In the only embryos of mammalia which I have examined, more than one row of cells is contained within the tube, and two or three are commonly met with (human fœtus, fœtus of ox). The cells are smaller than in the adult liver, the tubes not unfrequently being larger. In birds also (linnet, turkey, starling, fowl), two or three cells lie within the tube in some places, but in others only one is met with. In the embryo-chick numerous rows of cells are seen lying across the tube.

In those reptiles which I have observed, there are often several cells lying across the tube (frog, adder, field-snake); in fishes, so far as my observation goes, there is also room for many rows of cells (flounder, frog-fish, sturgeon, herring, cod, &c.).

It may be said, generally, that the tubes are most narrow amongst mammalia, and widest amongst fishes. The cells also are well defined in the former, but not often demonstrable in the latter class. The similarity of the arrangement of the cells in the tubes of the embryos of man and the higher animals, with the embryonic, as well as the permanent, condition which they assume in the lower vertebrata, and in many of the invertebrata, is a point which must excite great interest, and is one which has a parallel in the case of other glands and many tissues. There is perhaps no more striking example of this gradual progress from the lower and simpler form of structure to the higher and more elaborate than the one here adduced.

51

CHAPTER V.

OF THE ULTIMATE RAMIFICATIONS OF THE DUCTS, AND OF THEIR CONNEXION WITH THE CELL-CONTAINING NETWORK. -ANASTOMOSES OF THE DUCTS.-OF THE FINEST BRANCHES OF THE DUCTS, AND OF THEIR JUNCTION WITH THE SECRETING TUBES OF THE CELL-CONTAINING NETWORK; MAMMALIA; BIRDS; REPTILES; FISHES.

- EPITHELIUM OF

THE SMALLER DUCTS.-DR. HANDFIELD JONES' VIEWS.

NUMEROUS observers, who have carefully investigated the matter, differ widely in the conclusions they have arrived at with regard to the manner in which the most minute ducts commence in the liver; and are at issue with reference to their mode of connexion with the secreting structure, and the precise relation which they bear to the secreting cells. In his valuable paper published in the Philosophical Transactions for 1833, Mr. Kiernan describes and figures the anastomoses between branches of the hepatic ducts in the left triangular ligament of the human liver. He also refers to communications existing between the ducts in other situations, as in the membranous bridge stretching over the fissure of the umbilical vein, and upon the inferior surface of the diaphragm. In the same paper, this author gives a diagram of the manner in which he supposed the ducts to terminate in the lobules of the liver, and subjoins the following remarks :-"No such view of the ducts as that represented in this figure can be obtained in the liver. The interlobular ducts are in the figure seen anastomosing with each other. I have never seen these anastomoses, but I have seen the anastomoses of the ducts in the left lateral ligament, and, from the results of experiments related in this paper, I believe the interlobular ducts anastomose; I have never injected the lobular biliary plexus to the extent represented in this figure."

Since the appearance of this important communication, the subject has been much investigated both in this country and on the continent; but, as far as I can ascertain, no observer has yet succeeded in demonstrating the manner in which the ducts

commence, or has been able to show conclusively the precise relation which the hepatic cells bear to the biliary ducts. Various hypothetical views have been advanced.

Müller considered that the ducts terminated in blind extremities; E. H. Weber, in 1850, described ducts terminating in blind extremities upon the external surface of the cat's liver. Krukenberg, Schröder van der Kolk, Weber, Retzius, Theile, Backer, Leidy and others, have adopted the view that the hepatic cells lie within a basement membrane, and are of opinion that the cavity of the tubes in which the cells lie is continuous with that of the ducts. Lereboullet, in his memoir on the "Foie gras," published in 1853, advocates a similar view; but his representations are very diagrammatic, and for the most part taken from preparations examined by low powers. Almost all the drawings of authors, which represent the mode of origin of the ducts, are only offered as plans of the arrangement which they consider to exist, and are not pretended to be accurate copies of structure actually brought under observation.

