The organs of digestion are adapted to the work of dissolving the foods by
both their structure and arrangement. Most of them consist either of tubes
or cavities and these are so connected, one with the other, as to form a
continuous passageway entirely through the body. This passageway is known
as
*The Alimentary Canal. *--The alimentary canal has a length of about thirty
feet and, while it begins at the mouth, all but about eighteen inches of
it is found in the abdominal cavity. On account of its length it lies for
the most part in coils, the two largest ones being known as the small
intestine and the large intestine. Connected with the alimentary canal are
the glands that supply the liquids for acting on the food. The divisions
of the canal and most of the glands that empty liquids into it are shown
in Fig. 63 and named in the table below:
[Table]
*Coats of the Alimentary Canal.*--The walls of the alimentary canal, except
at the mouth, are distinct from the surrounding tissues and consist in
most places of at least three layers, or coats, as follows:
[Fig. 63]
Fig. 63--*Diagram of the digestive system.* 1. Mouth. 2. Soft palate. 3.
Pharynx. 4. Parotid gland. 5. Sublingual gland. 6. Submaxillary gland. 7.
Esophagus. 8. Stomach. 9. Pancreas. 10. Vermiform appendix. 11. Caecum. 12.
Ascending colon. 13. Transverse colon. 14. Descending colon. 15. Sigmoid
flexure. 16. Rectum. 17. Ileo-caecal valve. 18. Duct from liver and
pancreas. 19. Liver.
Diagram does not show comparative length of the small intestine.
1. An _inner coat_, or lining, known as the mucous membrane. This membrane
is not confined to the alimentary canal, but lines, as we have seen, the
different air passages. It covers, in fact, all those internal surfaces of
the body that connect with the external surface. It derives its name from
the substance which it secretes, called _mucus_. In structure it resembles
the skin, being continuous with the skin where cavities open to the
surface. It is made up of two layers--a thick underlayer which contains
blood vessels, nerves, and glands, and a thin surface layer, called the
_epithelium._ The epithelium, like the cuticle, is without blood vessels,
nerves, or glands.
2. A _middle coat_, which is muscular and which forms a continuous layer
throughout the canal, except at the mouth. (Here its place is taken by the
strong muscles of mastication which are separate and distinct from each
other.) As a rule the muscles of this coat are involuntary. They surround
the canal as thin sheets and at most places form two distinct layers. In
the inner layer the fibers encircle the canal, but in the outer layer they
run longitudinally, or lengthwise, along the canal.(57)
3. An _outer_ or _serous coat_, which is limited to those portions of the
canal that occupy the abdominal cavity. This coat is not found above the
diaphragm. It is a part of the lining membrane of the cavity of the
abdomen, called
[Fig. 64]
Fig. 64--*Diagram of the peritoneum.* 1. Transverse colon. 2. Duodenum. 3.
Small intestine. 4. Pancreas.
*The Peritoneum.*--The peritoneum is to the abdominal cavity what the
pleura is to the thoracic cavity. It forms the outer covering for the
alimentary canal and other abdominal organs and supplies the inner lining
of the cavity itself. It is also the means of holding these organs in
place, some of them being suspended by it from the abdominal walls (Fig.
64). By the secretion of a small amount of liquid, it prevents friction of
the parts upon one another.
*Digestive Glands.*--The glands which provide the different fluids for
acting on the foods derive their constituents from the blood. They are
situated either in the mucous membrane or at convenient places outside of
the canal and pass their liquids into it by means of small tubes, called
ducts. In the canal the food and the digestive fluids come in direct
contact--a condition which the dissolving processes require. Each kind of
fluid is secreted by a special kind of gland and is emptied into the canal
at the place where it is needed.
*The Digestive Processes.*--Digestion is accomplished by acting upon the
food in different ways, as it is passed along the canal, with the final
result of reducing it to the form of a solution. Several distinct
processes are necessary and they occur in such an order that those
preceding are preparatory to those that follow. These processes are known
as _mastication, insalivation, deglutition, stomach digestion_, and
_intestinal_ digestion. As the different materials become liquefied they
are transferred to the blood, and substances not reduced to the liquid
state are passed on through the canal as waste. The first two of the
digestive processes occur in
*The Mouth.*--This is an oval-shaped cavity situated at the very beginning
of the canal. It is surrounded by the lips in front, by the cheeks on the
sides, by the hard palate above and the soft palate behind, and by the
tissues of the lower jaw below. The mucous membrane lining the mouth is,
soft and smooth, being covered with flat epithelial cells. The external
opening of the mouth is guarded by the lips, and the soft palate forms a
_movable_ partition between the mouth and the pharynx. In a condition of
repose the mouth space is practically filled by the teeth and the tongue,
but the cavity may be enlarged and room provided for food by depressing
the lower jaw.
The mouth by its construction is well adapted to carrying on the processes
of mastication and insalivation. By the first process the solid food is
reduced, by the cutting and grinding action of the teeth, to a finely
divided condition. By the second, the saliva becomes mixed with the food
and is made to act upon it.
[Fig. 65]
Fig. 65--*The teeth.* _A._ Section of a single molar. 1. Pulp. 2. Dentine.
3. Enamel. 4. Crown. 5. Neck. 6. Root. _B._ Teeth in position in lower
jaw. 1. Incisors. 2. Canine. 3. Biscuspids. 4. Molars. _C._ Upper and
lower teeth on one side. 1. Incisors. 2. Canines. 3. Biscuspids. 4.
Molars. 5. Wisdom. _D._ Upper and lower incisor, to show gliding contact.
*Accessory Organs of the Mouth.*--The work of mastication and insalivation
is accomplished through organs situated in and around the mouth cavity.
These comprise:
1. _The Teeth._--The teeth are set in the upper and lower jaws, one row
directly over the other, with their hardened surfaces facing. In reducing
the food, the teeth of the lower jaw move against those of the upper,
while the food is held by the tongue and cheeks between the grinding
surfaces. The front teeth are thin and chisel-shaped. They do not meet so
squarely as do the back ones, but their edges glide over each other, like
the blades of scissors--a condition that adapts them to cutting off and
separating the food (_D_, Fig. 65). The back teeth are broad and
irregular, having surfaces that are adapted to crushing and grinding.
