10 CHAPTER II. GENERAL DESCRIPTION OF THE LIVER.-DIMENSIONS.-BULK. - WEIGHT.-SPECIFIC GRAVITY.-CHEMICAL COMPOSITION.- FOR much that is known of the anatomy of the liver, we are indebted to the labors of one of the greatest living anatomists, Mr. Kiernan, and almost every one is well acquainted with his most valuable researches upon the structure of this gland.* These investigations were made nearly a quarter of a century ago, at a time when the compound microscope was a very imperfect instrument, and principally with the aid of a lens alone. The liver-cells, the great agents in the secretion of bile, were hardly known, and the exact arrangement of the capillary vessels was not satisfactorily demonstrated until many years after. Yet very much of of that which Mr. Kiernan did, and the correctness of the conclusions which he arrived at, have been fully confirmed by later investigators. The latest researches appear to militate most strongly against the inferences of Mr. Kiernan with reference to the manner in which the ducts commence; but there is, I think, little doubt, that he was, in the main, right, and the conclusions which I have arrived at in investigating many of my own preparations were foreshadowed by him at a time when the means of observation were very imperfect in comparison to those which we may now employ. Dimensions, Weight, and Chemical Composition of the Healthy Liver. The liver in health measures about 12 inches from side to side, and 6 or 7 in its antero-posterior diameter. Its bulk corresponds to nearly 100 cubic inches, and its weight varies from 3 to 4 pounds, according to the quantity of blood which it may contain * On the Anatomy and Physiology of the Liver. Phil. Trans. 1833. at the time it is examined. Its specific gravity in health is about 1.05. The following is an analysis of a liver presumed to be healthy. The organ was taken from the body of a man 31 years of age, who was killed by falling from a second-floor window while in the enjoyment of perfect health. Per 100 of 68.58 ... ... In disease, the proportion of these constituents is liable to very great variation. In fatty degeneration an enormous amount of fatty matter may accumulate in the organ. In one remarkable case, I obtained from a liver 75-07 per cent. of solid matter, and of this 65.19 consisted of fatty matter.* In scrofulous degeneration of the liver, the albuminous materials and the water are increased, while the fatty matter is diminished in quantity. The liver is essentially composed of certain elementary structures, which are common to all secreting organs possessing ducts, although in the different glands these structures are variously modified to serve special ends, which differ in each individual organ. These essential structural elements are the following:-Cells, lying in a cavity composed of basement membrane, which have the power of separating certain peculiar substances from the blood, for which alone they have any attraction. Vessels, which bring the blood into close proximity with the cells, and Efferent Ducts, in direct continuity with the cavity in which the secreting cells are contained, which carry off the secretion after it has been elaborated. It is impossible to help being forcibly struck with the manner in which these anatomical elements are arranged in this organ, for the same disposition of the parts which economizes material and * Diseases of the Liver. Dr. Budd. Second edition. space in the greatest possible degree, provides for the most complete and perfect action of the cells upon the blood. While the latter fluid is made to pass slowly through an extensive series of tortuous channels, it is only separated from the cells by a delicate membrane of extreme tenuity, through the intervention of which it bathes the greater part of the surface of each individual cell. In the liver the capillary network is of a very unusual extent, a circumstance which alone would lead us to infer that the change exerted upon the blood as it passes through the organ is of a very important nature, and one which must be effected completely. The system of efferent ducts is extensive, and their course before they make their exit from the organ, a very intricate one. The coats of the larger ducts are largely supplied with arterial blood, and the arrangement of vessels around all of them is peculiar. Portal Canals and Hepatic Venous Canals.-In the compact mammalian liver there are two important series of channels, which in their ultimate distribution may be said to alternate with each other (fig. 2). Of these, one set contains branches of the portal vein, hepatic artery, and hepatic duct, while the other series contains branches of the hepatic vein alone, with the vascular branches which are supplied to their coats. Such an arrangement permits of a very equable and free distribution of the blood to the capillaries of the individual lobules, of which the organ is made up, and promotes its rapid removal after the proper changes have taken place in it; while at the same time the carrying off of the bile after its elaboration is effectually provided for, and the occurrence of other changes in it during its course to the gall bladder and common duct is facilitated. Upon the