rate, if a man takes no other food for a prolonged period, he will
have blood enclosed in his veins all the same). And clearly this disproves
the view of those who consider the elements unchangeable, as also,
for that matter, does the oil which is entirely used up in the flame
of the lamp, or the faggots which, in a somewhat longer time, turn
into fire. 

I said, however, that I was not going to enter into an argument with
these people, and it was only because the example was drawn from the
subject-matter of medicine, and because I need it for the present
treatise, that I have mentioned it. We shall then, as I said, renounce
our controversy with them, since those who wish may get a good grasp
of the views of the ancients from our own personal investigations
into these matters. 

The discussion which follows we shall devote entirely, as we originally
proposed, to an enquiry into the number and character of the faculties
of Nature, and what is the effect which each naturally produces. Now,
of course, I mean by an effect that which has already come into existence
and has been completed by the activity of these faculties- for example,
blood, flesh, or nerve. And activity is the name I give to the active
change or motion, and the cause of this I call a faculty. Thus, when
food turns into blood, the motion of the food is passive, and that
of the vein active. Similarly, when the limbs have their position
their position altered, it is the muscle which produces, and the bones
which undergo the motion. In these cases I call the motion of the
vein and of the muscle an activity, and that of the food and the bones
a symptom or affection, since the first group undergoes alteration
and the second group is merely transported. One might, therefore,
also speak of the activity as an effect of Nature- for example, digestion,
absorption, blood-production; one could not, however, in every case
call the effect an activity; thus flesh is an effect of Nature, but
it is, of course, not an activity. It is, therefore, clear that one
of these terms is used in two senses, but not the other.

3. It appears to me, then, that the vein, as well as each of the other
parts, functions in such and such a way according to the manner in
which the four qualities are mixed. There are, however, a considerable
number of not undistinguished men- philosophers and physicians- who
refer action to the Warm and the Cold, and who subordinate to these,
as passive, the Dry and the Moist; Aristotle, in fact, was the first
who attempted to bring back the causes of the various special activities
to these principles, and he was followed later by the Stoic school.
These latter, of course, could logically make active principles of
the Warm and Cold, since they refer the change of the elements themselves
into one another to certain diffusions and condensations. This does
not hold of Aristotle, however; seeing that he employed the four qualities
to explain the genesis of the elements, he ought properly to have
also referred the causes of all the special activities to these. How
is it that he uses the four qualities in his book "On Genesis and
Destruction," whilst in his "Meteorology," his "Problems," and many
other works he uses the uses the two only? Of course, if anyone were
to maintain that in the case of animals and plants the Warm and Cold
are more active, the Dry and Moist less so, he might perhaps have
even Hippocrates on his side; but if he were to say that this happens
in all cases, he would, I imagine, lack support, not merely from Hippocrates,
but even from Aristotle himself- if, at least, Aristotle chose to
remember what he himself taught us in his work "On Genesis and Destruction,"
not as a matter of simple statement, but with an accompanying demonstration.
I have, however, also investigated these questions, in so far as they
are of value to a physician, in my work "On Temperaments."

4. The so-called blood-making faculty in the veins, then, as well
as all the other faculties, fall within the category of relative concepts;
primarily because the faculty is the cause of the activity, but also,
accidentally, because it is the cause of the effect. But, if the cause
is relative to something- for it is the cause of what results from
it, and of nothing else- it is obvious that the faculty also falls
into the category of the relative; and so long as we are ignorant
of the true essence of the cause which is operating, we call it a
faculty. Thus we say that there exists in the veins a blood-making
faculty, as also a digestive faculty in the stomach, a pulsatile faculty
in the heart, and in each of the other parts a special faculty corresponding
to the function or activity of that part. If, therefore, we are to
investigate methodically the number and kinds of faculties, we must
begin with the effects; for each of these effects comes from a certain
activity, and each of these again is preceded by a cause.

5. The effects of Nature, then, while the animal is still being formed
in the womb, are all the different parts of its body; and after it
has been born, an effect in which all parts share is the progress
of each to its full size, and thereafter its maintenance of itself
as long as possible. 