Each tooth is composed mainly of a bone-like substance, called _dentine_,
which surrounds a central space, containing blood vessels and nerves,
known as the _pulp cavity_. It is set in a depression in the jaw where it
is held firmly in place by a bony substance, known as _cement_. The part
of the tooth exposed above the gum is the _crown_, the part surrounded by
the gum is the _neck_, and the part which penetrates into the jaw is the
_root_ (_A_, Fig. 65). A hard, protective material, called _enamel_,
covers the exposed surface of the tooth.
The teeth which first appear are known as the _temporary_, or milk, teeth
and are twenty in number, ten in each jaw. They usually begin to appear
about the sixth month, and they disappear from the mouth at intervals from
the sixth to the thirteenth year. As they leave, teeth of the second, or
_permanent_, set take their place. This set has thirty-two teeth of four
different kinds arranged in the two jaws as follows:
In front, above and below, are four chisel-shaped teeth, known as the
_incisors_. Next to these on either side is a tooth longer and thicker
than the incisors, called the _canine_. Back of these are two short,
rounded and double pointed teeth, the _bicuspids_, and back of the
bicuspids are three heavy teeth with irregular grinding surfaces, called
the _molars_ (_B_ and _C_, Fig. 65). Since the molar farthest back in each
jaw is usually not cut until maturity, it is called a _wisdom_ tooth. The
molars are known as the superadded permanent teeth because they do not
take the place of milk teeth, but form farther back as the jaw grows in
length.
[Fig. 66]
Fig. 66--*Diagram* showing directions of muscular fibers in tongue.
2. _The Tongue._--The tongue is a muscular organ whose fibers extend
through it in several directions (Fig. 66). Its structure adapts it to a
variety of movements. During mastication the tongue transfers the food
from one part of the mouth to another, and, with the aid of the cheeks,
holds the food between the rows of teeth. (By an outward pressure from the
tongue and an inward pressure from the cheek the food is kept between the
grinding surfaces.) The tongue has functions in addition to these and is a
most useful organ.
3. _The Muscles of Mastication._--These are attached to the lower jaw and
bring about its different movements. The _masseter_ muscles, which are the
heavy muscles in the cheeks, and the _temporal_ muscles, located in the
region of the temples, raise the lower jaw and supply the force for
grinding the food. Small muscles situated below the chin depress the jaw
and open the mouth.
[Fig. 67]
Fig. 67--*Salivary glands* and the ducts connecting them with the mouth.
4. _The Salivary Glands._--These glands are situated in the tissues
surrounding the mouth, and communicate with it by means of ducts (Fig.
67). They secrete the saliva. The salivary glands are six in number and
are arranged in three pairs. The largest, called the _parotid_ glands,
lie, one on either side, in front of and below the ears. A duct from each
gland passes forward along the cheek until it opens in the interior of the
mouth, opposite the second molar tooth in the upper jaw. Next in size to
the parotids are the _submaxillary_ glands. These are located, one on
either side, just below and in front of the triangular bend in the lower
jaw. The smallest of the salivary glands are the _sublingual_. They are
situated in the floor of the mouth, on either side, at the front and base
of the tongue. Ducts from the submaxillary and sublingual glands open into
the mouth below the tip of the tongue.
*The Saliva and its Uses.*--The saliva is a transparent and somewhat slimy
liquid which is slightly alkaline. It consists chiefly of water (about 99
per cent), but in this are dissolved certain salts and an active chemical
agent, or enzyme, called _ptyalin_, which acts on the starch. The ptyalin
changes starch into a form of sugar (maltose), while the water in the
saliva dissolves the soluble portions of the food. In addition to this the
saliva moistens and lubricates the food which it does not dissolve, and
prepares it in this way for its passage to the stomach. The last is
considered the most important use of the saliva, and dry substances, such
as crackers, which require a considerable amount of this liquid, cannot be
eaten rapidly without choking. Slow mastication favors the secretion and
action of the saliva.
*Deglutition.*--Deglutition, or swallowing, is the process by which food is
transferred from the mouth to the stomach. Though this is not, strictly
speaking, a digestive process, it is, nevertheless, necessary for the
further digestion of the food. Mastication and insalivation, which are
largely mechanical, prepare the food for certain chemical processes by
which it is dissolved. The first of these occurs in the stomach and to
this organ the food is transferred from the mouth. The chief organs
concerned in deglutition are the tongue, the pharynx, and the esophagus.
*The Pharynx* is a round and somewhat cone-shaped cavity, about four and
one half inches in length, which lies just back of the nostrils, mouth,
and larynx. It is remarkable for its openings, seven in number, by means
of which it communicates with other cavities and tubes of the body. One of
these openings is into the mouth, one into the esophagus, one into the
larynx, and one into each of the nostrils, while two small tubes (the
eustachian) pass from the upper part of the pharynx to the middle ears.
The pharynx is the part of the food canal that is crossed by the
passageway for the air. To keep the food from passing out of its natural
channel, the openings into the air passages have to be carefully guarded.
This is accomplished through the soft palate and epiglottis, which are
operated somewhat as valves. The muscular coat of the pharynx is made up
of a series of overlapping muscles which, by their contractions, draw the
sides together and diminish the cavity. The mucous membrane lining the
pharynx is smooth, like that of the mouth, being covered with a layer of
flat epithelial cells.
*The Esophagus*, or gullet, is a tube eight or nine inches long,
connecting the pharynx with the stomach. It lies for the most part in the
thoracic cavity and consists chiefly of a thick mucous lining surrounded
by a heavy coat of muscle. The muscular coat is composed of two layers--an
inner layer whose fibers encircle the tube and an outer layer whose fibers
run lengthwise.
*Steps in Deglutition.*--The process of deglutition varies with the kind of
food. With bulky food it consists of three steps, or stages, as follows:
1. By the contraction of the muscles of the cheeks, the food ball, or
bolus, is pressed into the center of the mouth and upon the upper surface
of the tongue. Then the tongue, by an upward and backward movement, pushes
the food under the soft palate and into the pharynx.
2. As the food passes from the mouth, the pharynx is drawn up to receive
it. At the same time the soft palate is pushed upward and backward,
closing the opening into the upper pharynx, while the epiglottis is made
to close the opening into the larynx. By this means all communication
between the food canal and the air passages is temporarily closed. The
upper muscles of the pharynx now contract upon the food, forcing it
downward and into the esophagus.
