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The formation of sugar in the liver is therefore a function composed of two distinct and successive processes, viz: first, the formation, in the hepatic tissue, of a glycogenic matter, having some resemblance to dextrine; and secondly, the conversion of this gly. cogenic matter into sugar, by a process of catalysis and transformation.

The sugar thus produced in the substance of the liver is absorbed from it by the blood circulating in its vessels. The mechanism of this absorption is probably the same with that which goes on in other parts of the circulation. It is a process of transudation and endosmosis, by which the blood in the vessels takes up the saccharine fluids of the liver, during its passage through the organ. While the blood of the portal vein, therefore, in an animal fed exclusively upon meat, contains no sugar, the blood of the hepatic vein, as it passes upward to the heart, is always rich in saccharine ingredients. This difference can easily be demonstrated by examining comparatively the two kinds of blood, portal and heptic, from the recently killed animal. The blood in its passage through the liver is found to have acquired a new ingredient, and shows, upon examination, all the properties of a saccharine liquid.

The sugar produced in the liver is accordingly to be regarded as a true secretion, formed by the glandular tissue of the organ, by a similar process to that of other glandular secretions. It differs from the latter, not in the manner of its production, but only in the mode of its discharge. For while the biliary matters produced in the liver are absorbed by the hepatic ducts and conducted downward to the gall-bladder and the intestine, the sugar is absorbed by the blood vessels of the organ and carried upward, by the hepatic veins, toward the heart and the general circulation.

The production of sugar in the liver during health is a constant process, continuing, in many cases, for several days after the animal has been altogether deprived of food. Its activity, however, like that of most other secretions, is subject to periodical augmentation and diminution. Under ordinary circumstances, the sugar, which is absorbed by the blood from the tissue of the liver, disappears very soon after entering the circulation. As the bile is transformed in the intestine, so the sugar is decomposed in the blood. We are not yet acquainted, however, with the precise nature of the changes which it undergoes after entering the vascular system. It is very probable, according to the views of Lehmann and Robin, that it is at first converted into lactic acid (C,H,O), which decomposes in

turn the alkaline carbonates, setting free carbonic acid, and form ing lactates of soda and potass. But whatever be the exact mode of its transformation, it is certain that the sugar disappears rapidly; and while it exists in considerable quantity in the liver and in the blood of the hepatic veins and the right side of the heart, it is not usually to be found in the pulmonary veins nor in the blood of the general circulation.

About two and a half or three hours, however, after the ingestion of food, according to the investigations of Bernard, the circulation of blood through the portal system and the liver becomes considerably accelerated. A larger quantity of sugar is then produced in the liver and carried away from the organ by the hepatic veins; so that a portion of it then escapes decomposition while passing through the lungs, and begins to appear in the blood of the arterial system. Soon afterward it appears also in the blood of the capillaries; and from four to six hours after the commencement of digestion it is produced in the liver so much more rapidly than it is destroyed in the blood, that the surplus quantity circulates throughout the body, and the blood everywhere has a slightly saccharine character. It does not, however, in the healthy condition, make its appearance in any of the secretions.

After the sixth hour, this unusual activity of the sugar producing function begins again to diminish; and the transformation of the sugar in the circulation going on as before, it gradually disappears as an ingredient of the blood. Finally, the ordinary equilibrium between its production and its decomposition is re-established, and it can no longer be found except in the liver and in that part of the circulatory system which is between the liver and the lungs. There is, therefore, a periodical increase in the amount of undecomposed sugar in the blood, as we have already shown to be the case with the fatty matter absorbed during digestion; but this increase is soon followed by a corresponding diminution, and during the greater portion of the time its decomposition keeps pace with its production, and it is consequently prevented from appearing in the blood of the general circulation.

There are produced, accordingly, in the liver, two different secretions, viz., bile and sugar. Both of them originate by transformation of the ingredients of the hepatic tissue, from which they are absorbed by two different sets of vessels. The bile is taken up by the biliary ducts, and by them discharged into the intestine; while the sugar is carried off by the hepatic veins, to be decomposed in the circulation, and become subservient to the nutrition of the blood.

CHAPTER X.

THE SPLEEN.

THE spleen is an exceedingly vascular organ, situated in the vicinity of the great pouch of the stomach and supplied abundantly by branches of the coeliac axis. Its veins, like those of the digestive abdominal organs, form a part of the great portal system, and conduct the blood which has passed through it to the liver, before it mingles again with the general current of the circulation. The spleen is covered on its exterior by an investing membrane or capsule, which forms a protective sac, containing the soft pulp of which the greater part of the organ is composed. This capsule, in the spleen of the ox, is thick, whitish and opaque, and is composed to a great extent of yellow elastic tissue. It accordingly possesses, in a high degree, the physical property of elasticity, and may be widely stretched without laceration; returning readily to its original size as soon as the extending force is relaxed.

