_Of Chylifaction and Nutrition._
In the preceding we have briefly considered the state of the fluid of
life, and mechanical instruments of their motion in the human machine.
In this we will consider how the whole is supported, and the vital lamp,
from time to time, becomes supplied.
Under this consideration we will examine the canal of food, and the
various changes the morsel undergoes from the time we take it in our
mouths, till the nutriment is reduced to blood, and the remaining dross
is expelled the body as useless.
The animal functions towards _chylifaction_ and _nutrition_, are
_mastication_, or chewing the _food_; _deglutition_, or swallowing;
_digestion_; _chylifaction_; _nutrition_; and the _excretion_ of the
_fæces_.
In order therefore to understand this clearly, it becomes necessary to
describe the organs which nature has formed for the requisite
performances thereof.
In the first place then, _smelling_ is that sensation which nature has
given to every animal, that has a choice of food, as the first
safe-guard to inform it of any thing agreeable or disagreeable, useful
or pernicious to its body; its seat is in the nose chiefly, but we find
by experience, that it has a friendly connection with our palate and
stomach; for the effluvia of any thing will either create a desire, or
give us the greatest aversion to every substance that throws out a
flavour.
It is performed by means of a subtile spreading of nerves, peculiarly
delicate; which continues through the membrane of the nose, the roof of
the mouth, gullet, and the very stomach.
_Taste_ is the next sensation, which nature has given us, not only as a
distinguisher of proper food, but a sensation from which we receive many
luxurious pleasures; and to the indulgence of this sensation, most evils
and plagues to mankind, take their origin.
The tongue is the principal instrument of that peculiar quality; but if
we examine somewhat closer into this affair, we shall find that the soul
of pleasure and pain of that sensation, as well as that of smelling, has
its seat in the stomach; for that which will taste pleasing and good at
the first approach, will soon lose its relish when the stomach is
gratified: and, if any thing tastes disagreeable, the stomach receives
it with reluctance, and will ever incline to discharge it again.
The tongue is an instrument (if I may be allowed the term) very
curiously constructed; it is moved by a variety of muscles, and serves
not only for tasting, but also as a labourer, to shovel and to turn our
meat between our grinders; so that nothing may escape being well
masticated, and intermixed with that fine digesting balsam, the spittle,
in order that it may be easily swallowed.
Besides this, it makes the most requisite instrument for the noble and
excellent faculty of speaking; which forms one of the principal
characteristics that distinguishes man from the brute creation.
The gullet or _oesophagus_, is the canal which conveys drink and food
from the mouth to the stomach; this canal is a muscular, tendinous, and
vascular tunic.
The commencement of it is in the mouth, and is called the _pharynx_; a
curious structure, that receives the food, and by its contractive
motion, and the help of the tongue, forces the aliment into the stomach.
The stomach is much like the bag of a Scotch bag-pipe; it lies
immediately under the _diaphragm_ or _midriff_, covered partly on the
right side with the liver, and on the left side with the spleen. The
left and superior part, is continued with the _oesophagus_; and the
right and inferior part, or orifice, commences the _intestines_.
The first orifice is called the mouth of the stomach; and the second the
_pylorus_, or _porter_: At the porter there is a curious valve which
lets the aliment out by small parcels into the intestines, where it
undergoes its various other changes.
The stomach has three teguments, a _muscular_, a _tendinous_, and
_nervous coat_; this _nervous coat_ has another slimy one, but this in
reality, is a delicate lining, interwoven with nerves, and the
ramifications of fine blood vessels.
The _intestines_ or guts, are a continuation of the stomach, they are a
canal which is generally reckoned six times as long as the subject it is
taken from; it is distinguished in small _tenuia_, and wide _crassa_.
Each again is divided into three parts: the _tenuia_, or small narrow
intestines, are the _duodenum_, or twelve finger-gut; the _jejunum_; the
_ileum_: The wide or _crassa_, is divided into the _cæcum_; the _colon_;
and the _rectum_.
Throughout the whole canal of intestines are numbers of little vessels,
called _lacteals_, which lead the chyle, extracted from the aliment,
into a _receptacle_, which is lodged in the _mesentery_, and from
thence, by another duct call’d the _thoracic duct_, is carried along the
back-bone upwards, and joins to the left subclavian vein, where the
_chyle_ gradually commences to be blood.
This short description we will let suffice, and now enter upon the
action itself.
The morsel now, which is designed for food, is taken into the mouth,
masticated with the teeth, turned about with the tongue; and as the
mouth is at work, the _saliva_ or spittle is squeezed from the salival
glands, and thus intermixed with the aliment; when enough chewed and
moistened with this saliva, it is conveyed to the _pharynx_, or swallow,
which receives it, and, by its contraction, forces it into the
_oesophagus_; and by a repeated contraction, is carried down into the
stomach.