3. In the esophagus the food is forced along by the successive
contractions of muscles, starting at the upper end of the tube, until the
stomach is reached.
Swallowing is doubtless aided to some extent by the force of gravity. That
it is independent of this force, however, is shown by the fact that one
may swallow with the esophagus in a horizontal position, as in lying down.
[Fig. 68]
Fig. 68--*Gastric Glands.* _A._ Single gland showing the two kinds of
secreting cells and the duct where the gland opens on to the surface. _B._
Inner surface of stomach magnified. The small pits are the openings from
the glands.
*The Stomach.*--The stomach is the largest dilatation of the alimentary
canal. It is situated in the abdominal cavity, immediately below the
diaphragm, with the larger portion toward the left side. Its connection
with the esophagus is known as the _cardiac orifice_ and its opening into
the small intestine is called the _pyloric orifice_. It varies greatly in
size in different individuals, being on the average from ten to twelve
inches at its greatest length, from four to five inches at its greatest
width, and holding from three to five pints. It has the coats common to
the canal, but these are modified somewhat to adapt them to its work.
_The mucous membrane_ of the stomach is thick and highly developed. It
contains great numbers of minute tube-shaped bodies, known as the _gastric
glands_ (Fig. 68). These are of two general kinds and secrete large
quantities of a liquid called the gastric juice. When the stomach is
empty, the mucous membrane is thrown into folds which run lengthwise over
the inner surface. These disappear, however, when the walls of the stomach
are distended with food.
_The muscular coat_ consists of _three_ separate layers which are named,
from the direction of the fibers, the circular layer, the longitudinal
layer, and the oblique layer (Fig. 69). The circular layer becomes quite
thick at the pyloric orifice, forming a distinct band which serves as a
valve.
[Fig. 69]
Fig. 69--*Muscles of the stomach* (from Morris' _Human Anatomy_). The layer
of Longitudinal fibers removed.
The outer coat of the stomach, called the _serous coat_, is a continuation
of the peritoneum, the membrane lining the abdominal cavity.
*Stomach Digestion.*--In the stomach begins the definite work of dissolving
those foods which are insoluble in water. This, as already stated, is a
double process. There is first a chemical action in which the insoluble
are changed into soluble substances, and this is followed immediately by
the dissolving action of water. The chief substances digested in the
stomach are the proteids. These, in dissolving, are changed into two
soluble substances, known as _peptones_ and _proteoses_. The digestion of
the proteids is, of course, due to the
*Gastric Juice.*--The gastric juice is a thin, colorless liquid composed of
about 99 per cent of water and about 1 per cent of other substances. The
latter are dissolved in the water and include, besides several salts,
three active chemical agents--hydrochloric acid, pepsin, and rennin.
_Pepsin_ is the enzyme which acts upon proteids, but it is able to act
only in an acid medium--a condition which is supplied by the _hydrochloric
acid_. Mixed with the hydrochloric acid it converts the proteids into
peptones and proteoses.
*Other Effects of the Gastric Juice.*--In addition to digesting proteids,
the gastric juice brings about several minor effects, as follows:
1. It checks, after a time, the digestion of the starch which was begun in
the mouth by the saliva.(58) This is due to the presence of the
hydrochloric acid, the ptyalin being unable to act in an acid medium.
2. While there is no appreciable action on the fat itself, the proteid
layers that inclose the fat particles are dissolved away (Fig. 79), and
the fat is set free. By this means the fat is broken up and prepared for a
special digestive action in the small intestine.
3. Dissolved albumin, like that in milk, is curded, or coagulated, in the
stomach. This action is due to the _rennin_. The curded mass is then acted
upon by the pepsin and hydrochloric acid in the same manner as the other
proteids.
4. The hydrochloric acid acts on certain of the insoluble mineral salts
found in the foods and reduces them to a soluble condition.
5. It is also the opinion of certain physiologists that cane sugar and
maltose (double sugars) are converted by the hydrochloric acid into
dextrose and levulose (single sugars).
After a variable length of time, the contents of the stomach is reduced to
a rather uniform and pulpy mass which is called _chyme_. Portions of this
are now passed at intervals into the small intestine.
*Muscular Action of the Stomach.*--The muscles in the walls of the stomach
have for one of their functions the mixing of the food with the gastric
juice. By _alternately_ contracting and relaxing, the different layers of
muscle keep the form of the stomach changing--a result which agitates and
mixes its contents. This action varies in different parts of the organ,
being slight or entirely absent at the cardiac end, but quite marked at
the pyloric end.
Another purpose of the muscular coat is to empty the stomach into the
small intestine. During the greater part of the digestive period the
muscular band at the pyloric orifice is contracted. At intervals, however,
this band relaxes, permitting a part of the contents of the stomach to be
forced into the small intestine. After the discharge the pyloric muscle
again contracts, and so remains until the time arrives for another
discharge.
In addition to emptying the stomach into the small intestine, these
muscles also aid in emptying the organ upward and through the esophagus
and mouth, should occasion require. Vomiting in case of poisoning, or if
the food for some reason fails to digest, is a necessary though unpleasant
operation. It is accomplished by the contraction of all the muscles of the
stomach, together with the contraction of the walls of the abdomen. During
these contractions the pyloric valve is closed, and the muscles of the
esophagus and pharynx are in a relaxed condition.(59)
[Fig. 70]
Fig. 70--*Passage from stomach* into small intestine. Illustration also
shows arrangement of mucous membrane in the two organs. _D._ Bile duct.
*The Small Intestine.*--This division of the alimentary canal consists of a
coiled tube, about twenty-two feet in length, which occupies the central,
lower portion of the abdominal cavity (Fig. 71). At its upper extremity it
connects with the pyloric end of the stomach (Fig. 70), and at its lower
end it joins the large intestine. It averages a little over an inch in
diameter, and gradually diminishes in size from the stomach to the large
intestine. The first eight or ten inches form a short curve, known as the
_duodenum_. The upper two fifths of the remainder is called the _jejunum_,
and the lower three fifths is known as the _ileum_. The ileum joins that
part of the large intestine known as the caecum, and at their place of
union is a marked constriction which prevents material from passing from
the large into the small intestine (Fig. 73). This is known as the
_ileo-caecal valve_.
