Page images
PDF
EPUB

artery. The distension of the capillaries with blood in a moderate degree would tend to force the bile towards the ducts, but if the bile accumulated to an unusual extent in the smaller ducts, or if it became so viscid that it would not flow readily through them; or if the bile were prevented from escaping readily from the common duct, the free circulation of the blood through the capillaries of the liver would be interfered with.

PORTAL CANALS.

The course which the portal canals take varies considerably, being sometimes highly tortuous and irregular, and in other instances so straight and even, that they appear to radiate from the transverse fissure of the liver towards the circumference of the lobes, in which case they are seen to alternate with the hepatic venous canals. This point is shown in fig. 31, which is a representation of the thin lobe of a rabbit's liver, in which the portal vein has been injected white, the artery red, and the duct dark blue, while the branches of the hepatic vein are yellow. The direction of the portal canals is also tolerably straight in the seal, but less so in the pig. In sections of human liver, the smallest channels seem to alternate in the manner described, and, as it were, to interdigitate with the hepatic venous canals (fig. 2). When the smaller trunks have been divided at right angles, a small hepatic vein is seen to be surrounded by several branches of the portal vein, artery, and duct, nearly equi-distant from it, and a short distance from each other, as represented in fig. 3, in which branches of the portal vein are marked a, and those of the hepatic vein b. In the thickest part of the liver the course of the portal canals is very tortuous, and a canal lies on many different planes at different parts of its course. When the lobules of the pig are divided, the hepatic vein is seen to be the centre of a space bounded by twigs of the portal vein, artery, and duct, which ramify upon the capsules of the lobules, and give off branches to the interior, as shown in figs. 7, 10, 11. The disposition of the branches of the portal vein is so regular, as to give the idea, when examined by low powers or with the naked eye, of a complete venous ring, in the centre of which is situated a branch of hepatic vein; but this is shown to be a fallacious appearance, by the careful exami

nation of well-prepared moist specimens with glasses which magnify about 40 diameters. By their mode of ramification the smaller portal vessels more or less imperfectly enclose little spaces, which vary somewhat in form in different animals, and according to the manner in which the section is made.

In the majority of animals the secreting structure and capillaries forming the substance of one lobule, communicate with those of the adjacent ones in the intervals between the channels by which the branches of the vessels reach the lobules. In the pig the complete isolation is dependent partly upon the disposition of the small vessels, but principally upon the presence of the fibrous capsule described in page 14. Branches of vessels which ramify in the portal canals are conducted into the spaces between the lobules, the interlobular fissures, and thus reach the margin of the lobules. The triangular interval left by the approximation of three lobules was termed by Mr. Kiernan the interlobular space.

GLISSON'S CAPSULE.

This structure since the time of its discoverer, has been described as a most important and essential constituent of the liver. Continuous with the proper capsule of the organ externally, it is said not only to form a sheath for the large vessels as they lie in the portal canals, but to be prolonged with them into the ultimate parts of the gland, and even to form, for each lobule a proper investment, or, as others have described it, a partition between contiguous lobules.

It is an important point to ascertain if the arrangement of this areolar tissue in the liver differs in any very essential particulars from that of other glandular organs.

Mr. Kiernan showed that, in the smaller portal canals, Glisson's capsule did not completely invest the vessels, but was only to be found upon that side of the vein on which the duct and artery were situated.

Most anatomists have failed to demonstrate a trace of areolar tissue within the lobules of the liver. Occasionally, a few fibres of a structure like fibrous tissue may, undoubtedly, be observed in uninjected specimens; but such an appearance is produced by physical alterations of the structures in the lobule, in the prepara

tion of the specimen, or it is the result of disease. In the lobules of the livers of all the animals which have fallen under my notice, it was impossible to demonstrate any fibrous structure whatever.

Even in the interlobular fissures of the human liver, and of others allied to it in structure, I have been unable to detect any fibrous structure. Bowman, Henle, and Vogel have altogether failed to detect any areolar tissue in this situation in the human liver.

Between the capsules of adjacent lobules in the pig's liver, the elements of areolar tissue undoubtedly exist in small quantity. The capsule can be easily demonstrated; its contents can be washed out. It is composed of delicate fibrous tissue quite distinct from the areolar tissue found in the interlobular spaces and portal canals. In the latter situation the ordinary elements of areolar tissue, white and yellow fibrous tissue, and not unfre-quently adipose tissue, are met with. It has, however, been pointed out in what important particulars the arrangement of the lobules of the pig's liver differs from that of other animals.

