_Oesophagus_, or _Gullet_.--This tube extends from the mouth to the
stomach, and is the medium through which the food is conveyed to the
latter organ. This tube is furnished with spiral muscles, which run in
different directions. By this arrangement, the food ascends or descends
at the will of the animal. The inner coat of the gullet is a
continuation of the same membrane that lines the mouth, nostrils, &c.
The gullet passes down the neck, inclining to the left side of the
windpipe, until it reaches the diaphragm, through a perforation of which
it passes, and finally terminates in the stomach. The food, having
undergone a slight mastication by the action of the teeth, is formed
into a pellet, and, being both moistened and lubricated with saliva,
passes down the gullet, by the action of the muscles, and falls
immediately into the paunch, or rumen; here the food undergoes a process
of maceration, or trituration. The food, after remaining in this portion
of the stomach a short time, and being submitted to the united action of
heat and moisture, passes into another division of the stomach, called
_reticulum_, the inner surface of which abounds in cells: at the bottom,
and indeed in all parts of them there are glands, which secrete from the
blood the gastric fluids. This stomach possesses a property similar to
that of the bladder, viz., that of contracting upon its contents. In the
act of contracting, it squeezes out a portion of the partly masticated
food and fluids; the former comes within the spiral muscles, is embraced
by them, and thus ascends the gullet, and passes into the mouth for
remastication. The soft and fluid parts continue on to the many plus and
true digestive stomach. The second stomach again receives a portion from
the paunch, and the process is continued.
Rumination and digestion, however, are mechanico-vital actions, and can
only be properly performed when the animal is in a healthy state.
Now, a portion of the food, we just observed, had ascended the gullet
by the aid of spiral muscles, and entered the mouth; it is again
submitted to the action of the grinders, and a fresh supply of saliva;
it is at length swallowed a second time, and goes through the same
routine as that just described, passing into the manyplus or manifolds,
as it is termed.
The manyplus abounds internally in a number of leaves, called laminae.
Some of these are attached to the upper and lower portion of the
division, and also float loose, and penetrate into the oesophagian
canal. The laminae have numerous projections on their surface, resembling
the papillae to be found on the tongue. The action of this stomach is one
of alternate contraction and expansion: it secretes, however, like the
other compartments of the stomach, its due share of gastric fluids, with
a view not only of softening its contents, but for the purpose of
defending its own surface against friction. The mechanical action of the
stomach is communicated to it partly by the motion of the diaphragm, and
its own muscular arrangement. It will readily be perceived, that by this
joint action the food is submitted to a sort of grinding process. Hence
any over-distention of the viscera, from either food or gas, will
embarrass and prevent the free and full play of this organ. The papillae,
or prominences, present a rough and sufficiently hard exterior to grind
down the food, unless it shall have escaped the reticulum in too fibrous
a form: foxgrass, cornstalks, and frosted turnips are very apt to make
sad havoc in this and other parts of the stomach, owing to their
unyielding nature; for the stomach, like other parts of the
organization, suffers from over-exertion, and a corresponding debility
ensues.
The fourth division of the stomach of the ox is called _abomasum_. It
somewhat resembles the duodenum of the horse in its function, it being
the true digestive stomach. It is studded with numerous nerves,
blood-vessels, and small glands. It is a laboratory admirably fitted up
by the Divine Artist, and is capable of carrying on the chemico-vital
process as long as the animal lives, provided its healthy functions are
not impaired. The glands alluded to secrete from the blood a powerful
solvent, called the _gastric juice_, which is the agent in reducing the
food to chyme and chyle. This, however, is accomplished by the united
agency of the bile and pancreatic juice. Both these fluids are conveyed
into the abomasum by means of small tubes or canals. Secretions also
take place from the inner membrane of the intestines, and, as the result
of the united action of all these fluids, aided by the muscular motion
just alluded to, which is also communicated to the intestines, a
substance is formed called _chyle_, which is the most nutritious portion
of the food, and has a milky appearance. The chyle is received into a
set of very minute tubes, called _lacteals_, which are exceedingly
numerous, and arise by open mouths from the inner surface of the
abomasum and intestines. They receive the chyle; from thence it passes
into a receptacle, and finally into the thoracic duct. The thoracic duct
opens into a vein leading directly to the heart; so that whatever
portion of the chyle is not actually needed by the organism is
thoroughly mixed with the general mass of blood. That portion of chyme
which is not needed, or cannot be converted into chyle, descends into
the intestines, and is finally carried out of the body by the rectum.