_The mucous membrane_ of the small intestine is richly supplied with blood
vessels and contains glands that secrete a digestive fluid known as the
_intestinal juice_. The membrane is thrown into many transverse, or
circular, folds which increase its surface and also prevent materials from
passing too rapidly through the intestine. One important respect in which
the small intestine differs from all other portions of the food canal is
that its surface is covered with great numbers of minute elevations known
as the villi. The purpose of these is to aid in the absorption of the
nutrients as they become dissolved (Chapter XI).
_The muscular coat_ of the small intestine is made up of two distinct
layers--the inner layer consisting of circular fibers and the outer of
longitudinal fibers. These muscles keep the food materials mixed with the
juices of the small intestine, but their main purpose is to force the
materials undergoing digestion through this long and much-coiled tube.
The outer, or _serous_, coat of the small intestine, like that of the
stomach, is an extension from the general lining of the abdominal cavity,
or peritoneum. In fact, the intestine lies in a fold of the peritoneum,
somewhat as an arm in a sling, while the peritoneum, by connecting with
the back wall of the abdominal cavity, holds this great coil of digestive
tubing in place (Fig. 64). The portion of the peritoneum which attaches
the intestine to the wall of the abdomen is called the _mesentery_.
Most of the liquid acting on the food in the small intestine is supplied
by two large glands, the liver and the pancreas, that connect with it by
ducts.
[Fig. 71]
Fig. 71--*Abdominal cavity* with organs of digestion in position.
*The Liver* is situated immediately below the diaphragm, on the right side
(Figs. 71 and 72), and is the largest gland in the body. It weighs about
four pounds and is separated into two main divisions, or lobes. It is
complex in structure and differs from the other glands in several
particulars. It receives blood from two distinct sources--the portal vein
and the hepatic artery. _The portal vein_ collects the blood from the
stomach, intestines, and spleen, and passes it to the liver. This blood is
loaded with food materials, but contains little or no oxygen. The _hepatic
artery_, which branches from the aorta, carries to the liver blood rich in
oxygen. In the liver the portal vein and the hepatic artery divide and
subdivide, and finally empty their blood into a single system of
capillaries surrounding the liver cells. These capillaries in turn empty
into a single system of veins which, uniting to form the _hepatic veins_
(two or three in number), pass the blood into the inferior vena cava (Fig.
72).
[Fig. 72]
Fig. 72--*Relations of the liver.* Diagram showing the connection of the
liver with the large blood vessels and the food canal.
The liver secretes daily from one to two pounds of a liquid called _bile_.
A reservoir for the bile is provided by a small, membranous sack, called
the _gall bladder_, located on the underside of the liver. The bile passes
from the gall bladder, and from the right and left lobes of the liver, by
three separate ducts. These unite to form a common tube which, uniting
with the duct from the pancreas, empties into the duodenum. Though usually
described as a digestive gland, the liver has other functions of equal or
greater importance (Chapter XIII).
*The Bile* is a golden yellow liquid, having a slightly alkaline reaction
and a very bitter taste. It consists, on the average, of about 97 per cent
of water and 3 per cent of solids.(60) The solids include bile pigments,
bile salts, a substance called cholesterine, and mineral salts. The
pigments (coloring matter) of the bile are derived from the hemoglobin of
broken-down red corpuscles (page 27).
Much about the composition of the bile is not understood. It is known,
however, to be necessary to digestion, its chief use being to aid in the
digestion and absorption of fats. It is claimed also that the bile aids
the digestive processes in some general ways--counteracting the acid of the
gastric juice, preventing the decomposition of food in the intestines, and
stimulating muscular action in the intestinal walls. No enzymes have been
discovered in the bile.
*The Pancreas* is a tapering and somewhat wedge-shaped gland, and is so
situated that its larger extremity, or head, is encircled by the duodenum.
From here the more slender portion extends across the abdominal cavity
nearly parallel to and behind the lower part of the stomach. It has a
length of six or eight inches and weighs from two to three and one half
ounces. Its secretion, the pancreatic juice, is emptied into the duodenum
by a duct which, as a rule, unites with the duct from the liver.
*The Pancreatic Juice* is a colorless and rather viscid liquid, having an
alkaline reaction. It consists of about 97.6 per cent of water and 2.4 per
cent of solids. The solids include mineral salts (the chief of which is
sodium carbonate) and four different chemical agents, or enzymes,--trypsin,
amylopsin, steapsin, and a milk-curding enzyme. These active constituents
make of the pancreatic juice the most important of the digestive fluids.
It acts with vigor on all of the nutrients insoluble in water, producing
the following changes:
1. It converts the starch into maltose, completing the work begun by the
saliva. This action is due to the _amylopsin_,(61) which is similar to
ptyalin but is more vigorous.
2. It changes proteids into peptones and proteoses, completing the work
begun by the gastric juice. This is accomplished by the _trypsin_, which
is similar to, but more active than, the pepsin.
3. It digests fat. In this work the active agent is the _steapsin_.
The necessity of a milk-curding enzyme, somewhat similar to the rennin of
the gastric juice, is not understood.
*Digestion of Fat.*--Several theories have been proposed at different times
regarding the digestion and absorption of fat. Among these, what is known
as the "solution theory" seems to have the greatest amount of evidence in
its favor. According to this theory, the fat, under the influence of the
steapsin, absorbs water and splits into two substances, recognized as
glycerine and fatty acid. This finishes the process so far as the
glycerine is concerned, as this is soluble in water; but the fatty acid,
which (from certain fats) is insoluble in water,(62) requires further
treatment. The fatty acid is now supposed to be acted on in one, or both,
of the following ways: 1. To be dissolved as fatty acid by the action of
the bile (since bile is capable of dissolving it under certain
conditions). 2. To be converted by the sodium carbonate into a form of
soap which is soluble in water.
