between the hemoglobin of the red cells and the air we breathe?
38. How is the color of the blood affected by respiration?
39. Where is the oxygen absorbed at each respiration carried to?
Where does the carbon dioxide in the blood come from to be given
off at each respiration?
40. What centre in the medulla controls inspiration and expira-
tion?
CHAPTER XI
THE ORGANS OF DIGESTION
THE digestive apparatus for the digestion of the food
we eat consists of the alimentary canal and accessory
organs.
The alimentary canal is a musculomembranous
tube, about thirty feet in length, extending from the
mouth to the anus, and lined by mucous membrane
throughout the entire length.
It is divided in different parts according to the
mechanical or chemical changes taking place during
the various stages of digestion: as the mouth, where
the teeth, tongue, and salivary glands perform the act
of mastication and insalivation; the pharynx and
esophagus, which receive, force, and convey the food
to the stomach, as in the act of swallowing or degluti-
tion; the stomach, in which the chief chemical changes
occur and the food is reduced to a semiliquid condition,
to be passed on to the small intestines; the small
intestines, where it is acted upon by the bile, pan-
creatic and intestinal juices which separate and
render absorbable the nutritive material; the large
intestines, to which that portion of the food which
is unabsorbable moves on to pass out through the
rectum and anus as feces or waste particles.
The accessory organs of digestion are: the teeth,
tongue, salivary glands parotid, submaxillary, and
sublingual the liver and pancreas.
252 THE ORGANS OF DIGESTION
The alimentary canal consists of the following:
Mouth. fDuodenum.
Pharynx. Small Intestine j Jejunum.
Esophagus. Uleum.
Stomach . Cecum .
Large Intestine olon -
Rectum.
Anal canal.
THE MOUTH, ORAL OR BUCCAL CAVITY
The mouth is the upper part of the alimentary canal.
It is bounded by the lips, cheeks, tongue, hard and
soft palate, alveolar processes of both jaws, with
their contained teeth, and opens behind, through the
isthmus faucium, into the pharynx. It is lined by
mucous membrane continuous in front with the skin,
behind with that of the fauces, its epithelium being
stratified.
The Teeth. The teeth in the human subject are
erupted in two sets, a temporary or deciduous, or milk
teeth, and a permanent or succedaneous set. The
former are 20 in number, 10 in each jaw; the latter,
32, 16 each above and below. Each tooth is made
up of three parts: the root, consisting of one or more
fangs, contained in the alveolus; the crown or body,
above the gum; and the neck, between the two. The
alveolar periosteum is reflected on to the fang as far
as the neck.
The twenty temporary teeth are divided into 4 inci-
sors, 2 canines, and 4 molars above and below. The 32
permanent teeth are : 4 incisors, 2 canines, 4 bicuspids,
and 6 molars in each jaw. The temporary teeth are
similar to but smaller than the permanent; of the
temporary molars, the hinder one is the largest of
THE MOUTH, ORAL OR BUCCAL CAVITY 253
all, and its place is afterward taken by the second
permanent bicuspid.
Of the permanent teeth the incisors are the 8 central
cutting teeth, 4 each above and below, the former
being the larger. They are bevelled at the expense
of the posterior surface. The canines (cuspidati) are
2 in each jaw, being situated 1 behind each lateral
incisor, the upper and larger being called the eye
teeth. The bicuspids (premolars or false molars),
4 in each jaw, lie 2 each behind the canines, the upper
being the larger. The molars (true molars or multi-
cuspidati) are the largest teeth, and number 6 in
each jaw, 3 each behind the posterior bicuspids above
and below. They present 4 tubercles on the upper,
5 on the lower crowns, and the root is subdivided into
from 2 to 5 fangs. The first molar is the largest and
broadest, the second smaller, and the third (wisdom
tooth) the smallest.
Fio. 95
Pulp Cavity. .
Root.
Vertical section of molar tooth.
A vertical section of a tooth shows it to be hollow,
the cavity being continuous with the aperture in the
fang and filled up with the soft dental pulp, and is
hence called the pulp cavity. The pulp is sensitive,
highly vascular, and consists of connective tissue,
with cells, vessels, and nerves. The hard substance
of each tooth consists of three parts: the iwry or
dentin, the enamel, and the crusta petrosa or cement.