The activities corresponding to the three effects mentioned are necessarily
three- one to each- namely, Genesis, Growth, and Nutrition. Genesis,
however, is not a simple activity of Nature, but is compounded of
alteration and of shaping. That is to say, in order that bone, nerve,
veins, and all other [tissues] may come into existence, the underlying
substance from which the animal springs must be altered; and in order
that the substance so altered may acquire its appropriate shape and
position, its cavities, outgrowths, attachments, and so forth, it
has to undergo a shaping or formative process. One would be justified
in calling this substance which undergoes alteration the material
of the animal, just as wood is the material of a ship, and wax of
an image. 

Growth is an increase and expansion in length, breadth, and thickness
of the solid parts of the animal (those which have been subjected
to the moulding or shaping process). Nutrition is an addition to these,
without expansion. 

6. Let us speak then, in the first place, of Genesis, which, as we
have said, results from alteration together with shaping.

The seed having been cast into the womb or into the earth (for there
is no difference), then, after a certain definite period, a great
number of parts become constituted in the substance which is being
generated; these differ as regards moisture, dryness, coldness and
warmth, and in all the other qualities which naturally derive therefrom.
These derivative qualities, you are acquainted with, if you have given
any sort of scientific consideration to the question of genesis and
destruction. For, first and foremost after the qualities mentioned
come the other so-called tangible distinctions, and after them those
which appeal to taste, smell, and sight. Now, tangible distinctions
are hardness and softness, viscosity, friability, lightness, heaviness,
density, rarity, smoothness, roughness, thickness and thinness; all
of these have been duly mentioned by Aristotle. And of course you
know those which appeal to taste, smell, and sight. Therefore, if
you wish to know which alterative faculties are primary and elementary,
they are moisture, dryness, coldness, and warmth, and if you wish
to know which ones arise from the combination of these, they will
be found to be in each animal of a number corresponding to its sensible
elements. The name sensible elements is given to all the homogeneous
parts of the body, and these are to be detected not by any system,
but by personal observation of dissections. 

Now Nature constructs bone, cartilage, nerve, membrane, ligament,
vein, and so forth, at the first stage of the animal's genesis, employing
at this task a faculty which is, in general terms, generative and
alterative, and, in more detail, warming, chilling, drying, or moistening;
or such as spring from the blending of these, for example, the bone-producing,
nerve-producing, and cartilage-producing faculties (since for the
sake of clearness these names must be used as well). 

Now the peculiar flesh of the liver is of this kind as well, also
that of the spleen, that of the kidneys, that of the lungs, and that
of the heart; so also the proper substance of the brain, stomach,
gullet, intestines, and uterus is a sensible element, of similar parts
all through, simple, and uncompounded. That is to say, if you remove
from each of the organs mentioned its arteries, veins, and nerves,
the substance remaining in each organ is, from the point of view of
the senses, simple and elementary. As regards those organs consisting
of two dissimilar coats, of which each is simple, of these organs
the coats are the are the elements- for example, the coats of the
stomach, oesophagus, intestines, and arteries; each of these two coats
has an alterative faculty peculiar to it, which has engendered it
from the menstrual blood of the mother. Thus the special alterative
faculties in each animal are of the same number as the elementary
parts; and further, the activities must necessarily correspond each
to one of the special parts, just as each part has its special use-
for example, those ducts which extend from the kidneys into the bladder,
and which are called ureters; for these are not arteries, since they
do not pulsate nor do they consist of two coats; and they are not
veins, since they neither contain blood, nor do their coats in any
way resemble those of veins; from nerves they differ still more than
from the structures mentioned. 

"What, then, are they?" someone asks- as though every part must necessarily
be either an artery, a vein, a nerve, or a complex of these, and as
though the truth were not what I am now stating, namely, that every
one of the various organs has its own particular substance. For in
fact the two bladders- that which receives the urine, and that which
receives the yellow bile- not only differ from all other organs, but
also from one another. Further, the ducts which spring out like kinds
of conduits from the gall-bladder and which pass into the liver have
no resemblance either to arteries, veins or nerves. But these parts
have been treated at a greater length in my work "On the Anatomy of
Hippocrates," as well as elsewhere. 

As for the actual substance of the coats of the stomach, intestine,
and uterus, each of these has been rendered what it is by a special
alterative faculty of Nature; while the bringing of these together,
the therewith of the structures which are inserted into them, the
outgrowth into the intestine, the shape of the inner cavities, and
the like, have all been determined by a faculty which we call the
shaping or formative faculty; this faculty we also state to be artistic-
nay, the best and highest art- doing everything for some purpose,
so that there is nothing ineffective or superfluous, or capable of
being better disposed. This, however, I shall demonstrate in my work
"On the Use of Parts." 

