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secreting structure of the liver, by the branches of the portal vein.*

Diameter of the smallest Arterial Branches.-The meshes of the network which the smallest branches of the artery contribute to form, are many times wider than those of the venous capillaries in the lobule, fig. 7; but the diameter of the small vessels of which this network is composed is very much less than that of the portal capillaries. I have measured the diameter of the smallest arterial branches in the pig and in the human subject. The average measurements were as follow; but these numbers must be considered rather as approximations to the truth, than as absolutely correct, because it is impossible to ascertain to what degree the vessels have been distended by the injection :

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Artery. That the capillaries of the lobule can be injected from the artery necessarily follows from the nature of the arrangement just described. In this way, Lieberkühn injected the lobules as well as from the portal or hepatic veins. As a general rule, the injection appears first quite at the portal surface of the lobule, and gradually extends towards the centre; but sometimes the central capillaries are injected from the artery very readily, while the marginal capillaries are quite free from injection. This has occurred to me both in the human subject and also in the pig; but it was confined to certain small portions of the liver, while in the greater part of the organ, small arteries could be readily traced into the marginal capillaries. In the situations referred to, the branches of the portal vein were very imperfectly injected, and the capillaries did not contain any injection, in consequence, probably, of being filled with fluid previous to injection, while the central capillaries were empty. The explanation of this circumstance I imagine to be this-that the injection having found its way by one channel into these central capillary vessels, gradually * Op. Cit. p. 748.

spread through them, until the appearance above referred to was produced, its course towards the central capillaries being, in the majority of instances, prevented by their distension with blood or serum. If the preparation exhibiting this arrangement, be com pared with others from the same liver, its mode of production can be clearly made out. I believe it to be the explanation of the result obtained by some anatomists, who have been led to conclude, from the appearance above indicated, that the small arteries emptied themselves into the capillaries nearer to the intralobular vein, or into this vessel itself.

The whole of the blood, therefore, which has become venous after passing through the arterial network, mixes with that of the portal vein, either before the trunk enters the liver, or into the large branches in the portal canals, or into the lobular capillaries; and probably takes part in the formation of bile.

Arrangement in Gall-Bladder, &c.-The arrangement of the artery and veins is different in the gall-bladder, and in the transverse fissure of the liver (fig. 25) and large portal canals. In these situations even very small branches of the artery are accompanied by two small branches of the vein. This arrangement appears to facilitate the free return of blood from these parts, and, it seems to me, may be looked upon as a provision for permitting that amount of stretching or pressure to which these vessels must necessarily be subjected, without interfering with the free circulation of blood through them. As the united capacity of the veins is twice that of the artery, the free and equable circulation in these small vessels is insured under all circumstances. The chance of congestion occurring in the portal canals is still further provided against by the communication of the arterial network with the capillaries of the lobule.

HEPATIC DUCT.

The branches of the duct lie in the portal canals with branches of the vein and artery. At least one branch of the duct accompanies each branch of the portal vein, but frequently there are two or three (fig. 12). From the branch or branches accompanying the vein, several smaller ones pass off to the secreting structure. These do not anastomose so as to encircle the lobules.

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derately good injection of the ducts, they are seen in great number, although it is not always easy to isolate them, in consequence of the quantity of areolar tissue with which they are surrounded. In order to inject them, it is necessary to force out the bile from the ducts in the first instance, in the manner described in Chapter I. It is, however, difficult to understand how the smaller ducts of a liver could be successfully injected without these tubes being rendered sufficiently distinct.

These ducts are imbedded in areolar tissue, which is abundant in the parts where they are found. In the transverse fissure of the adult, the ducts lie nearer to the hepatic substance than to the coats of the portal vein; but they can be easily removed, without cutting into it. I have been able to trace straight branches from the vasa aberrantia directly into the hepatic substance. The further course of these branches is that of an ordinary duct.

In the foetus, these curious ducts are much less numerous; their course is less tortuous, and they occur in small patches, in which the branches are seen to be very numerous, and the anastomoses very frequent. The epithelium is more abundant, and the cells large, and dark in colour; so that the injection does not run so readily as in the adult. The quantity of areolar tissue about them is much less than in the adult. The vasa aberrantia lie so close to the hepatic tissue, that it is almost impossible to remove them without a thin layer of the latter, into which they are prolonged at numerous points. There are, nevertheless, many blind extremities connected with them.

