The stomach (Ventriculus) is the large dilatation of the alimentary
canal between the œsophagus and the small intestine. It is a sharply
curved, U-shaped sac, the right branch being, however, much shorter than
the left one. The convexity is directed ventrally. When moderately
distended, there is often a slight constriction which indicates the
division into right and left sacs. It is relatively small, and is
situated in the dorsal part of the abdominal cavity behind the diaphragm
and liver, mainly to the left of the median plane.
[Illustration:
FIG. 259.—STOMACH OF HORSE, VISCERAL SURFACE, WITH FIRST PART OF
DUODENUM.
Fixed _in situ_ when full but not distended. The posterior gastric
artery and its larger branches with two satellite veins are shown.
]
It presents for description two surfaces, two curvatures, and two
extremities. The =parietal surface= (Facies parietalis) is convex and is
directed forward, upward, and toward the left; it lies against the
diaphragm and liver. The =visceral surface= (Facies visceralis), also
convex, faces in the opposite direction; it is related to the terminal
part of the large colon, the pancreas, the small colon, and the small
intestine. The =lesser curvature= (Curvatura minor) is very short,
extending from the termination of the œsophagus to the junction with the
small intestine. When the stomach is _in situ_, its walls are here in
contact, and the cardia and pylorus close together. The =greater
curvature= (Curvatura major) is very extensive. From the cardia it is
first directed upward and curves over the left extremity; it then
descends, passes to the right, crosses the median plane, and curves
upward to end at the pylorus. Its left portion is related to the spleen,
while its ventral portion rests on the left divisions of the great
colon. The =left extremity= or =saccus cæcus= is a rounded cul-de-sac
which lies under the upper ends of the fourteenth, fifteenth, and
sixteenth ribs and the diaphragm.[90] It is related to the pancreas
behind and the base of the spleen externally. The =right= or =pyloric
extremity= is much smaller and is continuous with the duodenum, the
junction being indicated by a marked constriction. It lies on the liver,
a little to the right of the median plane, and a little lower than the
cardiac opening. About two or three inches (ca. 5 to 8 cm.) from the
pylorus there is a constriction which marks off the =antrum pylori= from
the rest of the right sac. The =œsophageal orifice= or =cardia= is
situated at the left extremity of the lesser curvature, but about eight
to ten inches (ca. 20 to 25 cm.) from the left extremity. The œsophagus
joins the stomach very obliquely. The opening is closed by the sphincter
cardiæ and numerous folds of mucous membrane. The =pyloric orifice=
communicates with the duodenum. Its position is indicated externally by
a distinct constriction. Internally it presents a circular ridge
produced by a ring of muscular tissue—the =sphincter pylori=.
The stomach is held in position mainly by the pressure of the
surrounding viscera and by the œsophagus. The following peritoneal folds
connect it with the adjacent parts:
1. The =gastro-phrenic ligament= (Lig. gastrophrenicum) connects the
great curvature, from the cardia to the left extremity, with the crura
of the diaphragm. This leaves a narrow area uncovered with peritoneum,
and here the stomach is attached to the diaphragm by loose connective
tissue.
2. The =small= or =gastro-hepatic omentum= (Omentum minus) connects the
lesser curvature and the first part of the duodenum with the liver below
the œsophageal notch and the portal fissure.
3. The =gastro-splenic omentum= (Lig. gastrolienale) passes from the
left part of the great curvature to the hilus of the spleen.
4. The =great= or =gastro-colic omentum= (Omentum majus) connects the
ventral part of the great curvature and the first curve of the duodenum
with the terminal part of the great colon and the initial part of the
small colon.
5. The =gastro-pancreatic fold= (Plica gastro-pancreatica) extends from
the left sac above the cardia to the duodenum. It is attached dorsally
to the liver and vena cava, ventrally to the pancreas.
[Illustration:
FIG. 260.—EVERTED STOMACH OF HORSE FROM WHICH THE MUCOUS MEMBRANE HAS
BEEN REMOVED.
_O_, Œsophagus; _D_, duodenum; _b_, circular layer; _c′_, internal
oblique fibers; _c″_, loop around cardia; _c‴_, transition of
internal to external oblique fibers; _d_, fibers connecting the two
branches of the cardiac loop; _p_, antral sphincter; _p′_, pyloric
sphincter. (Ellenberger-Baum, Anat. d. Haustiere.)