There it is again moistened with _saponaceous liquid_, or _pancreatic
juice_; by which and by a perpetual motion of the stomach, it is brought
into a state of digestion; then by small degrees entered through the
_pylorus_ or porter, into the first division of the gut, the _duodenum_.
This gut is about twelve fingers long; and whilst the aliment is there,
it is intermixed with the gall, which is a liquor separated by the
liver, and contained in the gall bladder; this liquor, the gall, is
carried into the _duodenum_, by a small duct, called the _ductus
cysticus_; where also enters another kind of liquor called the
_pancreatic juice_.
When the aliment is thus prepared, and fit for a particular state of
dissolution, it is carried into, and through the _jejunum_. This gut is
in length about twelve or thirteen hands breadth, and its motion
somewhat brisk; through which the aliment passes pretty quick, and
hence, generally is somewhat empty.
As it passes through this part, the chyle is separated from it by the
lacteals, which are small vessels that separate the chyle from the
aliment, and abound there more than in any other part of the gut.
From thence it comes into the _ileum_; that is the longest of all the
divisions of the guts, and is in length about twenty-one hands breadth;
it has a great many circumvolutions, and next to the _jejunum_, has many
lacteals to separate the chyle.
Now the aliment comes into the wide gut, and gradually becomes fæces;
and first, the _cæcum_: This part is rather an appendix only, and hangs
from the main part like a finger to a glove. The use of this gut has
been much controverted by anatomists; it seems however, very probable,
that this appendix is designed to keep the aliment in for further
digestion, as it now begins to putrefy, and becomes fæces or excrement.
From thence it enters the _colon_, which is a long, and very winding
intestine; it runs up along and about the liver, touches the gall
bladder, and the spleen; from thence it descends again to the _os
sacrum_. It has but few lacteals, and is, as it were, the last drainer
of the fæces: It is this intestine which is the seat of the Colic, and
of most other complaints of the belly.
Next to this comes the last and straightest, the _rectum_; this gut is
closely adherent to the _sacrum_, and ends in the fundament; which is
provided with muscles to open and shut the _anus_, in order to contain
the fæces, and discharge it.
The mechanism of chylifaction in the human body differs from the brute
creation in general, except that most contemptible of the whole, the
hog; to which it bears a very near resemblance, insomuch that there is
very little distinction.
Both have that advantage over the generality of terrestrial animals,
that they are confined to no particular food; which favours greatly the
luxury of the one, and the beastiality of the other.
The brute creation are generally distinguished into _carnivorous_ and
_granivorous_: The first is that kind which feeds upon flesh; and the
latter upon grain and vegetables. Upon examination however, we find,
that the stomach and guts are peculiarly adapted to their food; and that
grass agrees no more with the dog, than mutton does with the horse.
But man is so happily made, that any thing which is food, is proper for
him, and he may become used to it; and thus is either carnivorous or
granivorous.
The whole canals, from the stomach to the anus, is in a continual
vermicular motion, which is called the _peristaltic motion_; by this the
aliments are dissolved, and disunited; and as they pass along, are
drained by the lacteals, of their _nutriment_ or _chyle_.
These lacteals are, by means of a membrane (with which they are
surprisingly interwoven, and connected to the whole canal) called the
_mesentery_, lead regularly into one common cistern, lodged almost in
the middle of the intestines, in that membrane, called the _receptacle
of chyle_; and from thence the _chyle_ is carried by a duct up along the
back-bone, called the _ductus thoracicus_, into the left _subclavian
vein_, where it gradually commences to be blood.
By this mechanism we are nourished, and the substance of our food
converted into blood, and transported through the whole animal machine,
for the support of every part of its wonderful composition.
As all animals which feed upon flesh, are more subject to diseases,
nature has provided them with these advantages: that when any thing is
obnoxious to their nature, and received into their stomach, or their
being over loaded, it can discharge itself of so troublesome a burthen,
by vomiting, which is effected thus: when the inner coat of the stomach,
which is irritable and nervous, is stimulated by whatever is obnoxious,
it will cause in the whole stomach, a contraction; and by that, force
its contents to the shortest direction of evacuation, namely, by the
canal of the oesophagus, through the mouth. This expulsion is peculiar
to carnivorous animals only.
Purging or discharging by the fundament is common to all animals of
whatever kind; and is performed by an irritation in the intestines, by
which the peristaltic motion is increased; to this I must add a
reversion of the secretion of the lacteal vessels, by which the humours
are increased, the motion accelerated, and the fæces discharged, without
giving any nourishment to the body, and consequently the system
diminished.
Digestion is that act by which the aliment or food is prepared, so as to
produce a good chyle, and consequently good blood, for the nourishment
of the body. Though no animal has a more delicate stomach than man, yet
it must be observed, that none has a stomach better adapted for all
kinds of food.
Whence therefore in the common course of life, temperance and gentle
exercise is what nature requires to maintain health. But nothing becomes
more obnoxious to that blessing, than gluttony, voluptuousness, and
idleness.
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