The manner in which the gastric fluids act on alimentary matter, is by
solution and chemical action; for cornstalks and foxgrass, that cannot
be dissolved by ammonia or alcohol, yield readily to the solvent power
of the gastric secretion. Bones and other hard substances are reduced to
a pulpy mass in the stomach of a dog; while, at the same time, many
bodies of delicate texture remain in the stomach, and ultimately are
ejected, without being affected by the gastric fluids. This different
action on different subjects is analogous to the operation of chemical
affinity, and corroborates the theory that digestion is effected by
solution and chemical action.
_The Spleen_, or _Milt_, is an oblong, dark-colored substance, having
attachments to the paunch. It is composed of blood-vessels, nerves, and
lymphatics, united by cellular structure. It appears to serve as a
reservoir for the blood that may be designed for the secretions of bile
in the liver. P. M. Roget says, "Any theory that assigns a very
important function to the spleen will be overturned by the fact, that in
many animals the removal of this organ, far from being fatal, or
interrupting, in any sensible manner, the continuance of the functions,
seems to be borne with perfect impunity." Sir E. Home, Bichat, Leuret,
Lassaigne, and others, suppose that "the spleen serves as a receptacle
for the superfluous quantity of fluid taken into the stomach."
_The Liver_ is a dense gland, of a lobulated structure, situated below
the diaphragm, or "skirt." It is supplied, like other organs, with
arterial blood, by vessels, called _hepatic_ arteries, which are sent
off from the great aorta. It receives also a large amount of venous
blood, which is distributed through its substance by a separate set of
vessels, derived from the venous system. The veins which receive the
blood that has circulated in the usual manner unite together into a
large trunk, called vena portae, (gate vein,) and this vein, on entering
the liver, ramifies like an artery, and ultimately terminates in the
branches of the hepatic veins, which transmit the blood, in the ordinary
course of circulation, to the vena cava, (hollow vein.) Mr. Kiernan
says, "The hepatic veins, together with the lobules which surround them,
resemble, in their arrangement, the branches and leaves of a tree, the
substance of the lobules being disposed around the minute branches of
the veins like the parenchyma of a leaf around its fibres. The hepatic
veins may be divided into two classes, namely, those contained in
lobules, and those contained in canals formed by lobules. The first
class is composed of interlobular branches, one of which occupies the
centre of each lobule, and receives the blood from a plexus formed in
the lobule by the portal vein; and the second class of hepatic veins is
composed of all those vessels contained in canals formed by the lobules,
and including numerous small branches, as well as the large trunks
terminating in the inferior cava. The external surface of every lobule
is covered by an expansion of '_Glisson's capsule_,' by which it is
connected to, as well as separated from, contiguous lobules, and in
which branches of the hepatic duct, portal veins, and hepatic artery
ramify. The ultimate branches of the hepatic artery terminate in the
branches of the portal vein, where the blood they respectively contain
is mixed together, and from which mixed blood the bile is secreted by
the lobules, and conveyed away by the hepatic ducts. The remaining blood
is returned to the heart by the hepatic veins, the beginnings of which
occupy the centre of each lobule, and, when collected into trunks, pour
their contents into the inferior cava. Hence the blood which has
circulated through the liver, and has thereby lost its arterial
character, is, in common with that which is returning from other parts,
poured into the vena portae, and contributes its share in furnishing
materials for the biliary secretion. The hepatic artery furnishes
nutrition to the liver itself."
