Encyclopaedia Britannica > Volume 3, Anatomy-Astronomy

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ANATOMY.
From this description, for the particulars of which we
are indebted to the accurate account of Sir E. Home, it
results that the stomach of the camel differs from that of
Waiter- ^ie 0X ant* ot^er horned ruminants chiefly in the pos-
ells of thesession of the quadrilateral cells in the second stomach
i 4" I'l trro i a Vvw 4-1-.^. ^ 1 1 1
Compara¬
tive
Anatomy-
109
iuadrilo-
uiar sto-
'rop of
inis.
;amel. Into these the water is conveyed by the animal when
drinking, and in these it remains. By the action of the
muscular band (m), the aperture between the Paunch and
Kingshood is opened, and the water is directed into it so
as to fill its cells. When these are filled the surplus runs
off into those of the first stomach, where at least those on
the left side of the long band may be regarded as part of
the general cellular structure. These cells are represent¬
ed of a large size in fig. 5, in which they appear like ob¬
longs with rounded corners. They are always larger on
the left side of the band, in the Paunch, than those in the
second stomach.
Both in the bullock and in the camel, and in all the
Ruminants, the first and second stomachs only are covered
by mucous epidermis.
In the Cetaceous animals similar complication of the
gastric cavities is observed. In the bottle-nose porpoise,
Yt acea which may be tak(:n asan example, the oesophagus, which
LTAC A’ is large and capacious, terminates in a spheroidal or ovoi-
dal flask-like bag (fig. 6, a, a) with an aperture a little be¬
low the cardiac, consisting of rose-like annular folds, and
leading into a second cavity. This, which corresponds
to the ruminant kingshood, is nearly spherical in shape
(fig. 7, b), and presents valvular folds more circular than in
the ruminants, and intersected by others so as to give it
the honeycomb appearance characteristic of this cavity.
From this another aperture leads into the smallest cavity
of the three (c, fig. 7) ; and thence into the fourth, which
is long, cylindrical, and slightly incurvated. The third
cavity is remarkable for presenting in its inner membrane
numerous apertures of mucous glands.
Birds are distinguished by possessing a stomach con¬
sisting of three cavities. The first is the crop, which may
be regarded as a mere expansion of the oesophagus, and
confined chiefly to land birds. It is filled not only with
food, but with small stones; and its chief purpose seems,
by mechanical comminution, to supply the place of the
teeth in dividing the granular aliment, and bruising or
killing the animals swallowed. It is found chiefly in the
granivorous birds. It is wanting in the ostrich, in the
piscivorous birds, and most of the Graliae.
I he second is the glandular crop or subsidiary stomach
(ventriculus succenturiatus, bulbus glandulosus), a mem-
brano-glandular sac, which may be also regarded as an
oesophageal dilatation. It is larger when the crop is want¬
ing ; and though, when conjoined with it, it is always very
glandular, and may be therefore regarded as a chemical
solvent of alimentary substances, it appears to supply the
want of the crop, which is certainly chiefly a mechanical
apparatus. rIhe glandular crop or subsidiary stomach is
remarkable for the number and size of the glandular bodies
contained between its mucous and muscular tunics. These
glands, though variable in shape, are generally conical;
and some consist of several glands conjoined in one com¬
mon peduncle (Plate XXXVI. fig. 3). All of them are hol¬
low, and secrete a fluid which is discharged by one or
nioie minute apertures, and which is of essential importance
m the solution of the food. In some instances, as in the
mencan ostrich (fig. 2), they are few in number, and oc-
cupy only a small part of the posterior wall of the oesophagus.
ie gizzard or proper stomach of birds may be consi¬
dered as a horny mucous membrane, somewhat cartilagi¬
nous, continuous with that of the oesophagus, and covered
V two strong thick muscles, the fibres of which converge
to a point. (Plate XXXVI. fig. 1.) In the carnivorous and
piscivoious <7e«era of birds, especially those in which the
dandular
izzard.
crop is wanting, the gizzard loses its muscular character, Compara-
and is converted into a membranous pouch. live
I he stomach of Reptiles does not present those dila- Anatomy,
tations observed in Birds; and when it changes its dia-^^
meter or capacity, it is only progressively and insensibly,
Its general diameter, nevertheless, is proportionally larger
than m the two classes already noticed. Most generally
without cul desac, its shape is spheroidal, more or less ob¬
long; its membranous walls are thin and transparent- its
muscular layer almost imperceptible; the cellular identified
with the mucous tissue; the situation of the cardia indeter¬
minate ; and the pylorus, without valve, is distinguished
by a simple tapering contraction of the gastric walls, and
the appearance of the structure proper to the intestines.
In this class of animals, further, digestion appears to
be less regulated by fixed principles than in the other
two. It is evidently not confined exclusively to the sto¬
mach. The oesophagus of the turtle is provided with
numerous large, firm, pointed processes, which in all
probability contribute to the mechanical division of the
food, so much the more requisite as the Cheloniad Rep¬
tiles are toothless. Except in the crocodile, the Sau-
rial Reptiles are destitute of large arch or proper car¬
diac cavity. In the Ophidial or Serpentine Reptiles
the stomach has the figure merely of a dilated sac be¬
tween the oesophagus and intestines, and presents no cur¬
vature. It is probably in connection with this modifica¬
tion of structure that we find animals remain for days in
the oesophagus of serpents; and this tube appears to be
to a certain extent capable of digesting aliment as much
as the stomach. The best mark of distinction in such
circumstances is the cessation of epidermis.
In no class of animals does the stomach vary more in Stomach
shape, structure, and situation, than in Fishes; and per-of fishes,
haps the general character of the alimentary canal in this
class is most justly given by representing it as deviating
from those attributes of regularity which we find in the
higher classes. While in the Mammalia and Birds it is
always distinguished by its spheroidal or pyriform enlarge¬
ment between two tapering extremities, and by being
much more dilated than any other part of the alimentary
canal, in the Reptile class it begins to part with this cha¬
racter ; and it loses it altogether in the Fishes. In most
of the finny tribes it is often not more capacious than the
oesophagus ; and it is distinguished from this tube only by
the villous character of its internal membrane. In gene¬
ral, also, the situation of this pyriform dilated sac is trans¬
versely across the body in the Mammiferous class. In
the feathered tribes this character also is slightly set
aside, and partly from the alteration in shape, partly from
that of position, the stomach occupies less of the trans¬
verse diameter than of the longitudinal extent of the
body. Among the Reptiles this character, though still
retained in the Cheloniad, and even in the Saurial, is
gradually enfeebled in the Ophidial ; and in the Fishes
it may be said to be entirely obliterated, since the organ
occupies much more longitudinal extent than transverse
width of the body.
The first character of the alimentary tube of fishes is
the width or capacity combined with shortness of the oeso¬
phagus. The latter character is manifestly associated
with the absence of lungs and consequent want of chest;
so that between the throat and abdominal cavity, the in¬
terval, which corresponds only to the space occupied by
the heart, is extremely abridged. The oesophagus con¬
sists, as in the other classes, of a mucous membrane sur¬
rounded by a muscular tunic; but the mucous mem¬
brane is distinguished by the firmness and whiteness of its
corion, which in some genera approaches to the consistence
of horn or cartilage, and by the presence of conical pa-
pillce, sometimes of great hardness, and which appear to