The emulsification of fat is known to occur in the small intestine. By
this process the fat is separated into minute particles which are
suspended in water, but not changed chemically, the mixture being known as
an _emulsion_. While this is believed by some to be an actual process of
digestion, the advocates of the solution theory claim that it is a process
accompanying and aiding the conversion of fat into fatty acid and
glycerine.(63)
*The Intestinal Juice* is a clear liquid with an alkaline reaction,
containing water, mineral salts, and certain proteid substances that may
act as enzymes. It assists in bringing about an alkaline condition in the
small intestine and aids in the reduction of cane sugar and maltose to the
simple sugars, dextrose and levulose. Since it is difficult to obtain this
liquid in sufficient quantities for experimenting, its uses have not been
fully determined. Recent investigators, however, assign to it an important
place in the work of digestion.
*Work of the Small Intestine.*--The small intestine is the most important
division of the alimentary canal. It serves as a receptacle for holding
the food while it is being acted upon; it secretes the intestinal juice
and mixes the food with the digestive fluids; it propels the food toward
the large intestine; and, in addition to all this, serves as an organ of
absorption.
Digestion is practically finished in the small intestine, and a large
portion of the reduced food is here absorbed. There is always present,
however, a variable amount of material that is not digested. This,
together with a considerable volume of liquid, is passed into
*The Large Intestine.*--The large intestine is a tube from five to six feet
in length and averaging about one and one half inches in diameter. It
begins at the lower right side of the abdominal cavity, forms a coil which
almost completely surrounds the coil of small intestine, and finally
terminates at the surface of the body (Figs. 2, 71 and 73). It has three
divisions, known as the caecum, the colon, and the rectum.
[Fig. 73]
Fig. 73--*Passage from small into large intestine.* At the ileo-caecal valve
is the narrowest constriction of the food canal.
_The caecum_ is the pouch-like dilatation of the large intestine which
receives the lower end of the small intestine. It measures about two and
one half inches in diameter and has extending from one side a short,
slender, and blind tube, called the _vermiform appendix_. This structure
serves no purpose in digestion, but appears to be the rudiment of an organ
which may have served a purpose at some remote period in the history of
the human race. The caecum gradually blends into the second division of the
large intestine, called the colon.
_The colon_ consists of four parts, described as the ascending colon, the
transverse colon, the descending colon, and the sigmoid flexure, or
sigmoid colon. The first three divisions are named from the direction of
the movement of materials through them and the last from its shape, which
is similar to that of the Greek letter sigma ({~GREEK CAPITAL LETTER SIGMA~}).
_The rectum_ is the last division of the large intestine It is a nearly
straight tube, from six to eight inches in length, and connects with the
external surface of the body.
The general structure of the large intestine is similar to that of the
small intestine, and, like the small intestine, it is held in place by the
peritoneum. It differs from the small intestine, however, in its lining of
mucous membrane and in the arrangement of the muscular coat. The mucous
membrane presents a smooth appearance and has no villi, while the
longitudinal layer of the muscular coat is limited to three narrow bands
that extend along the greater length of the tube (Fig. 74). These bands
are shorter than the coats, and draw the large intestine into a number of
shallow pouches, by which it is readily distinguished from the small
intestine (Fig. 71).
[Fig. 74]
Fig. 74--*Section of large intestine*, showing the coats. 1. Serous coat.
2. Circular layer of muscle. 3. Submucous coat. 4. Mucous membrane. 5.
Muscular bands extending lengthwise over the intestine.
*Work of the Large Intestine.*--The large intestine serves as a receptacle
for the materials from the small intestine. The digestive fluids from the
small intestine continue their action here, and the dissolved materials
also continue to be absorbed. In these respects the work of the large
intestine is similar to that of the small intestine. It does, however, a
work peculiar to itself in that it collects and retains undigested food
particles, together with other wastes, and ejects them periodically from
the canal.
*Work of the Alimentary Muscles.*--The mechanical part of digestion is
performed by the muscles that encircle the food canal. Their uses, which
have already been mentioned in connection with the different organs of
digestion, may be here summarized: They supply the necessary force for
masticating the food. They propel the food through the canal. They mix the
food with the different juices. At certain places they partly or
completely close the passage until a digestive process is completed. They
may even cause a reverse movement of the food, as in vomiting. All of the
alimentary muscles, except those around the mouth, are involuntary. Their
work is of the greatest importance.
*Other Purposes of the Digestive Organs.*--The digestive organs serve other
important purposes besides that of dissolving the foods. They provide
favorable conditions for passing the dissolved material into the blood.
They dispose of such portions of the foods as fail, in the digestive
processes, to be reduced to a liquid state. A considerable amount of waste
material is also separated from the blood by the glands of digestion
(especially the liver), and this is passed from the body with the
undigested portions of food. Then the food canal (stomach in particular)
is a means of holding, or storing, food which is awaiting the processes of
digestion. Considering the number of these purposes, the digestive organs
are remarkably simple, both in structure and in method of operation.
HYGIENE OF DIGESTION
Many of the ills to which flesh is heir are due to improper methods of
taking food and are cured by observing the simple rules of eating. Habit
plays a large part in the process and children should, for this reason, be
taught early to eat properly. Since the majority of the digestive
processes are involuntary and the food, after being swallowed, is
practically beyond control, careful attention must be given to the proper
mastication of the food and to such other phases of digestion as are under
control.
*Necessity for Thorough Mastication.*--Mastication prepares the food for
the digestive processes which follow. Unless the food has been properly
masticated, the digestive fluids in the stomach and intestines cannot act
upon it to the best advantage. When the food is carefully chewed, a larger
per cent of it is actually digested--a point of importance where economy in
the use of food needs to be practiced.
A fact not to be overlooked is that one cannot eat hurriedly and practice
thorough mastication. The food must not be swallowed in lumps, but reduced
to a finely divided and pulpy mass. This requires time. The one who
hurries through the meal is necessarily compelled to bolt his food. Thirty
minutes is not too long to give to a meal, and a longer period is even
better.
Perhaps the most important result of giving plenty of time to the taking
of food is that of _stimulating the digestive glands to a proper degree of
activity_. That both the salivary and gastric glands are excited by the
sight, smell, and thought of food and, through taste, by the presence of
food in the mouth, has been fully demonstrated. Food that is thoroughly
masticated and relished will receive more saliva and gastric juice, and
probably more of other juices, than if hastily chewed and swallowed. This
has a most important bearing upon the efficiency of the digestive
processes.