7. Passing now to the faculty of Growth let us first mention that
this, too, is present in the foetus in utero as is also the nutritive
faculty, but that at that stage these two faculties are, as it were,
handmaids to those already mentioned, and do not possess in themselves
supreme authority. When, however, the animal has attained its complete
size, then, during the whole period following its birth and until
the acme is reached, the faculty of growth is predominant, while the
alterative and nutritive faculties are accessory- in fact, act as
its handmaids. What, then, is the property of this faculty of growth?
To extend in every direction that which has already come into existence-
that is to say, the solid parts of the body, the arteries, veins,
nerves, bones, cartilages, membranes, ligaments, and the various coats
which we have just called elementary, homogeneous, and simple. And
I shall state in what way they gain this extension in every direction,
first giving an illustration for the sake of clearness. 

Children take the bladders of pigs, fill them with air, and then rub
them on ashes near the fire, so as to warm, but not to injure them.
This is a common game in the district of Ionia, and among not a few
other nations. As they rub, they sing songs, to a certain measure,
time, and rhythm, and all their words are an exhortation to the bladder
to increase in size. When it appears to them fairly well distended,
they again blow air into it and expand it further; then they rub it
again. This they do several times, until the bladder seems to them
to have become large enough. Now, clearly, in these doings of the
children, the more the interior cavity of the bladder increases in
size, the thinner, necessarily, does its substance become. But, if
the children were able to bring nourishment to this thin part, then
they would make the bladder big in the same way that Nature does.
As it is, however, they cannot do what Nature does, for to imitate
this is beyond the power not only of children, but of any one soever;
it is a property of Nature alone. 

It will now, therefore, be clear to you that nutrition is a necessity
for growing things. For if such bodies were distended, but not at
the same time nourished, they would take on a false appearance of
growth, not a true growth. And further, to be distended in all directions
belongs only to bodies whose growth is directed by Nature; for those
which are distended by us undergo this distension in one direction
but grow less in the others; it is impossible to find a body which
will remain entire and not be torn through whilst we stretch it in
the three dimensions. Thus Nature alone has the power to expand a
body in all directions so that it remains unruptured and preserves
completely its previous form. 

Such then is growth, and it cannot occur without the nutriment which
flows to the part and is worked up into it. 

8. We have, then, it seems, arrived at the subject of Nutrition, which
is the third and remaining consideration which we proposed at the
outset. For, when the matter which flows to each part of the body
in the form of nutriment is being worked up into it, this activity
is nutrition, and its cause is the nutritive faculty. Of course, the
kind of activity here involved is also an alteration, but not an alteration
like that occurring at the stage of genesis. For in the latter case
something comes into existence which did not exist previously, while
in nutrition the inflowing material becomes assimilated to that which
has already come into existence. Therefore, the former kind of alteration
has with reason been termed genesis, and the latter, assimilation.

9. Now, since the three faculties of Nature have been exhaustively
dealt with, and the animal would appear not to need any others (being
possessed of the means for growing, for attaining completion, and
for maintaining itself as long a time as possible), this treatise
might seem to be already complete, and to constitute an exposition
of all the faculties of Nature. If, however, one considers that it
has not yet touched upon any of the parts of the animal (I mean the
stomach, intestines, liver, and the like), and that it has not dealt
with the faculties resident in these, it will seem as though merely
a kind of introduction had been given to the practical parts of our
teaching. For the whole matter is as follows: Genesis, growth, and
nutrition are the first, and, so to say, the principal effects of
Nature; similarly also the faculties which produce these effects-
the first faculties- are three in number, and are the most dominating
of all. But as has already been shown, these need the service both
of each other, and of yet different faculties. Now, these which the
faculties of generation and growth require have been stated. I shall
now say what ones the nutritive faculty requires. 

10. For I believe that I shall prove that the organs which have to
do with the disposal of the nutriment, as also their faculties, exist
for the sake of this nutritive faculty. For since the action of this
faculty is assimilation, and it is impossible for anything to be assimilated
by, and to change into anything else unless they already possess a
certain community and affinity in their qualities, therefore, in the
first place, any animal cannot naturally derive nourishment from any
kind of food, and secondly, even in the case of those from which it
can do so, it cannot do this at once. Therefore, by reason of this
law, every animal needs several organs for altering the nutriment.
For in order that the yellow may become red, and the red yellow, one
simple process of alteration is required, but in order that the white
may become black, and the black white, all the intermediate stages
are needed. So also, a thing which is very soft cannot all at once
become very hard, nor vice versa; nor, similarly can anything which
has a very bad smell suddenly become quite fragrant, nor again, can
the converse happen. 

