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CHAPTER VI.

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SOME GENERAL OBSERVATIONS UPON THE ANATOMY PHYSIOLOGY OF THE LIVER:-THE VERTEBRATE LIVER.THE VERTEBRATE AND INVERTEBRATE LIVER COMPARED.THE LIVER AND KIDNEY COMPARED.-POSITION OF THE LIVER AS A SECRETING ORGAN. -SUMMARY.

The Vertebrate Liver.-The vertebrate liver may be looked upon as consisting essentially of two distinct systems of channels arranged so as to form solid* networks which mutually interlace with each other. In one of these networks lie the secreting cells, often arranged so as to form only a single row, which is, therefore, surrounded on all sides by the blood containing the elements from which the bile is formed. The fluid bile escaping from the cells into the network of tubes, flows towards the surface of the lobule where the ducts lie. In the other network flows the blood, but in a precisely opposite direction to that which the bile takes.

The

The portal blood reaches the capillaries of the lobule at many different points upon its surface, and all the blood from these numerous sources at the circumference, must pass through capillary vessels which converge towards the single radicle of the hepatic vein in the centre; and hence it follows that the circulation of this blood must be much more rapid in the central part of the lobule than near the portal surface. This perfectly harmonizes with many anatomical facts which I have before alluded to. blood which has just arrived from the intestines, loaded with recently absorbed constituents, flows very slowly, in order to give time for the action of the liver cells upon it, and for the removal of some of these substances from it. When these have been separated, its slow course is no longer necessary, and as it becomes purer it flows with increasing velocity, until it is at last poured into the large inferior cava.

By the very simple and beautiful combination of the elementary * This term was first employed by Mr. Bowman, vide article, "Mucous Membrane," Todd's Cyclopedia of Anatomy and Physiology.

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tissues of the liver, so as to form minute portions resembling each other, or lobules, the most complete changes in the circulating blood are ensured, while the circulation is retarded in the least possible degree compatible with the alteration to be produced; and at the same time the anatomical elements, through the intervention of which these changes are effected, are combined in such a manner as to occupy the smallest possible amount of space.

The cells in all parts of the network have, no doubt, the power of forming bile, but in different degrees; those quite close to the centre of the lobule, have often been observed to be filled with yellow granules. There is no reason for supposing that the secretion is passed from cell to cell, and at last into the duct; for if this were the case, we should at least expect to find greater difference in the cells near the ducts than really exists; and it must be remembered that in many animals in a state of perfect health, no difference whatever is to be demonstrated. The cells at the circumference of the lobules of the human liver usually contain a much larger quantity of oil than those nearer the centre, which is just what we should anticipate when we consider that the portal blood, rich with the freshly absorbed constituents of the food, first reaches these marginal cells. The direction of the circulation is the reverse of that which we should expect to find if the secretion were transmitted from cell to cell, as most authorities in the present day insist upon. For if this were really the case, it seems to me that we should be forced to admit that the blood flowed through the superficial part of the capillary network to no purpose. On the other hand, the anatomy of the organ almost forces upon us the conclusion, that the cells nearest to the duct take the most active part in the secretion, and that the blood gets gradually rendered more free of substances from which the bile is formed, as it travels along the capillary tubes from circumference to centre, with a gradually increasing rapidity of movement. Lastly, the assumed absence of any channel in which bile could flow, ceases to be a tenable objection, since it has been shown to be comparatively easy, provided that certain precautions be observed in conducting the operation, to force injection into a channel which does exist. For if, by artificial means, fluid can be made to pass in an opposite direction, within the tubes of this

cell-containing network, up to the hepatic vein, how very sure may we feel of the existence of such a channel along which the bile flows during the life of the organ.

This large and important organ presents us with an example of gland structure in which the different tissues of which it is composed are arranged in the most advantageous manner it is possible to conceive. The secerning cells and the blood are brought into the closest proximity possible. The blood containing the crude elements is separated from the cells whose office it is to elaborate these constituents, only by the intervention of the thin capillary walls and a membranous partition, under ordinary circumstances not visible, and so delicate that its existence is disputed by many great authorities. The areolar tissue, which serves to support the capillaries of most glands, is here absent, and the wall of the capillary comes into close contact, and in many instances is incorporated with the thin transparent membranous tube in which the liver cells lie.