*Order of Taking Food.*--There has been evolved through experience a rather
definite order of taking food, which our knowledge of the process of
digestion seems to justify. The heavy foods (proteids for the most part)
are eaten first; after which are taken starchy foods and fats; and the
meal is finished off with sweetmeats and pastry.(64) The scientific
arguments for this order are the following:
1. By receiving the first of the gastric flow the proteids can begin
digesting without delay. Since these are the main substances acted on in
the stomach, the time required for their digestion is shortened by eating
them first.
2. Sugar, being of the nature of predigested starch, quickly gets into the
blood and _satisfies the relish_ for food. The result of taking sugar
first may be to cause one to eat less than he needs and to diminish the
activity of the glands.
3. Fat or grease, if taken first, tends to form a coating over the walls
of the stomach and around the material to be digested. This prevents the
juices from getting to and mixing with the foods upon which they are to
act.
4. Starch following the proteids, for the most part, does not so quickly
come in contact with the gastric juice. This enables the ptyalin of the
saliva to continue its action for a longer time than if the starch were
eaten first.
*Liquids during the Meal.*--Liquids as ordinarily taken during the meal are
objectionable. They tend to diminish the secretion of the saliva and to
cause rapid eating. Instead of eating slowly and swallowing the food only
so fast as the glands can supply the necessary saliva, the liquid is used
to wash the food down. Water or other drinks should be taken after the
completion of the meal or when the mouth is completely free from food.
Even then it should be taken in small sips. While the taking of a small
amount of water in this way does no harm, a large volume has the effect of
weakening the gastric juice. Most of the water needed by the body should
be taken between meals.
*The State of Mind* has much to do with the proper digestion of the food.
Worry, anger, fear, and other disturbed mental states are known to check
the secretion of fluids and to interfere with the digestive processes.
While the cultivation of cheerfulness is important for its general
hygienic effects, it is of especial value in relation to digestion.
Intense emotions, either during or following the meal, should if possible
be avoided. The table is no place for settling difficulties or
administering rebuke. The conversation, on the other hand, should be
elevating and joy giving, thereby inducing a desirable reactionary
influence upon the digestive processes.
*Care of the Teeth.*--The natural teeth are indispensable for the proper
mastication of the food. Of especial value are the molars--the teeth that
grind the food. The development of the profession of dentistry has made
possible the preservation of the teeth, even when naturally poor, as long
as one has need of them. To preserve the teeth they must be kept clean.
They should be washed at least once a day with a soft-bristled brush, and
small particles of food, lodged between them, should be removed with a
wooden pick. The biting of hard substances, such as nuts, should be
avoided, on account of the danger of breaking the enamel, although the
chewing of tough substances is considered beneficial.
Decayed places in the teeth should be promptly filled by the dentist. It
is well, even when decayed places are not known to exist, to have the
teeth examined occasionally in order to detect such places before they
become large. On account of the expense, pain, and inconvenience there is
a tendency to put off dental work which one knows ought to be done.
Perhaps in no other instance is procrastination so surely punished. The
decayed places become larger and new points of decay are started; and the
pain, inconvenience, and expense are increased proportionately.
*The Natural Appetite* should be followed with reference to both the kind
and the amount of food eaten. No system of knowledge will ever be devised
which can replace the appetite as an aid in the taking of food. _It is_
_nature's means of indicating the needs of the body_. The natural appetite
may be spoiled, however, by overeating and by the use of highly seasoned
foods, or by indulging in stimulants during the meal. It is spoiled in
children by too free indulgence in sweetmeats. By cultivating the natural
appetite and heeding its suggestions, one has at his command an almost
infallible guide in the taking of food.
*Preparation of Meals.*--The cooking of food serves three important
purposes. It renders the food more digestible, relieving the organs of
unnecessary work; it destroys bacteria that may be present in the food,
diminishing the likelihood of introducing disease germs into the body; and
it makes the food more palatable, thereby supplying a necessary stimulus
to the digestive glands. While the methods employed in the preparation of
the different foods have much to do with the ease with which they are
digested and with their nourishing qualities, the scope of our subject
does not permit of a consideration of these methods.
*Quantity of Food.*--Overeating and undereating are both objectionable from
a hygienic standpoint. Overeating, by introducing an unnecessary amount of
food into the body, overworks the organs of digestion and also the organs
of excretion. It may also lead to the accumulation of burdensome fat and
of harmful wastes. On the other hand, the taking of too little food
impoverishes the blood and weakens the entire body. As a rule, however,
more people eat too much than too little, and to quit eating before the
appetite is fully satisfied is with many persons a necessary precaution.
The power of self-control, valuable in all phases of life, is
indispensable in the avoidance of overeating.
*Frequency of Taking Food.*--Eating between meals is manifestly an
unhealthful practice. The question has also been raised as to whether the
common habit of eating three times a day is best suited to all classes of
people. Many people of weak digestive organs have been benefited by the
plan of two meals a day, while others adopt the plan of eating one heavy
meal and two light ones. Either plan gives the organs of digestion more
time to rest and diminishes the liability of overeating. On the other
hand, those doing heavy muscular work can hardly derive the energy which
they need from less than three good meals a day. Though no definite rule
can be laid down, there is involved a hygienic principle which all should
follow: _Meals should not overlap_. The stomach should be free from food
taken at a previous meal before more is introduced into it. When this
principle is not observed, material ferments in the stomach, causing
indigestion and other disorders. It should be noted, however, that the
overlapping may be due to overeating as well as to eating too frequently.
*Dangers from Impure Food.*--Food is frequently the carrier of disease
germs and for this reason requires close inspection (page 128). Typhoid
fever, a most dangerous disease, is usually contracted through either
impure food or impure water (Chapter XXIII). One safeguard against disease
germs, as stated above, is thorough cooking. Too much care cannot be
exercised with reference to the water for drinking purposes. Water which
is not perfectly clear, which smells of decaying material, or which forms
a sediment on standing is usually not fit to drink. It can, however, be
rendered comparatively harmless by boiling. The objections which many
people have to drinking boiled water are removed when it is boiled the day
before it is used, so as to give it time to cool, settle, and replace the
air driven off by the boiling.