How, then, could blood ever turn into bone, without having first become,
as far as possible, thickened and white? And how could bread turn
into blood without having gradually parted with its whiteness and
gradually acquired redness? Thus it is quite easy for blood to become
flesh; for, if Nature thicken it to such an extent that it acquires
a certain consistency and ceases to be fluid, it thus becomes original
newly-formed flesh; but in order that blood may turn into bone, much
time is needed and much elaboration and transformation of the blood.
Further, it is quite clear that bread, and, more particularly lettuce,
beet, and the like, require a great deal of alteration, in order to
become blood. 

This, then, is one reason why there are so many organs concerned in
the alteration of food. A second reason is the nature of the superfluities.
For, as we are unable to draw any nourishment from grass, although
this is possible for cattle, similarly we can derive nourishment from
radishes, albeit not to the same extent as from meat; for almost the
whole of the latter is mastered by our natures; it is transformed
and altered and constituted useful blood; but, not withstanding, in
the radish, what is appropriate and capable of being altered (and
that only with difficulty, and with much labour) is the very smallest
part; almost the whole of it is surplus matter, and passes through
the digestive organs, only a very little being taken up into the veins
as blood- nor is this itself entirely utilisable blood. Nature, therefore,
had need of a second process of separation for the superfluities in
the veins. Moreover, these superfluities need, on the one hand, certain
fresh routes to conduct them to the outlets, so that they may not
spoil the useful substances, and they also need certain reservoirs,
as it were, in which they are collected till they reach a sufficient
quantity, and are then discharged. 

Thus, then, you have discovered bodily parts of a second kind, consecrated
in this case to the [removal of the] superfluities of the food. There
is, however, also a third kind, for carrying the pabulum in every
direction; these are like a number of roads intersecting the whole
body. 

Thus there is one entrance- that through the mouth- for all the various
articles of food. What receives nourishment, however, is not one single
part, but a great many parts, and these widely separated; do not be
surprised, therefore, at the abundance of organs which Nature has
created for the purpose of nutrition. For those of them which have
to do with alteration prepare the nutriment suitable for each part;
others separate out the superfluities; some pass these along, others
store them up, others excrete them; some, again, are paths for the
transit in all directions of the utilisable juices. So, if you wish
to gain a thorough acquaintance with all the faculties of Nature,
you will have consider each one of these organs. 

Now in giving an account of these we must begin with those effects
of Nature, together with their corresponding parts and faculties,
which are closely connected with the purpose to be achieved.

11. Let us once more, then, recall the actual purpose for which Nature
has constructed all these parts. Its name, as previously stated, is nutrition, and the definition corresponding to the name is: an assimilation of that which nourishes to that which receives nourishment. And in order that this may come about, we must assume a preliminary process of adhesion, and for that, again, one of presentation. For whenever the juice which is destined to nourish any of the parts of the animal is emitted from the vessels, it is in the first place dispersed all through this part, next it is presented, and next it adheres, and becomes completely assimilated. 

The so-called white [leprosy] shows the difference between assimilation
and adhesion, in the same way that the kind of dropsy which some people
call anasarca clearly distinguishes presentation from adhesion. For,
of course, the genesis of such a dropsy does not come about as do
some of the conditions of atrophy and wasting, from an insufficient
supply of moisture; the flesh is obviously moist enough,- in fact
it is thoroughly saturated,- and each of the solid parts of the body
is in a similar condition. While, however, the nutriment conveyed
to the part does undergo presentation, it is still too watery, and
is not properly transformed into a juice, nor has it acquired that
viscous and agglutinative quality which results from the operation
of innate heat; therefore, adhesion cannot come about, since, owing
to this abundance of thin, crude liquid, the pabulum runs off and
easily slips away from the solid parts of the body. In white [leprosy],
again, there is adhesion of the nutriment but no real assimilation.
From this it is clear that what I have just said is correct, namely,
that in that part which is to be nourished there must first occur
presentation, next adhesion, and finally assimilation proper.