In the human liver, although there is a considerable quantity of areolar tissue surrounding and adhering to the large vessels, at the point where they enter the liver, and in the larger portal canals, this gradually becomes less, and is, at last, quite lost as we approach the smaller portal canals.

The quantity of this fibrous tissue in the large portal canals (Glisson's capsule) varies very much in different animals. In the rodent animals which I have examined, it seems reduced to a minimum (rabbit, rat, mouse). In the seal it is very sparing in quantity.

Structures which may be mistaken for fibrous tissue.—There can be no doubt of the presence of much areolar tissue in the large portal canals of the human subject; but there are, nevertheless, other structures imbedded in this, such as lymphatics and vasa aberrantia, all of which would be included under the head of Glisson's capsule, by a casual observer. In the horse, the external coats of the large duct and portal vein are incorporated at the point where they touch, and here the areolar tissue clearly belongs to the external coat of these vessels. In the rabbit very little areolar tissue is found to accompany or invest the vessels of the liver. This may be readily demonstrated by injecting a

VESSELS, ETC., MISTAKEN FOR FIBROUS TISSUE.

21

rabbit's liver with plain size. The whole organ becomes transparent, and the course of the small distended veins can be most readily traced. In this animal the capillaries of the liver are large, and injection runs into them very readily. One cannot fail to be struck by the almost total absence of anything which can be described as Glisson's capsule, or fibrous tissue, in this beautiful liver. A little condensed areolar tissue is seen only around the largest veins, and external to this ramifies an abundant plexus of lymphatics. All the vessels are clear and well-defined. The artery can be readily separated from the portal vein, and neither it nor the duct are bound up with the vein in a fibrous sheath. The larger ducts can be readily distinguished from the other vessels, and their small branches, if not injected, can be traced for a long distance by the characteristic epithelium in their interior. In the smaller portal canals the veins and ducts are entirely destitute of any external coat, but their course may, nevertheless, be traced very distinctly.

When the ducts of a liver (human, rabbit, seal, and other animals,) have been injected with Prussian blue, and the portal vein distended with plain size, a transverse section of a portal canal exhibits no structures but the distended veins and ducts, with branches of the artery. The external coat of the large vein is well seen, and consists of a thin layer of condensed areolar tissue; but the duct and artery are not invested with it. In the small portal canals the vein is quite destitute of this external coat, and it lies in immediate contact with the basement membrane of the peripheral portion of the cell-containing network, or with that of the finest ducts, which often form a plexus around it. In fig. 29, which represents a transverse section of a portal canal of the seal's liver, the smaller ducts, of which a few branches only are very imperfectly injected, f, are seen passing round the artery, e, and portal vein, a small branch of which is shown at d. These small ducts in many preparations seem to enclose the artery and duct, as it were, in a sheath, connected with the coat of the portal vein, but the real nature of this apparent connecting sheath is clearly shown in the preparation from which the drawing was taken.

The difficulty of deciding positively upon this point, however,

in every specimen submitted to examination, is, perhaps, greater than would be supposed from what I have just observed; by manipulation the small vessels in uninjected specimens become so altered that it would be impossible to distinguish them from fibrous tissue. On the other hand, it might be urged, that in injected specimens the fibrous tissue is so compressed by distended vessels as to be almost invisible. Even if this were the case, in numerous places where these vessels were slightly separated from each other, one could hardly fail to recognize fibrous tissue if it existed.

The areolar coat of the vessels and ducts is not prolonged upon the small branches which are given off from the trunks in the larger portal canals; and, after many careful examinations of the arrangement and connexions of the vessels in the portal canals of many animals, I have been unable to observe anything in the distribution of the areolar tissue around the vessels of the liver differing from its arrangement in many other situations. The vessels of the kidney and of other glands, as is well known, are invested at their entrance, like those of the liver, with much areolar tissue, which is gradually lost as the vessels approach the secreting portion of the gland.

Manner in which the mapping-out into distinct portions is produced. The cause of the mapping-out of the lobules of the liver in all animals has been considered to be due to the extensive distribution of Glisson's capsule between them; but in these situations, as has been already remarked, many observers like myself have altogether failed in their attempts to demonstrate such a structure, with the single exception of the pig. The fibrous appearance in the interlobulur spaces of the uninjected livers of animals generally, I have shown to be due to the collapsed state of small branches of the vein, artery, and especially of the duct, which are very numerous.

The divisions, or apparent septa, between the ultimate portions of hepatic substance in the human liver (independent of alterations in vascular distension so well described by Kiernan) seem to be produced in part by the arrangement of the smallest branches of the duct, and partly by that of the vein and artery. In the seal and hedgehog the markings are not very distinct, and the

« PreviousContinue »