*Care of the Bowels.*--In considering the hygiene of the alimentary canal,
the fact that it is used as a means of separating the impurities from the
body must not be overlooked. Frequently, through lack of exercise,
negligence in evacuating the bowels, or other causes, a weakened condition
of the canal is induced which results in the retention of impurities
beyond the time when they should be discharged. This is a great annoyance
and at the same time a menace to the health.
In most cases this condition can be relieved, and prevented from
recurring, by observing the following habits: 1. Have a regular time each
day for evacuating the bowels. This is a most important factor in securing
the necessary movements. 2. Drink a cup of cold water on rising in the
morning and on retiring at night. 3. Eat generously of fruits and other
coarse foods, such as corn bread, oatmeal, hominy, cabbage, etc. 4.
Practice persistently such exercises as bring the abdominal muscles into
play. These exercises strengthen indirectly the muscles of the canal. 5.
Avoid overwork, especially of the nervous system.
*Alcohol and Digestion.*--Though exciting temporarily a greater flow of the
digestive fluids, alcoholic drinks taken in any but very small quantities
are considered detrimental to the work of digestion. Large doses retard
the action of enzymes, inflame the mucous lining of the stomach,(65) and
bring about a diseased condition of the liver. It may be noted, however,
that the bad effects of alcoholic beverages upon the stomach, the liver,
and the body in general are less pronounced when these are taken as a part
of the regular meals.
*Effects of Tea and Coffee.*--In addition to the stimulating agent
caffeine, tea and coffee contain a bitter, astringent substance, known as
tannin. On account of the tannin these beverages tend to retard digestion
and to irritate the lining of the stomach--effects that may be largely
obviated by methods of preparing tea and coffee which dissolve little of
the tannin. (They should be made without continued boiling or steeping.)
The caffeine may do harm through its stimulating effect upon the nervous
system (page 56) and through the introduction of a special waste into the
body. In chemical composition caffeine closely resembles a waste, called
uric acid, and in the body is converted into this substance. If one is in
a weakened condition, the uric acid may fail to be oxidized to urea, as
occurs normally, or to be thrown off as uric acid. In this case it
accumulates in the body, causing rheumatism and related diseases. It thus
happens that while some people may use tea and coffee without detriment,
others are injured by them.
*Summary.*--The main structure in the digestive system is the alimentary
canal. This provides cavities where important dissolving processes take
place, and tubes for joining these cavities, while glands connecting with
the canal supply the necessary liquids for changing and dissolving the
foods. The general plan of digestion is that of passing the food through
the canal, beginning with the mouth, and of acting on it at various
places, with the final result of reducing most of it to the liquid state.
The digestive fluids supply water which acts as a solvent and carries the
active chemical agents, or enzymes, that convert the insoluble foods into
substances that are soluble. The muscles in the walls of the canal perform
the mechanical work of digestion, while the nervous system controls and
regulates the activity of the various organs concerned in this work.
Exercises.--1. State the general purpose of digestion. How does digested
food differ from that not digested?
2. Name all the divisions of the alimentary canal in the order in which
the food passes through them.
3. What other work besides digestion is carried on by the alimentary
canal?
4. What is gained by the mastication of the food? Why should mastication
precede the other processes of digestion?
5. What is the work of the tongue in digestion?
6. State the purposes served by the gastric juice.
7. Give reasons for regarding the small intestine as the most important
division of the food canal.
8. At what places, and by the action of what liquids, are fats, proteids,
and starch digested?
9. What enzymes are found in the pancreatic juice? What is the digestive
action of each?
10. Describe the work performed by the muscles of the stomach, the mouth,
the esophagus, and the small intestine.
11. What advantages are derived from the use of cooked food?
12. State the advantages of drinking pure water.
13. If all the food that one needs to take at a single meal can be
thoroughly masticated in fifteen minutes, why is it better to spend a
longer time at the table?
14. What is meant by the overlapping of meals? What bad results follow?
How avoided?
PRACTICAL WORK
Examine a dissectible model of the human abdomen (Fig. 75), noting the
form, location, and connection of the different organs. Find the
connection of the esophagus with the stomach, of the stomach with the
small intestine, and of the small intestine with the large intestine.
Sketch a general outline of the cavity, and locate in this outline its
chief organs.
Where it is desirable to learn something of the actual structure of the
digestive organs, the dissection of the abdomen of some small animal is
necessary. On account of unpleasant features likely to be associated with
such a dissection, however, this work is not recommended for immature
pupils.
[Fig. 75]
Fig. 75--Model for demonstrating the abdomen and its contents.
*Dissection of the Abdomen.* (Optional)--For individual study, or for a
small class, a half-grown cat is perhaps the best available material. It
should be killed with chloroform, and then stretched, back downward, on a
board, the feet being secured to hold it in place.
The teacher should make a preliminary examination of the abdomen to see
that it is in a fit condition for class study. If the bladder is
unnaturally distended, its contents may be forced out by slight pressure.
The following materials will be needed during the dissection, and should
be kept near at hand: a sharp knife with a good point, a pair of heavy
scissors, a vessel of water, some cotton or a damp sponge, and some fine
cord. During the dissection the specimen should be kept as clean as
possible, and any escaping blood should be mopped up with the cotton or
the sponge. The dissection is best carried out by observing the following
order:
1. Cut through the abdominal wall in the center of the triangular space
where the ribs converge. From here cut a slit downward to the lower
portion of the abdomen, and sideward as far as convenient. Tack the
loosened abdominal walls to the board, and proceed to study the exposed
parts. Observe the muscles in the abdominal walls, and the fold of the
_peritoneum_ which forms an apron-like covering over the intestines.
2. Observe the position of the stomach, liver, spleen, and intestines, and
then, by pushing the intestines to one side, find the kidneys and the
bladder.
3. Study the liver with reference to its location, size, shape, and color.
On the under side, find the gall bladder, from which a small tube leads to
the small intestine. Observe the portal vein as it passes into the liver.
As the liver is filled with blood, neither it nor its connecting blood
vessels should be cut at this time.
4. Trace out the continuity of the canal. Find the esophagus where it
penetrates the diaphragm and joins the stomach. Find next the union of the
stomach with the small intestine. Then, by carefully following the coils
of the small intestine, discover its union with the large intestine.