The bile, having been secreted, accumulates in the gall-bladder, where
it is kept for future use. When the healthy action of the fourth stomach
is interrupted, the bile is supposed to be reabsorbed,--it enters into
the different tissues, producing yellowness of the eyes; the malady is
then termed _yellows_, _jaundice_, &c. Sometimes the passage of the bile
is obstructed by calculi, or gall-stones; they have been found in great
numbers in oxen.
_The Pancreas_ is composed of a number of lobules or glands; a small
duct proceeds from each; they unite and form a common canal, which
proceeds towards, and terminates in, the fourth stomach. The pancreatic
juice appears to be exceedingly analogous, both in its sensible
properties and chemical composition, to the saliva.
"The recent researches of MM. Bouchardat, Sandras, Mialhe, Bareswil, and
Bernard himself, have placed beyond a doubt the existence of a ferment,
in some of the fluids which mix with the alimentary mass, destined to
convert starchy matters into sugar. They have proved that the gastric
juice has for its peculiar office the solution and digestion of azotized
substances. There remained to be ascertained the real agent for the
digestion of fatty matters; that is to say, the agent in the formation
of chyle out of those substances.
"M. Bernard has proved that this remarkable office is performed by the
pancreatic juice; he has demonstrated the fact by three conclusive
proofs.
"1. The pancreatic juice, pure and recently formed, forms an emulsion
with oils and fats with the greatest facility. This emulsion may be
preserved for a long time, and the fatty substance soon undergoes a
fermentation which separates its constituent acids.
"2. The chyle only begins to appear in the lacteals below that part of
the intestinal tube where the pancreatic juice enters it to mix with the
alimentary matters.
"3. In disorders of the pancreas, we find that the fatty matters,
contained in the food, pass entire in the evacuations."
The above is an extract from the report of a body composed of several
members of the French Academy of Sciences. "M. Bernard" (continues the
report) "has exhibited to us the first of these experiments, and has
furnished us with the means of repeating it with the several varieties
of the gastric juice. We have not the slightest doubt on the subject. It
is incontestable that fatty substances are converted into an emulsion by
this juice, in a manner easy and persistent, and it is no less true that
the saliva, the gastric juice, and the bile are destitute of this
property.
"The second demonstration can be given in various modes; but the author
has discovered, in the peculiar arrangement of the digestive apparatus
of the rabbit, an unexceptional means of obtaining it with the greatest
precision, and at will. The pancreatic juice enters the intestinal tube
of this animal about fourteen inches below the point where the bile is
poured in. Now, as long as the food is above the region where it mixes
with the pancreatic juice, there appears to be no formation and
separation of a milky chyle; nothing shows that the fatty matters are
reduced to an emulsion. On the contrary, as soon as the pancreatic juice
mixes with the alimentary matters, we observe the fat to be converted
into an emulsion, and a milky chyle to fill the corresponding lacteals.
Nothing can give an idea of the result of these experiments, which have
all the accuracy of a chemical operation performed in the laboratory,
and all the beauty of the most perfect injection.
"We are not, therefore, surprised that divers pathological cases,
hitherto imperfectly understood, should come to confirm the views of M.
Bernard, by proving that, in diseases of the pancreas, fatty matters
have been observed to pass unchanged in the dejections.
"The committee cannot hesitate to conclude that the author has perfectly
demonstrated his physiological propositions; that he has completed the
general characters of the theory of digestion, and that he has made
known the mode of formation of the fatty matter of the chyle, and the
manner of the digestion of the fatty matters."
_The Kidneys._--Their office is, to secrete from the blood the useless
or excrementitious fluids in the form of urine. When the skin is
obstructed, the secretion is augmented, and profuse perspiration lessens
it. From a cavity in the centre of each kidney a canal or tube proceeds,
by which the urine is conveyed into the bladder. These tubes are named
_ureters_. As the ureters enter the bladder, they pass forward, a short
distance between its coats; which effectually prevents the urine from
taking a retrograde course. The urine is expelled by the muscular power
which the bladder possesses of contracting upon its contents.