254 THE ORGANS OF DIGESTION
The period of eruption of the temporary teeth are
(C. S. Tome) :
Lower central incisors 6 to 9 months
Upper incisors 8 to 10 months
Lower lateral incisors and first molars . 15 to 21 months
Canines 16 to 20 months
Second molars 20 to 24 months
The period of eruption of the permanent teeth are :
First molars 6j years
Two middle incisors 7th year
Two lateral incisors 8th year
First bicuspid 9th year
Second bicuspid 10th year
Canine llth to 12th year
Second molars 12th to 13th year
Third molars 17th to 21st year
The Tongue (Lingua). The tongue is the organ of the
special sense of taste, 1 also assisting in insalivation, mas-
tication, deglution, and articulate speech. It is situated
in the floor of the mouth, in the interval between the
horizontal rami of the mandible. It is attached to the
hyoid bone at the base by the genioglossus and hyo-
glossus muscles and the hyoglossal membrane; with the
epiglottis by three folds, the glosso-epiglottic folds, of
mucous membrane; with the soft palate by means
of the anterior pillars of the fauces; and with the
pharynx by the superior constrictor muscles and the
mucous membrane.
The muscles controlling the tongue are the extrinsic,
which are inserted into the tongue, their terminal
fibers contained within the substance, namely, the
styloglossus, the hyoglossus, the palatoglossus, the
genioglossus, and part of the superior constrictor of
the pharynx (pharyngoglossus). The intrinsic muscles
of the tongue are : the superior lingualis, the chondro-
glossus, the transverse lingualis, the vertical lingualis,
and the inferior lingualis.
1 See chapter Sense of Taste, page 406.
THE MOUTH, ORAL OR BUCCAL CAVITY 255
The arteries of the tongue are derived from the
lingual, the facial, ascending pharyngeal (all branches
of the external carotid artery). The veins open into
the internal jugular. The lymphatic vessels from the
FIG. 96
UVULA
PHARYNX
Dorsal surface of the tongue. (Testut.)
256 THE ORGANS OF DIGESTION
anterior half of the tongue drain into the submaxillary
nodes. Those draining the posterior half end in the
deep cervical nodes; along the internal jugular vein.
Nerves of Tongue. (See Nerve System, pages 374
and 406.)
The, Palate. The palate forms the roof of the
mouth, and consists of a front part or hard, and a
back part or soft palate. The periosteum of the hard
palate (see Bones) is covered by and intimately con-
nected with the mucous membrane of the mouth.
In the middle line is a raphe ending in front at a
small papilla, which marks the anterior palatine fossa
which receives the terminal part of the anterior
palatine and nasopalatine nerves. The mucous mem-
brane is pale and corrugated, covered with squamous
epithelium, and furnished with a number of palatal
glands which lie between it and the bone.
The soft palate partially separates the mouth and
pharynx. It consists of muscular, connective, and
adenoid tissue, with vessels, nerves, and mucous
glands, all enclosed in a fold of mucous membrane.
Above it is joined to the back of the hard palate;
laterally it blends with the pharynx; below it is free;
in front it is concave, with a median ridge; and behind
it is convex. Its mucous membrane is continuous with
that of the roof of the mouth and of the posterior nares.
From its lower border a conical process depends,
the uvula, from whose base descend the pillars of the
soft palate, the anterior, formed by the palaloglossi
muscles, to the sides of the base of the tongue; the
posterior, formed by the palatopharyngei, to the sides
of the pharynx. These pillars are covered by mucous
membrane and separated below by the tonsil, the
space being called the isthmus of the fauces.
The tonsils (tonsilla palatina) are two in number,
situated on each side of the fauces, and lie between
the anterior and posterior palatine pillars, and are
about J inch long and \ inch wide and thick, but vary
much in size.
THE MOUTH, ORAL OR BUCCAL CAVITY 257
The Salivary Glands. There are three pairs,
parotid, submaxillary, and sublingual.
The parotid gland, the largest, weighs J to 1 ounce,
and lies on the face below and in front of the ear. Its
outer surface, lobulated, is covered by the skin and
fascia, and partly by the platysma and several lym-
phatic glands.
FIG. 97
The salivary glands. (Gray.)
The duct (Stenson's) is about 2J inches long and f
inch in diameter, and opens opposite the second
molar tooth, into the mouth, thence runs backward
beneath the mucous membrane, through the buccina-
tor, and across the masseter to the front of the gland.
It conveys the saliva to the mouth.
The submaxillary gland is of an irregular form, weighs
about 2 drams, and lies below the jaw and above the
17
258 THE ORGANS OF DIGESTION
digastric muscle. It is covered by the skin, platysma,
and fasciae, and grooves the inner surface of the lower
jaw.
The submaxillary duct (Wharton's) is 2 inches long,
and opens at the top of a papilla close to the frenum
of the tongue into the mouth. Thence it runs back
between the sublingual gland and the geniohyoglossus,
then between the mylohyoid and the hyoglossus and
geniohyoglossus.