]
The stomach of the equidæ is relatively small, its capacity varying from
two to four gallons (ca. 8 to 15 liters).
The size, form, and position of the stomach are subject to
considerable variation. When the stomach is nearly empty the saccus
cæcus contains only gas and is strongly contracted; the middle portion
(physiological fundus) contains the ingesta and preserves its rounded
character, while the pyloric portion is contracted. When distended the
middle portion settles down some four or five inches, pushing back
coils of the small colon and small intestine which may lie between the
great curvature and the large colon, and also pushing to the left or
right the left dorsal part of the great colon; the spleen, small
colon, and small intestines are pushed back by the distention of the
left sac.
=Structure.=—The wall is composed of four coats—serous, muscular,
submucous, and mucous.
The =serous coat= (Tunica serosa) covers the greater part of the organ
and is closely adherent to the muscular coat except at the curvatures.
It partially bridges over the lesser curvature, and covers here elastic
tissue which assists in retaining the bent form of the stomach. The
peritoneal folds have been described.
The =muscular coat= consists of three incomplete layers, an external of
longitudinal, a middle of circular, and an internal of oblique fibers.
The layer of =longitudinal fibers= (Stratum longitudinale) is very thin
and exists only along the curvatures and at the antrum. At the lesser
curvature it is continuous with the longitudinal fibers of the
œsophagus. On the antrum pylori it forms a well developed complete
layer. The layer of =circular fibers= (Stratum circulare) exists only on
the right sac. At the pyloric orifice it forms a thick ring—the =pyloric
sphincter=. Another ring, the antral sphincter, is found at the left end
of the antrum pylori. The =oblique fibers= (Fibræ obliquæ) are arranged
in two layers; the external stratum covers the left sac and is a
continuation (in part) of the longitudinal fibers of the œsophagus; the
internal stratum is found also on the left sac, and exchanges fibers
with the circular and external oblique layers. It forms a remarkable
loop around the cardiac orifice, constituting a powerful =cardiac
sphincter= (Sphincter cardiæ).
[Illustration:
FIG. 261.—FRONTAL SECTION OF STOMACH AND FIRST PART OF DUODENUM OF
HORSE.
_C_, Cardiac orifice. Photograph of specimen fixed _in situ_.
]
The =submucous coat= is a layer of loose connective tissue which
connects the muscular and mucous coats; in it the vessels and nerves
ramify before entering the mucosa.
The =mucous coat= is clearly divided into two parts. That which lines
the left sac resembles the œsophageal mucous membrane, and is termed
=œsophageal= or =cuticular=. It is white in color, destitute of glands,
and covered with a thick, squamous, stratified epithelium. At the
cardiac orifice it presents numerous folds which occlude the
opening.[91] It terminates abruptly at an elevated, denticulated,
sinuous line, termed the =cuticular ridge= (Margo plicatus). Below and
to the right of this line the mucous membrane has a totally different
character, being soft and velvety to the touch, and covered by a mucoid
secretion. It is glandular, and three zones may be recognized, although
no sharp line of demarcation exists. A narrow zone next to the cuticular
ridge has a yellowish-gray color, and contains short tubular =cardiac
glands= (Cardiac gland region). Next to this is a large area which has a
mottled reddish-brown color, and contains =fundus glands= (fundus gland
region). This part of the mucous membrane is thick and very vascular,
and corresponds to the fundus of the stomach in man and the dog. The
remainder of the mucous membrane has a reddish-gray color and contains
branched, tubular, =pyloric glands= (pyloric gland region); it
corresponds to the pyloric portion of man and the dog.
The folding of the stomach wall at the lesser curvature produces a
prominent ridge which projects into the cavity of the stomach. Circular
ridges occur at the antral and pyloric sphincters.
=Blood-vessels and Nerves.=—The stomach receives blood from all the
branches of the cœliac artery. The gastric veins drain into the portal
vein. The nerves are derived from the vagus and sympathetic nerves.
[Illustration:
FIG. 262.—DIAGRAM OF ZONES OF MUCOUS MEMBRANE OF STOMACH OF HORSE.
]