under-surface of the liver is a large groove, the longitudinal fissure, from which passes off transversely upon the surface of the right lobe another depression, the transverse fissure. At this point the portal vein, the hepatic artery, and nerve-fibres, enter the substance of the organ; and the hepatic duct and numerous lymphatic vessels emerge from it. These structures all lie in close proximity to each other, and, in the larger channels, are bound together by a certain quantity of areolar tissue. From the transverse fissure may be traced numerous tubular passages throughout the entire substance of the liver, those most distant being of course the smallest. They all contain branches of the vessels above enumerated, and in this way no portion of hepatic tissue is at a greater distance from one of these canals than the one-hirtieth of an inch. These tubular passages have been named portal canals, by Mr. Kiernan. From the oblique notch on the posterior thick border, can be traced another series of channels, which contain branches of the hepatic vein, and every part of the liver is brought into an equally close relation with one of these canals, which have been termed by the same observer hepatic venous canals. Such is this beautiful arrangement, of which there is no other example, by which the vessels are brought into close relation with every portion of this large and compact gland; and thus an even distribution of the blood to each individual part is insured. From this general survey of the arrangement of the vessels of the liver, we pass naturally to the consideration of the lobules. LOBULES OF THE LIVER. If the surface of a liver be examined, it is perceived to be mapped out, as it were, into a number of little spaces, about the size of a small hemp-seed, but differing slightly from each other in dimensions and form. The appearance of these markings is different in the livers of different animals. In the pig each little space is completely circumscribed, but, in the human subject, they seem to fit into, or in a manner appear to dovetail with each other. In other mammalian animals the markings are more or less distinct than in the human subject, but in no one are these spaces defined in the perfect manner in which they are in the pig. If a section be made in various directions in different parts of the organ, a similar mapping out is seen, although, perhaps, it is not quite so distinct as upon the surface. The structure of all these little component masses of the liver is similar. Each little mass, whether it be perfectly circumscribed, as in the pig, or less completely so as is the case in other animals, is termed a "lobule." A lobule, then, contains all the essential elements for the secretion of bile, and for effecting those changes in the blood which this fluid is known to undergo in its passage through the capillary vessels. Lobules of the Pig's Liver.-A lobule contains all the elements of the liver. The entire organ is a collection of lobules. Every lobule may be regarded as an elementary liver. The small liver of the mouse differs from the large liver of the elephant in a certain degree with respect to the size of the lobules, but enormously with regard to their number. Now, the lobule is most perfectly seen in the pig's liver, and also in that of the polar bear, according to Müller, and in that of the Octodon Cummingii (one of the Rodents), according to the observations of Hyrtl; and these livers, in their anatomical arrangement, differ vastly from those of other animals with which I am acquainted, as regards the separation of the lobules from each other. With care, a portion of the liver of the pig may be separated into a number of small roundish masses, like very small peas. Each is invested with a firm, fibrous membrane of its own; and, if it be pressed strongly, it bursts, and its contents may be squeezed out. Upon the surface of this lobule ramify small twigs of the portal vein, hepatic artery, and hepatic duct; many branches from these perforate the capsule at various points, and are distributed in the interior of the lobule. The capsule of the lobule is composed of a variety of fibrous tissue. The fibres are exceedingly delicate, and so arranged as to surround small apertures, through which branches of the vessels and duct pass to the interior. The capsules of adjacent lobules are connected together by the branches of these vessels, and, in many instances, by a small quantity of fibrous tissue. Many observers describe the lobules of the pig's liver as being surrounded with an expansion of the capsule of Glisson, an arrangement which is sanctioned by Kölliker, who considers that the fibrous material forms partitions between the lobules instead of forming a distinct and separate capsule to each lobule. If a piece of fresh pig's liver be injected with alcohol, to which a few drops of a solution of soda have been added, from the portal vein, and then the whole soaked in the same solution for a few days until it has become hard, the lobules can be readily separated in the manner above described. In very thin sections, from well prepared specimens in which the vessels have been injected, the separate outlines of the capsules of adjacent lobules may be dis |