The specimen was cut along the greater curvature and laid open by
reflecting the anterior wall. _C_, is the cardia. The arrow points
to the reticulo-omasal orifice. The spiral twist of the œsophageal
groove was of necessity partly undone by reflecting the wall; its
lips are drawn apart, showing some of the peculiar birdclaw-like
papillæ in the lower part.
]
The cavity of the omasum is occupied to a considerable extent by about a
hundred longitudinal folds, the =laminæ= (Laminæ omasi), which spring
from the dorsal and lateral walls. The largest of these—a dozen or more
in number—have a convex attached edge, and a ventral, free, slightly
concave edge which reaches to within a short distance of the ventral
wall of the sac. If these are drawn apart or a cross-section is made, it
will be seen that there is a second order of shorter laminæ, and a third
order still shorter; finally, there is a series of very low folds or
lines. The food is pressed into thin layers in the narrow spaces between
the laminæ (Recessus interlaminares), and reduced to a fine state of
division by being ground down by the numerous rounded horny papillæ
which stud the surfaces of the folds. The ventral wall of the omasum
forms a groove (Sulcus omasi), which connects the reticulo-omasal
opening with the omaso-abomasal opening; it is about three or four
inches (ca. 8 to 10 cm.) long, and is directed backward and a little
downward and to the right. It is free from leaves, but marked usually by
slight folds and small papillæ; it forms a direct path from the
reticulum to the abomasum for fluid and finely divided food. In the neck
of the omasum there are thick folds, and a number of the peculiar
papillæ already mentioned as occurring in the lower part of the
œsophageal groove. The =omaso-abomasal orifice= (Ostium
omaso-abomasicum) is oval, and is about four inches (ca. 10 cm.) long.
It is bounded in front by a thick muscular pillar which fades out above
on the lateral walls of the omasum. Laterally there are two folds
(Valvulæ terminales), which probably prevent regurgitation of the
contents of the abomasum.
The cavity of the abomasum is divided by a constriction into two areas.
The first of these (fundus gland region) is lined with a soft glandular
mucous membrane, which forms about a dozen large, slightly spiral folds
(Plicæ spirales). The second part (pyloric region) is much narrower and
resembles in appearance the corresponding region of the horse’s stomach.
A small cardiac gland zone surrounds the omaso-abomasal orifice. The
pyloric orifice is small and round.
[Illustration:
FIG. 291.—MUSCULATURE OF ŒSOPHAGEAL GROOVE AND ADJACENT PART OF
STOMACH OF OX, SEEN FROM WITHIN AFTER REMOVAL OF MUCOUS MEMBRANE.
_a_, Œsophagus; _b_, inner muscular layer of atrium; _d_,
reticulo-omasal orifice; _e_, _e′_, muscle of lips of groove; _g_,
lower end of muscle of larger lip, which curves around the
reticulo-omasal orifice in part and spreads out in the inner
muscular layer of the reticulum in part; _h_, lower end of muscle of
smaller lip which in part curves around the reticulo-omasal orifice
and is in part continuous with the inner muscular layer of the
omasal groove and the omasum; _i_, inner muscular layer of
reticulum. (Ellenberger-Baum, Anat. d. Haustiere.)
]
=Structure.=—The =serous coat= invests all of the free surface of the
stomach. The surface of the rumen which is attached to the dorsal
abdominal wall is of course uncovered, as well as the adjacent area to
which the spleen is attached. The furrows are bridged over by the
peritoneum and superficial muscle-fibers, and contain fat and (in most
cases) branches of the gastric arteries. The =lesser omentum= passes
from the visceral surface of the liver to the right (parietal) face of
the omasum and the pyloric portion of the abomasum. There is a fold
between the ventral curvature of the omasum and the dorsal curvature of
the abomasum. The =great omentum= is formed by the peritoneum leaving
the stomach along the left longitudinal and posterior transverse furrows
and the right surface of the rumen below the longitudinal furrow, also
the ventral curvature of the abomasum. Its superficial (ventral) part is
attached on the right to the duodenum, while the deep (dorsal) part
blends with the mesentery. It conceals the greater part of the intestine
on the right side, with the exception of the duodenum, and covers the
ventral sac of the rumen almost entirely. It is not lace-like, as in the
horse, and contains a large amount of fat in animals in good condition.
The =epiploic foramen= (of Winslow) is almost sagittal in direction.
The =muscular coat of the rumen= consists of an external longitudinal,
and a thicker internal circular layer. The latter forms the bulk of the
chief pillars, where it is about one-half to one inch (ca. 1 to 2 cm.)
thick. Scattered bundles of striped muscle-fibers radiate from the
cardia in the wall of the atrium.
The =muscular coat of the reticulum= consists of two chief layers which
begin and end at the œsophageal groove; they pass in a circular or
oblique fashion around the sac, the fibers of the two layers crossing
each other at an acute angle. The walls of the cells contain a central
muscular layer. The lips of the œsophageal groove contain a thick layer
of longitudinal fibers, which are largely continuous at or cross each
other below the reticulo-omasal orifice. The bottom of the groove has
two layers of oblique, unstriped muscle-fibers, with a variable outer
layer of striped muscle continuous with that of the œsophagus.
The =muscular coat of the omasum= consists of a thin external
longitudinal layer and a thick internal circular layer. At the omasal
groove there is an additional inner layer of longitudinal fibers. The
laminæ contain three muscular strata—a central layer of vertical fibers,
flanked on either side by a layer of longitudinal fibers, which are
continuous at the attached border with the muscularis mucosæ.
[Illustration: