Читать книгу The American Reformed Cattle Doctor - George H. Dadd - Страница 15

Table of Contents

Œsophagus, 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 laminæ. Some of these are attached to the upper and lower portion of the division, and also float loose, and penetrate into the œsophagian canal. The laminæ have numerous projections on their surface, resembling the papillæ 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 papillæ, 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 portæ, (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 portæ, 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."