The sublingual gland, the smallest of the salivary
glands, lies at the side of the frenum of the tongue
and against the inner surface of the lower jaw, beneath
the mucous membrane. It is almond-shaped, weighs
1 dram, and its ducts (of Rivini), ten to twenty in
number, open separately, one or two joining to form
the duct of Bartholin, which joins Wharton's duct.
THE PHARYNX
The pharynx is a musculomembranous tube, conical
in shape, between the oral cavity and the esophagus;
communicating with the posterior nares, the oral
cavity, the larynx, the two Eustachian tubes. It is
attached above to the periosteum of the petrous
portion of the temporal bone and the basilar process
of the occipital bone. The raphe of the constrictor
muscles is attached to the pharyngeal tubercle of the
basilar process of the occipital bone. It is bounded
above by the body of the sphenoid and basilar process
of the occipital; below, it is continuous with the
esophagus; anteriorly, it is incomplete, and is attached
to the Eustachian tube, the internal pterygoid plate,
the pterygomandibular ligament, the posterior portion
of the mylohyoid ridge, the mucous membrane of
the mouth, the base of the tongue, the hyoid bone,
the thyroid and cricoid cartilages; posteriorly, the
prevertebral fascia, and areolar tissue connect it to
THE PHARYNX
259
the cervical portion of the vertebral column, anterior
to the longus colli and rectus capitis anticus muscles,
the areolar tissue is contained in the retropharyngeal
FIG. 98
Sagittal section of face and neck, showing external wall of right
nasal fossa. (Testut.)
space; laterally, it is connected to the styloid process
and its muscles. The constrictor muscles surround it
and aid in deglutition. It is 4^ inches long, and for
260 THE ORGANS OF DIGESTION
purposes of studying, divided into a nasal, oral, and
laryngeal portion.
The nasal part or nasopharynx, lies posterior to the
nares and above the soft palate.' In front are the
posterior nares (choanae); behind, the pharyngeal
tonsil, consisting of lymphoid tissue seen above the
orifices of the Eustachian tubes in the median line.
The floor of the nasopharynx is continuous with the
nasal fossae, anteriorly, and behind is the sloping
portion of the soft palate. On its lateral wall is the
orifice of the Eustachian tube, level with the inferior
turbinated bone and one-third to one-half inch pos-
terior.
The oral part extends from the soft palate to the
level of hyoid bone. It opens into the oral cavity,
through the fauces, bounded on either side by the
anterior and posterior pillars, between which are the
tonsils.
The laryngeal part is continuous with the oral
portion above, and below at the level of the cricoid
cartilage is^continuous with the esophagus. Anteriorly,
it presents the aperture of the larynx, bounded in
front by the epiglottis, and laterally by the aryteno-
epiglottic folds.
The pharynx is lined with mucous membrane
continuous with that lining the Eustachian tube, the
nasal fossae, the mouth, and the larynx. In the naso-
pharynx it is covered by stratified ciliated epithelium;
in the oral and laryngeal portions it is of the
stratified squamous variety.
THE ESOPHAGUS (GULLET)
The esophagus is the tube connecting the pharynx
with the stomach, and extends from the level of the
sixth cervical vertebra through the diaphragm, entering
the stomach opposite the tenth or eleventh dorsal
THE ESOPHAGUS
261
vertebra, a distance of 9 or 10 inches, and from the
incisor teeth to the beginning of the esophagus is
about 6 inches; th\is making the distance from the
FIG. 99
FIRST THORA-
CIC VERTEBRA
FWELFTH THORA
CIC VERTE
Esophagus and stomach in their natural relations to the vertebral
column and the aorta. (Testut.)
262
THE ORGANS OF DIGESTION
incisor teeth to the cardiac opening of the stomach
15 to 16 inches. It is the narrowest part of the alimen-
tary canal, and presents two constrictions, one at its
commencement, the other at the diaphragm.
FIG. 100
CARDIAC ENC
Stomach and duodenum, the liver and most of the intestines having been
removed. The pyloric end of the stomach should be represented as turned
directly backward. (Testut.)
THE STOMACH (GASTER)
The stomach lies in the epigastrium, left hypo-
chondrium, and sometimes the mesogastrium. It is
the most dilated portion of the alimentary canal.
Its shape is pyriform, the left or larger portion is
THE STOMACH
263
FIG. 101
el ,RFACE OF MUCOSA
called the cardia, and below this is the fundus, the
right end is termed the pylorus. The right opening
of the stomach is called the pyloric orifice, and the
left the esophageal orifice, the former opens into the
duodenum, and the latter, the
esophagus. It is 10 to 12 inches
in length, 4 to 5 inches in the
vertical direction, and weighs 4
to 5 ounces. Its capacity is
from 3 to 6 pints.