The alteration in volume which the cells may be made to undergo by artificial means, indicates how very simply a channel may be produced when the organ is in its natural condition. The two sets of tubes mutually adapt themselves to each other; and if the secreting tubes be unusually empty, the vascular tubes admit more blood. It has been already pointed out what a very large proportion of fluid the capillary vessels of the liver are capable of containing over and above what may be assumed to be their normal quantity, and also how the distension of the vessels, by forcing in fluid in one direction, promotes the passage of liquid through the cell-containing network in the opposite direction. Careful reflection upon some of these circumstances assists much in the comprehension of many of the morbid changes which take place in the liver.

The bile having arrived at the smallest ducts, without doubt undergoes further changes. It is not improbable that the oxygen brought by the arterial blood which flows in the vascular network. through the meshes of which the ducts pass, exerts some influence upon it. This, however, has not been proved. The most important alteration which the bile now undergoes, is concentration. Water is removed, and the proportion of solid constituents gra

dually increases as the bile passes slowly along the thin-walled, tortuous ducts. It is this change which serves to explain the comparatively slight increase of diameter of the ducts, in proportion to the great number of branches which they receive. Thus the apparent disproportion between the vast amount of secreting structure and the small efferent ducts, vanishes, when we consider how highly concentrated the bile becomes before it reaches the large duct. Bile contains a larger proportion of soluble constituents than any other liquid secretion.

The manner in which this concentration is effected has been alluded to. In those animals provided with a gall-bladder, it has been shown experimentally by Bidder and Schmidt that much fluid is absorbed while the bile remains within this viscus. It is interesting to observe, that the arrangement of vessels and lymphatics in the portal canals and transverse fissure of the liver, is similar to that which is met with in the gall-bladder. The bile is brought into very close relation with the vessels, by entering the little cavities, parietal sacculi, in the coats of the ducts, and in the vasa aberrantia, which are always surrounded by numerous branches of the vein and artery, and by lymphatic vessels.

In some instances the inspissation of the bile is carried to an abnormal extent, and small, tolerably hard granules of biliary matter are eventually produced. These often form the nuclei of gall-stones, and sometimes accumulate in great numbers in a branch of the duct. Although a calculus might become impacted in a duct, the free transmission of the bile is fully provided for by the numerous anastomosing branches, just external to the duct itself, which have been described in Chapter III.

The Vertebrate and Invertebrate Liver compared.-It has been remarked by many writers, that there is a great difference of structure between the livers of the vertebrata and the invertebrata; but the more carefully the vertebrate liver is examined in its minute details, the stronger becomes the evidence that both resemble each other in many essential particulars. Nay, in some of the lowest of the invertebrata, the general minute anatomy of the liver closely accords with that of the organ in the highest animals. In the large intestine of the common frog, a small entozoon of the Trematode order is very often met with. In some of these, which

were taken from the body of a starved frog, the biliary follicles and their ducts were very distinct. The cells in their interior contained colouring matter, and but very little oil, consequently the follicles were somewhat shrunken, and could be examined very satisfactorily. The follicle terminates in a cœcal extremity, but is prolonged at the opposite end into a very narrow duct, which is soon joined by the ducts from other follicles. Suppose the cœcal extremities extended, and communicating with each other, so as to form a network, and an appearance closely resembling the drawing of the connexion between the ducts and cell-containing network in fig. 27 would result. The cells appear to contain colouring matter and oil, either in a granular state or in the form of distinct globules, in all classes of animals; their general character is very similar, and they lie in the tube or follicle which contains them without order or regularity, their form being influenced to a considerable extent by mutual pressure. It is impossible to help seeing a similarity in the arrangement of the cœcal follicles, with their narrow necks, around some of the ducts in the higher animals, and comparing this with the follicular condition of the entire organ many of the mollusca and crustacea.

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In the different members of the animal creation, the liver-cell preserves certain constant peculiarities; and in the development of the young animal, this cell soon attains the characters which it bears throughout life, except that it contains a greater number of nuclei at this early period. The comparison is exceedingly interesting, as shewing the great similarity in the essential arrangement of the liver in animals occupying such opposite positions in the animal scale.

The Liver and Kidney compared.-The bile and the urine, the two most important secretions in the body, have often been contrasted, and their opposite or complementary nature has been pointed out. It is interesting to compare the anatomy of the glands which secrete these dissimilar fluids.

The kidney is an organ destined for the rapid removal of substances, the most important of which, at least, exist pre-formed in the blood, largely diluted with water. A tortuous tube of basement membrane, closed at one end, lined in its interior with cells, but having a free central cavity, is the essential apparatus by