Strictly speaking, then, nutriment is that which is actually nourishing,
while the quasi-nutriment which is not yet nourishing (e.g. matter
which is undergoing adhesion or presentation) is not, strictly speaking,
nutriment, but is so called only by an equivocation. Also, that which
is still contained in the veins, and still more, that which is in
the stomach, from the fact that it is destined to nourish if properly
elaborated, has been called "nutriment." Similarly we call the various
kinds of food "nutriment," not because they are already nourishing
the animal, nor because they exist in the same state as the material
which actually is nourishing it, but because they are able and destined
to nourish it if they be properly elaborated. 

This was also what Hippocrates said, viz., "Nutriment is what is engaged
in nourishing, as also is quasi-nutriment, and what is destined to
be nutriment." For to that which is already being assimilated he gave
the name of nutriment; to the similar material which is being presented
or becoming adherent, the name of quasi-nutriment; and to everything
else- that is, contained in the stomach and veins- the name of destined
nutriment. 

12. It is quite clear, therefore, that nutrition must necessarily
be a process of assimilation of that which is nourishing to that which
is being nourished. Some, however, say that this assimilation does
not occur in reality, but is merely apparent; these are the people
who think that Nature is not artistic, that she does not show forethought
for the animal's welfare, and that she has absolutely no native powers
whereby she alters some substances, attracts others, and discharges
others. 

Now, speaking generally, there have arisen the following two sects
in medicine and philosophy among those who have made any definite
pronouncement regarding Nature. I speak, of course, of such of them
as know what they are talking about, and who realize the logical sequence
of their hypotheses, and stand by them; as for those who cannot understand
even this, but who simply talk any nonsense that comes to their tongues,
and who do not remain definitely attached either to one sect or the
other- such people are not even worth mentioning. 

What, then, are these sects, and what are the logical consequences
of their hypotheses? The one class supposes that all substance which
is subject to genesis and destruction is at once continuous and susceptible
of alteration. The other school assumes substance to be unchangeable,
unalterable, and subdivided into fine particles, which are separated
from one another by empty spaces. 

All people, therefore, who can appreciate the logical sequence of
an hypothesis hold that, according to the second teaching, there does
not exist any substance or faculty peculiar either to Nature or to
Soul, but that these result from the way in which the primary corpuscles,
which are unaffected by change, come together. According to the first-mentioned
teaching, on the other hand, Nature is not posterior to the corpuscles,
but is a long way prior to them and older than they; and therefore
in their view it is Nature which puts together the bodies both of
plants and animals; and this she does by virtue of certain faculties
which she possesses- these being, on the one hand, attractive and
assimilative of what is appropriate, and, on the other, of what is
foreign. Further, she skilfully moulds everything during the stage
of genesis; and she also provides for the creatures after birth, employing
here other faculties again, namely, one of affection and forethought
for offspring, and one of sociability and friendship for kindred.
According to the other school, none of these things exist in the natures
[of living things], nor is there in the soul any original innate idea,
whether of agreement or difference, of separation or synthesis, of
justice or injustice, of the beautiful or ugly; all such things, they
say, arise in us from sensation and through sensation, and animals
are steered by certain images and memories. 

Some of these people have even expressly declared that the soul possesses
no reasoning faculty, but that we are led like cattle by the impression
of our senses, and are unable to refuse or dissent from anything.
In their view, obviously, courage, wisdom, temperance, and self-control
are all mere nonsense, we do not love either each other or our offspring,
nor do the gods care anything for us. This school also despises dreams,
birds, omens, and the whole of astrology, subjects with which we have
dealt at greater length in another work, in which we discuss the views
of Asclepiades the physician. Those who wish to do so may familiarize
themselves with these arguments, and they may also consider at this
point which of the two roads lying before us is the better one to
take. Hippocrates took the first-mentioned. According to this teaching,
substance is one and is subject to alteration; there is a consensus
in the movements of air and fluid throughout the whole body; Nature
acts throughout in an artistic and equitable manner, having certain
faculties, by virtue of which each part of the body draws to itself
the juice which is proper to it, and, having done so, attaches it
to every portion of itself, and completely assimilates it; while such
part of the juice as has not been mastered, and is not capable of
undergoing complete alteration and being assimilated to the part which
is being nourished, is got rid of by yet another (an expulsive) faculty.