The cardiac orifice is the
highest part of the stomach, and
lies behind the seventh costal
cartilage, 1 inch to the left of
the sternum. The pyloric orifice
lies about 2 inches to the right
of the midline, on a level with
the upper border of the first
lumbar vertebra; it is guarded
by a valve, the pylorus. Be-
tween the two orifices the
stomach is sickle-shaped and
presents an upper concave bor-
der, the lesser curvature, and
a lower convex border, the
greater curvature. The pyloric
orifice is anterior and inferior
to the fundus and is in relation
with the quadrate lobe of the
liver and belly wall. The
stomach presents two surfaces,
-, Cardiac gland in longitudinal
an anterosuperior and postero- eec tion. (?. H. G.)
inferior.
The stomach has a serous (peritoneal) coat, a
muscular coat comprising a longitudinal, circular, and
oblique layer, an areolar coat of loose tissue (submucous
coat), and a mucous coat, lined with columnar epithe-
lium. The latter is thickest near the pylorus, thinnest at
264
THE ORGANS OF DIGESTION
the fundus, and presents, in the empty condition of
the organ, numerous ridges or rugce, which run longi-
The regions of the abdomen and their contents. Edges of costal cartilages
in dotted outline. (Gray.)
tudinally along the great curvature. Studded over its
surface are many small polygonally shaped depres-
sions which are the enlarged mouths of the gastric
THE SMALL INTESTINE 265
tubular glands. These are of two kinds, called pyloric
and cardiac glands they secrete the gastric juice;
some are simply tubular, while others have several
branches opening into a common duct. The pyloric
glands are most numerous at the smaller end, but the
cardiac glands (see Fig. 101, page 263) are found all
over the stomach, the ducts of the latter being shorter.
In the cardiac glands, between the basement membrane
and the lining epithelium, are numerous peptic or
parietal cells, the others being known as the central
or chief cells. Between the glands the mucous mem-
brane contains lymphoid tissue, collected here and
there into little masses resembling the solitary intes-
tinal glands, and called the lenticular glands. Beneath
the membrane is a muscularis mucosae. (See Fig. 82,
page 202, for blood-supply of the stomach.)
THE SMALL INTESTINE
The Duodenum. The duodenum is about 10 inches
long, and runs in a curved direction from the pylorus
of the stomach to the jejunum, which it joins on the
left side of the second lumbar vertebra. The concavity
of the curve looks toward the left and embraces the
head of the pancreas. It is divided, for description,
into four parts or portions.
The Jejunum and Ileum. The jejunum includes
the first two-fifths of the remaining part of the small
intestine, running from the left side of the first or
second lumbar vertebra to the beginning of the ileum.
Its coats are thicker and more vascular, and are of a
deeper color and larger caliber than the ileum.
The remainder of the small intestine is the ileum,
which ends by opening into the inner side of the com-
mencement of the large gut in the right iliac fossa.
The Structure of the Small Intestines. The wall
of the small intestine, including the duodenum, con-
266 THE ORGANS OF DIGESTION
sists of a serous, a muscular, a submucous, and a
mucous coat.
The Serous Coat. This is derived from the peri-
toneum and surrounds the bowel completely, except
in the duodenum, where only the first portion is com-
pletely covered. Along its mesenteric border, where
the mesentery (a fold of peritoneum) is attached, is
an uncovered interval for the entrance and exit of
arteries, veins, nerves, and lymphatics which pass
between the layers of mesentery.
The Muscular Coat. This consists of an outer
longitudinal and an inner circular set of muscle
fibers. These muscular layers propel the food along
the intestines, as well as assist by their action in mixing
it with the intestinal juices during active digestion.
The Submucous Coat. This is composed of areolar
tissue and holds the mucous and muscular coats
together. It contains the branches of the nutrient
arteries to the bowel, previous to their distribution to
the mucous coat, also the lymph channels and nerves.
The lymph nodules are lodged in this layer; they are
pear-shaped with their apex lying in the mucous
membrane. These are called solitary follicles and
Peyer's patches. The submucous coat in the small
intestines extends up into the valvulse conniventes.
In the duodenum the duodenal glands are lodged in
the submucous coat.
The Mucous Membrane. This is lined with columnar
epithelium. It is soft and velvety in appearance.