Office of the Vasa Aberrantia and Sacculi.-The circumstances which have been alluded to, appear to me to militate strongly against the notion of the vasa aberrantia being modified, and anastomosing mucous glands. They seem rather to point to the ductal nature of these curious channels; and I think it not improbable that they are really altered secreting tubes, and at one time formed a part of the secreting structure of the liver. As the portal vein becomes larger at the termination of intrauterine life, it is not unreasonable to suppose that the hepatic tissue, close to it, would recede; and that the most superficial portion would become so modified, as to be no longer adapted for

secretion. The cells would become altered, the ducts shortened and contorted, until a condition such as that above described might at last be produced.

In the very thin edge of a horse's liver, which was composed principally of fibrous tissue, I have been enabled to trace the gradual alteration of the ducts through many intermediate stages to the ultimate complete disappearance of secreting cells, until at length nothing remains but a branched tube without any cells in the interior. The ducts, discovered by Mr. Kiernan in the triangular ligament, in the pons hepatis, and in other situations, are probably produced by a similar change.

To the sacculi the office of secreting the mucus which is found in the bile has been assigned by most observers who have investigated this part of the subject, but, from a careful examination of their arrangement, I cannot help doubting the correctness of this inference. Cavities opening into a tube, by a narrow neck, often not more than the 1-5000th of an inch in diameter, seem hardly adapted for the secretion and pouring out of a highly viscid mucus (figs. 37, 38). If these so-called "glands" were the seat of the formation of mucus, one would not expect that injection would so readily enter them, and, from their arrangement, it is clear that it would be difficult to force the mucus out of them; again, the complicated and highly tortuous ducts, which Weber termed very properly "vasa aberrantia," possess no characters which entitle them to be regarded as anastomosing mucous glands, a view which has been advocated by Theile. They are most readily injected, and their walls are much thinner than those of ordinary ducts.

It must be remarked that an abundant venous plexus surrounds the vasa aberrantia (fig. 25) and the larger ducts, and that the bile in the larger ducts, by means of the sacculi, is brought much nearer to the vessels than would be the case in simple tubes with thick fibrous walls. If these little sacculi were mucous glands, one would be led to expect that the bile of animals, in which they were numerous, would be more viscid than that of animals in which they were few in number. I have ascertained that the bile of the rabbit, in which animal these sacculi are almost absent, contains as much mucus as that of the pig, in which they are exceedingly numerous, and arranged entirely round the

ducts. It seems to me that we may regard these appendages connected with the ducts as diverticula, in which the bile may be retained temporarily, while it becomes inspissated, and probably undergoes other changes. In fact, I think that we may look upon them as little gall-bladders appended to the ducts.

In the rabbit and guinea-pig these sacculi are very slightly developed. I have not seen them in the ducts of the fishes and reptiles which I have examined.

Coats of the Larger Ducts.-The coats of the larger ducts are composed of condensed fibrous tissue; but there is reason for supposing that they contain a few muscular fibre-cells, although there is no evidence of a distinct muscular coat, at least in the human subject. In some fishes I have seen an internal layer composed of circular fibres, and an external coat of longitudinal fibres (figs. 65 m, 66 a). In the human subject I have observed, but not very distinctly, indications of a somewhat similar arrangement.

Epithelium.-The epithelium of the larger ducts is of the columnar variety. The cells are large and well-formed, usually exhibiting a distinct nucleus. They are frequently tinged with yellow colouring matter, and often contain yellow granules. In the small ducts, this epithelium becomes shorter, and it approaches more nearly to the tesselated variety. This change in the character of the epithelium is a gradual one. In ducts less than the 1-500th of an inch in diameter the epithelium consists of small round flattened granular cells.

The small branches resulting from the division of the trunks in the interlobular fissures, or upon the surfaces of the lobules in the case of the pig's liver, are composed of basement membrane, and only lined with a single layer of epithelium. These small ducts may, without difficulty, be traced up to the secreting structure of the lobule, but the manner in which they commence, and the relation which they bear to the liver-cells, have long been matters of dispute among anatomists. It has been held by some of the latest authorities, that the ducts commence by blind extremities at the margin of the lobules; by others, that they are open and impinge against the secreting cells; while, according to the researches of some observers, they commence as very narrow inter

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