869. _Why do we eat food?_
Because the atoms of which our bodies are composed are _continually
changing_. Those atoms that have fulfilled the purposes of nature are
removed from the system, and, therefore, new matter must be introduced
to supply their place.
870. _Why do we eat animal and vegetable food?_
Because their substances are composed of _oxygen_, _hydrogen_,
_carbon_, and _nitrogen_--the four chemical elements of which the human
system is formed. They are, therefore, capable of nourishing the body,
after undergoing digestion.
871. _Why do we masticate our food?_
Because mastication is _the first process towards the digestion of
food_. Before animal or vegetable substances can nourish us, their
condition must be entirely changed, their _organic_ states must be
dissolved, and they must become simple matter, in a homogeneous mass,
consisting of the four chemical elements necessary to nutrition, and
they must again be restored to an organic condition.
872. _Why does saliva enter the mouth when we are eating?_
Because, in addition to the _mechanical_ grinding of the food by the
action of the teeth, it is necessary that it should undergo certain
chemical modifications to adapt it to our use. There are placed,
therefore, in various parts of the body, _glands_, which secrete
peculiar fluids, that have a chemical influence upon the food.
The first of these glands are the _salivary glands of the mouth_, which
pour out a clear watery fluid upon the food we eat, and which fluid has
been found to possess a property which contributes to the digestion of
food.
The moisture afforded by the salivary secretion is also necessary to
enable us to swallow the food.
[Verse: "And the Lord said unto him, Who hath made man's mouth?
or who maketh the dumb, or the seeing, or the blind? have not I the
Lord?"--EXODUS IV.]
873. _Why does the salivary juice enter the mouth just at the moment
that we are eating?_
Because the glands, which are buried in the muscles of the mouth, and
which in their form are much like bunches of currants, are always full
of salivary secretion. There are nerves which are distributed from the
brain to these glands, and when other nerves which belong to the senses
of taste, of sight, or of feeling, are excited by the presence of food,
_a stimulus_ is imparted to the salivary glands, through the nerves
that surround them, their cells collapse, and the juice which they
contain is poured out through their stems, or ducts, into the mouth.
874. _How do we know that impressions imparted to one set of nerves,
may be imparted to another set, so as to put any particular organ in
action._
Because very frequently _the mere sight_ of rich fruit, or acid
substances, _will cause the saliva to flow freely_. In this case it
is evident that the salivary glands _could not see or know_ that such
substances were present. An impression must, therefore, be made upon
the brain, _through the organ of vision_, and the desire to taste the
substances being awakened, a nervous stimulus is _imparted to the
glands of the mouth_, and they at once commence their action, _as if
food were present_.
875. _Why does food descend into the stomach?_
Because, after the teeth, the tongue, and the muscles of the mouth
generally, have rolled the food into a soft bolus, it is conveyed to
the back of the mouth, where it is set upon the opening of the throat
(_oesophagus_). It does not then descend through the throat by its own
gravity, because the throat is generally in a compressed or collapsed
state, like an empty tube; and we know that persons can eat or drink
when with their heads downwards. The oesophagus is formed of a number
of muscular threads, or rings, and _each little thread is like a hand
ready to grasp at the morsel that is coming_. As soon as the bolus is
presented at the top of the throat, these little muscular hands lay
hold of it, and transmit it downward, passing it from one to another,
until it is conveyed through the long passage, to the door of the
stomach, which it enters.
[Verse: "Remove far from me poverty and lies; give me neither
poverty nor riches; feed me with food convenient for me."--PROVERBS
XXX.]
[Illustration: Fig. 49.--SECTION OF THE STOMACH, &c.]
A. The inner coat of the _stomach_. (The stomach is here
represented cut through its length, so that we can see its inside.)
B. The lower extremity of the throat, or _oesophagus_, through
which food enters the stomach.
C. The passage out of the stomach, called the _pylorus_, where a
muscular contraction prevents the escape of undigested food.
D. The _duodenum_, and the ducts through which the _bile_ and
_pancreatic_ juices enter and mingle with our food.
876. _Why do we not feel the food being transmitted through the throat?_
Because the nerves of the body differ in their powers: some are
nerves of _feeling_, some of _motion_, and others are nerves of the
_senses_. The nerves of feeling are most abundantly distributed to
those parts _where feeling is most useful and necessary to us_. But the
faculty of feeling our food undergoing digestion would be no service
to us whatever; therefore the nerves of _motion_ are plentifully
distributed to the throat and stomach, but very few of the nerves of
_feeling_--just as many as will tell us when we eat anything _too hot_,
or _too cold_, or that the stomach is _out of order_.
877. _Why do we feel uneasy after eating to excess?_
Because the stomach is _distended_, and presses upon the other organs
by which it is surrounded.
[Verse: "Who satisfieth thy mouth with good things; so that thy
youth is renewed like the eagles."--PSALM CIII.]
878. _Why do we feel drowsy after eating heartily?_
Because, while the stomach is in action, _a great proportion of the
blood of the body is drawn towards it_, and as the blood is withdrawn
from the other parts of the body, they fall into a state of languor.
879. _Why does blood flow more freely to the stomach during digestion?_
Because the energy of an organ is _increased by the flow of blood_,
which supplies the _material_ of which our organs are composed, and in
which the _vital essence_, supporting life, resides.
880. _Why does excess in eating bring on indigestion?_
Because the power of the stomach to digest food is _governed by the
amount of food required by the system_. It seems to be an instinct of
the stomach to hold back food which is in excess, and by indications
of pain and disturbance to warn its master that _excess has been
committed_.
881. _Why is food digested in the stomach?_
Because it enters the stomach in the form of a paste, produced by the
action of the mouth; and directly food enters, the _gastric juice_,
which is formed by glands embedded in the coats of the stomach,
trickles down its sides. This is a more _powerful solvent_ than the
salivary juice--it is like the same kind of fluid, only much stronger,
and it soon turns the food from a rough and crude _paste_ into a
_greyish cream_ (chyme). The heat of the stomach assists the operation,
and the muscular threads of the coats move the cream along, in the same
manner that the muscles of the oesophagus brought down the food.
The cream is passed towards the door which leads outward from the
stomach (_pylorus_); but if, in the midst of the cream, there are any
undissolved particles of food, it closes upon them, and they return
again to the stomach to be further changed.
[Verse: "When thou hast eaten and art full, then thou shalt bless
the Lord thy God for the good land which he hath given thee."--DEUT.
VIII.]
882. _Why does indigestion bring on bilious attacks?_
Because the _liver_ secretes a fluid to assist in the digestion of
food. The liver is a gland--a similar organ to the glands of the
mouth--and it forms _bile_ in the same manner that they form the
salivary juice. Only the liver is a _much larger gland_, and a much
greater quantity of blood passes through it. The liver pours its
secretion into the biliary duct (Fig. 49) to mix with the grey cream
as it passes onward, and to further dissolve it. But when the stomach
is excited by food which it cannot dissolve, and when the owner of the
stomach, disregarding its remonstrances, will persist in over-eating,
or in eating things that disagree with the system, then _the liver
and the stomach sympathise_, and the muscular threads, or hands, that
prevail all through the alimentary organs, instead of moving _onward_,
move backward, and _throw some bile into the stomach_ to assist to
dissolve and remove the excessive or improper food.