In the carnivorous animals, on the other hand, the capsule of the spleen is thinner, and more colorless and transparent. It contains here but very little elastic tissue, being composed mostly of smooth, involuntary muscular fibres, connected in layers by a little. intervening areolar tissue. In the herbivorous animals, accordingly, the capsule of the spleen is simply elastic, while in the carnivora it is contractile.

In both instances, however, the elastic and contractile properties of the capsule subserve a nearly similar purpose. There is every reason to believe that the spleen is subject to occasional and perhaps regular variations in size, owing to the varying condition of the abdominal circulation. Dr. William Dobson' found that the size of the organ increased, from the third hour after feeding up to the fifth; when it arrived at its maximum, gradually decreasing after that period. When these periodical congestions take place,

'In Gray, on the Structure and Uses of the Spleen. London, 1854, p. 40.

the organ becoming turgid with blood, the capsule is distended; and limits, by its resisting power, the degree of tumefaction to which the spleen is liable. When the disturbing cause has again passed away, and the circulation is about to return to its ordinary condition, the elasticity of the capsule in the herbivora and its contractility in the carnivora, compress the soft vascular tissue within, and reduce the organ to its original dimensions. This contractile action of the investing capsule can be readily seen in the dog or the cat, by opening the abdomen while digestion is going on, exposing the spleen and removing it, after ligature of its vessels. When first exposed, the organ is plump and rounded, and presents externally a smooth and shining surface. But as soon as it has been removed from the abdomen and its vessels divided, it begins to contract sensibly, becomes reduced in size, stiff, and resisting to the touch; while its surface, at the same time, becomes uniformly wrinkled, by the contraction of its muscular fibres.

In its interior, the substance of the spleen is traversed everywhere by slender and ribbon-like cords of fibrous tissue, which radiate from the sheath of its principal arterial trunks, and are finally attached to the internal surface of its investing capsule. These fibrous cords, or trabeculæ, as they are called, by their frequent branching and mutual interlacement, form a kind of skeleton or framework by which the soft splenic pulp is embraced, and the shape and integrity of the organ maintained. They are composed of similar elements to those of the investing capsule, viz., elastic tissue and involuntary muscular fibres, united with each other by a varying quantity of the fibres of areolar tissue.

The interstices between the trabecule of the spleen are occupied by the splenic pulp; a soft, reddish substance, which contains, beside a few nerves and lymphatics, capillary blood vessels in great profusion, and certain whitish globular bodies, which may be regarded as the distinguishing anatomical elements of the organ, and which are termed the Malpighian bodies of the spleen.

The Malpighian bodies are very abundant, and are scattered throughout the splenic pulp, being most frequently attached to the sides, or at the point of bifurcation of some small artery. They are readily visible to the naked eye in the spleen of the ox, upon fresh section of the organ, as minute, whitish, rounded bodies, which may be separated, by careful manipulation, from the surrounding parts. In the carnivorous animals, on the other hand, and in the human subject, it is more difficult to distinguish them by the un

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aided eye, though they always exist in the spleen in a healthy condition. Their average diameter, according to Kölliker, is of an inch. They consist of a closed sac, or capsule, containing in its interior a viscid, semi-solid mass of cells, cell-nuclei, and homogeneous substance. Each Malpighian body is covered, on its exterior, by a network of fine capillary blood vessels; and it is now perfectly well settled, by the observations of various anatomists (Kölliker, Busk, Huxley, &c.), that blood vessels also penetrate into the substance of the Malpighian body, and there form an internal capillary plexus.

The spleen is accordingly a glandular organ, analogous in its minute structure to the solitary and agminated glands of the small intestine, and to the lymphatic glands throughout the body. Like them, it is a gland without an excretory duct; and resembles also, in this respect, the thyroid and thymus glands and the supra-renal capsules. All these organs have a structure which is evidently glandular in its nature, and yet the name of glands has been sometimes refused to them because they have, as above mentioned, no duct, and produce apparently no distinct secretion. We have already seen, however, that a secretion may be produced in the interior of a glandular organ, like the sugar in the substance of the liver, and yet not be discharged by its excretory duct. The veins of the gland, in this instance, perform the part of excretory ducts. They absorb the new materials, and convey them, through the medium of the blood, to other parts of the body, where they suffer subsequent alterations, and are finally decomposed in the circula

tion.

The action of such organs is consequently to modify the constitution of the blood. As the blood passes through their tissue, it absorbs from the glandular substance certain materials which it did not previously contain, and which are necessary to the perfect constitution of the circulating fluid. The blood, as it passes out from the organ, has therefore a different composition from that which it possessed before its entrance; and on this account the name of vascular glands has been applied to all the glandular organs above mentioned, which are destitute of excretory ducts, and is eminently applicable to the spleen.

The precise alteration, however, which is effected in the blood during its passage through the splenic tissue, has not yet been discovered. Various hypotheses have been advanced from time to time, as to the processes which go on in this organ; many of them

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