5. Within the first coil of the small intestine, as it leaves the stomach,
find the _pancreas_. Note its color, size, and branches. Find its
connection with the small intestine.
6. Beginning at the cut portion of the abdominal wall, lift the thin
lining of the peritoneum and carefully follow it toward the back and
central portion of the abdomen. Observe whether it extends back of or in
front of the kidneys, the aorta, and the inferior vena cava. Find where it
leaves the wall as a _double_ membrane, the _mesentery_, which surrounds
and holds in place the large and small intestines. Sketch a coil of the
intestine, showing the mesentery.
7. Find in the center of the coils of small intestine a long, slender body
having the appearance of a gland. This is the beginning of the _thoracic
duct_ and is called the _receptacle of the chyle_. From this the thoracic
duct rapidly narrows until it forms a tiny tube difficult to trace in a
small animal.
8. Cut away about two inches of the small intestine from the remainder,
having first tied the tube on the two sides of the section removed. Split
it open for a part of its length, and wash out its contents. Observe its
coats. Place it in a shallow vessel containing water, and examine the
mucous membrane with a lens to find the _villi_. Make a drawing of this
section, showing the coats.
9. Study the connection of the small intestine with the large. Split them
open at the place of union, wash out the contents, and examine the
ileo-caecal valve.
10. Observe the size, shape, and position of the kidneys. Do they lie in
front of or back of the peritoneum? Do they lie exactly opposite each
other? Note the connection of each kidney with the aorta and the inferior
vena cava by the renal artery and the renal vein. Find a slender tube, the
_ureter_, running from each kidney to the bladder. Do the ureters connect
with the top or with the base of the bladder? Show by a sketch the
connection of the kidneys with the large blood vessels and the bladder.
*To demonstrate the Teeth.*--Procure from the dentist a collection of
different kinds of teeth, both sound and decayed.
(_a_) Examine external surfaces of different kinds of teeth, noting
general shape, cutting or grinding surfaces, etc. Make a drawing of an
incisor and also of a molar.
(_b_) After soaking some of the teeth for a couple of days in warm water
saw one of them in two lengthwise, and another in two crosswise, and
smooth the cut surfaces with fine emery or sand paper. Examine both kinds
of sections, noting arrangement and extent of dentine, enamel, and pulp.
Make drawings.
(_c_) Examine a decayed tooth. Which substance of the tooth appears to
decay most readily? Why is it necessary to cut away a part of the tooth
before filling?
(_d_) Test the effect of acids upon the teeth by leaving a tooth over
night in a mixture of one part hydrochloric acid to four parts water, and
by leaving a second tooth for a couple of days in strong vinegar. Examine
the teeth exposed to the action of acids, noting results.
*To show the Importance of Mastication.*--Fill two tumblers each half full
of water. Into one put a lump of rock salt. Into the other place an equal
amount of salt that has been finely pulverized. Which dissolves first and
why?
*To illustrate Acid and Alkaline Reactions.*--To a tumbler half full of
water add a teaspoonful of hydrochloric or other acid, as vinegar. To a
second tumbler half full of water add an equal amount of cooking soda.
Taste each liquid, noting the sour taste of the acid, and the alkaline
taste of the soda. Hold a piece of red litmus paper in the soda solution,
noting that it is turned blue. Then hold a piece of blue litmus paper in
the acid solution, noting that it is turned red. Add acid to the soda
solution, and soda to the acid solution, until the conditions are
reversed, testing with the red and blue litmus papers.
Hold, for a minute or longer, a narrow strip of red litmus paper in the
mouth, noting any change in the color of the paper. Repeat, using blue
litmus paper. What effect, if any, has the saliva upon the color of the
papers? Has the mouth an acid or an alkaline reaction?
*To show the Action of Saliva on Starch.*--1 (Optional). Prepare starch
paste by mixing half a teaspoonful of starch in half a pint of water and
heating the mixture to boiling. Place some of this in a test tube and thin
it by adding more water. Then add a small drop of iodine solution (page
136) to the solution of starch. It should turn a deep blue color. This is
the test for starch.
Now collect from the mouth, in a clean test tube, two or three
teaspoonfuls of saliva. Add portions of this to small amounts of fresh
starch solution in two test tubes. Let the tubes stand for five or ten
minutes surrounded by water having about the temperature of the body. Test
for changes that have occurred as follows:
(_a_) To one tube add a little of the iodine solution. If it does not turn
blue, it shows that the starch has been converted into some other
substance by the saliva, (_b_) To the other tube add a few drops of a very
dilute solution of copper sulphate. Then add sodium (or potassium)
hydroxide, a few drops at a time, until the precipitate which first forms
dissolves and turns a deep blue. Then gradually heat the upper portion of
the liquid to boiling. If it turns an orange or yellowish red color, the
presence of a form of sugar (maltose or dextrose) is proved. See page 136.
2. Hold some powdered starch in the mouth until it completely dissolves
and observe that it gradually acquires a sweetish taste. This shows the
change of starch into sugar.
*To illustrate the Action of the Gastric Juice.*--Add to a tumbler two
thirds full of water as much scale pepsin (obtained from a drug store) as
will stay on the end of the large blade of a penknife. Then add enough
hydrochloric acid to give a slightly sour taste. Place in the artificial
gastric juice thus prepared some boiled white of egg which has been finely
divided by pressing it through a piece of wire gauze. Also drop in a
single large lump. Keep in a warm place (about the temperature of the
body) for several hours or a day, examining from time to time. What is the
general effect of the artificial gastric juice upon the egg?
*To illustrate Effect of Alcohol upon Gastric Digestion.*--Prepare a
tumbler half full of artificial gastric juice as in the above experiment,
and add 10 cubic centimeters of this to each of six clean test tubes
bearing labels. To five of the tubes add alcohol from a burette as
follows: (1) .5 c.c., (2) 1 c.c., (3) 1.5 c.c., (4) 2 c.c., and (5) 3
c.c., leaving one tube without alcohol. Now add to each tube about 1/4
gram of finely divided white of egg from the experiment above, and place
all of the tubes in a beaker half full of water. Keep the water a little
above the temperature of the body for several hours, examining the tubes
at intervals to note the progress of digestion. Inferences.