The membrane is highly vascular near the beginning
of the duodenum, and gradually becomes paler as the
lower portion of the bowel is reached. The membrane
is thrown into folds called valvulse conniventes. Each
fold is simply two layers of membrane folded upon
itself and held together by fibrous tissue. They
increase the absorbing surface of the intestinal canal
and retard the progress of the food, according to some
authors. They measure J to J an inch in width, and
THE SMALL INTESTINE
267
FIG. 103
extend to about one-half to two-thirds of the circum-
ference of the bowel. The villi in the mucous mem-
brane are described under absorption (see page 287).
The Intestinal Glands or Glands of Lieberkuhn. These
are found in the mucous membrane throughout the
small intestines. They are minute tubular depressions
seen at the base of the villi and communicate with
the surface of the mucous
membrane, upon which they
pour out a special secretion
from the columnar cells which
line them; the latter rest on
a thin basement membrane,
which is surrounded by capil-
lary vessels.
The Duodenal or Brunner's
Glands. They are found only
in the duodenum. They are
small, branched, tubular glands
situated in the submucouscoat
and open upon the mucous
membrane of the duodenum
by very small ducts.
The Lymph Nodules. They
are divided into solitary fol-
licles and Peyer's patches.
Their bodies are in the sub-
mucous coat and their apices
in the mucous membrane.
The solitary follicles are
found throughout the mucous
and submucous layers of the small and large intes-
tines. They are small, round, whitish collections of
areolar tissue rich in leukocytes or white corpuscles,
and blood capillaries, and communicate through their
base with the lacteals of the villi. Each consists of
a lighter, central area, the germinal centre, where the
leukocytes are reproducing, and an outer darker zone,
Intestinal gland in longitudinal
section. (Testut.)
268
THE ORGANS OF DIGESTION
where the cells are more numerous and closely packed
(Gray).
FIG. 104
Duodenal gland. * (Frey.)
FIG. 105
Aggregated lymph nodule (Peyer's patch). (Testut.)
Peyer's patches are regarded as collections of solitary
follicles, seen as oval or rounded patches, placed
THE LARGE INTESTINE 269
lengthwise with the bowel, measuring from J to 4
inches in length. Usually ten to sixty are present.
They are found mostly in the ileum. Peyer's patches
are highly inflamed in typhoid fever, and ulcerate,
giving rise to hemorrhage and perforation of the bowel
in severe attacks of the disease. (See Fig. 83, page
204, for blood-supply of the small intestines.)
THE LARGE INTESTINE
The large intestine is that part of the alimentary
canal which extends from the end of the ileum to the
anus; it is about 5| feet long. It commences by a
dilated part, the cecum, in the right iliac fossa, ascends
to the under surface of the liver, then runs transversely
across the abdomen to the vicinity of the spleen,
descends to the left iliac fossa, and forms the sigmoid
flexure, and finally passes along back of the pelvis to
end at the anus.
The Cecum. The cecum is the large cul-de-sac
which is the beginning of the large intestine, and is
about 3 inches broad and 2 J inches long. It is variously
situated, being found upon and external to the psoas;
upon the iliacus muscle it lies internal, on the pelvic
brim, or entirely within the pelvis. In any of these
positions it is entirely surrounded by peritoneum.
The vermiform appendix comes off from the inner
and back part of the cecum, near its lower end, and
extends upward and inward behind it. This is a
piece of gut of the diameter of a goose-quill, varying
from 3 to 6 inches in length, curved upon itself, and
ending in a blind extremity. It tapers gradually to
its end, which is blunt, is completely invested by the
peritoneum, which forms for it a mesentery (meso-
appendix), and at its connection with the cecum is
guarded by an imperfect valve (valve of Gerlach) . This
is not always constant.
The ileocecal valve guards the opening of the small
270 THE ORGANS OF DIGESTION
intestine into the large gut. This junction is oblique
and situated about 2 \ inches above the lower extremity
of the cecum. It is a double fold lying transversely
to the long axis of the colon. Each fold of the valve
is made up of the mucous and submucous coats,
reinforced by some circular fibers from the muscular
coat, of each portion of the gut, and is covered on the
side toward the ileum with villi.
The Colon. The ascending colon runs from the
cecum, above the ileocecal valve, upward to the under
surface of the liver on the right side of the gall-bladder,
and then turns forward and to the left to form the
hepatic flexure. The peritoneum rarely forms for it
a mesocolon; generally it covers only the front part
and the sides. It occupies the right lumbar and
hypochondriac regions.
The transverse colon arches across the abdomen, the
convexity looking toward the belly wall, and makes
a sudden turn backward and downward beneath the
spleen, forming the splenic flexure, and is completely
invested by the peritoneum, which holds it to the
anterior aspect of the pancreas and second portion