Henle, Gerlach, and Natalis Guillot look upon the finest gall-ducts as communicating with spaces between the hepatic cells, into which the bile escapes, and from which it is received by the most minute ducts.

Handfield Jones and Kölliker describe the hepatic cells as forming a solid network composed of columns of cells, not bounded by any basement membrane, but lying in the meshes of the capillary network. The former excellent observer concludes that the ducts terminate by blind extremities, which lie amongst the cells at the peripheral parts of the lobule. The small cells lining these ducts are considered by Dr. Handfield Jones to be the chief agents concerned in the secretion of bile, and he looks upon the function of the hepatic cells as totally distinct from this. Professors Busk and Huxley, and Dr. Carpenter, appear to concur in this view, which places the liver in the same category as the supra-renal capsules, follicles of Peyer, spleen, &c. "In fact, startling as this view may at first appear, a very clear transition between the Peyerian follicles, &c., and the liver, is afforded by the tonsils; which, on the one hand, are identical with Peyer's follicles, in so far as they are solid vascular networks, whose

meshes are filled with a morphologically indifferent tissue, while, on the other hand, without differing from the liver in this respect, they resemble it in having these elements arranged around diverticula of the intestinal mucous membrane." Finally, we suggest that the liver itself is but a huge tonsil-a vascular gland, with what might be termed a false duct."*

Kölliker offers the supposition, that the finest ducts impinge upon the columns of the network of hepatic cells, and makes the following remarks with reference to this point :-"Often as I have sought for a direct communication of the finest canals with the hepatic networks, I have not directly observed it; which is indeed by no means surprising, if we consider the softness of the parts with which we have to do; but unfortunately the result is a hiatus in the minute anatomy of the parts, which can hardly be made good by hypotheses. As such, however, I would offer the supposition, that the finest ducts impinge directly upon the columns of the network of hepatic cells, as the diagram shows, so that their cavity is terminated by hepatic cells.”+

The conflicting opinions of observers appear to have been based upon inference and hypothesis rather than upon direct observation, and are embodied in diagrammatic figures. Some authors, agreeing in the main with Kiernan and the older observers, regard the liver as arranged upon the type of true glands with permanent ducts, while the latest authorities have endeavoured to establish the view, that this important organ is more nearly related to the ductless glands.‡

My own observations have been made upon the livers of several different animals, and I have tried very numerous methods of preparation, some with considerable success. The results of the examination of injected specimens precisely accord with the observations made many months before upon uninjected preparations.

The chief point which I hope to establish, with reference to the origin of the minute ducts, is the following:

* Kölliker's Manual of Human Histology, translated and edited by G. Busk, F.R.S., and T. Huxley, F R.S., p. 126, note by the editors; 426 in the Appendix. + Op. Cit. p. 118.

An excellent abstract of the views upon the structure of the liver will be found in a paper by Professor Weber, "über den Bau der Leber," in the third part of the "Berichte der königlich Sächsischen Gesellschaft der Wissenschaften zu Leipzig," for the year 1849, and in Professor Kölliker's "Mikroskopiche Anatomie, 1852.”

That the smallest biliary ducts* are directly continuous with the tubular network of basement membrane in which the liver-cells lie; for, in favourable specimens, injection, forced in from the duct, will pass into every part of the tubular network, even quite to the centre of the lobule. It is possible to inject the capillary network in the same preparation as that in which the ducts and cell-containing network are injected.

OF THE ULTIMATE RAMIFICATIONS OF THE DUCTS, AND OF THEIR CONNEXION WITH THE CELL-CONTAINING NETWORK.

Anastomoses of the Ducts.-The anastomoses of the large trunks, and of the branches given off by the larger interlobular ducts, are more numerous than, from the observations of anatomists, one would be led to suppose. These anastomoses, however, occur principally between the trunks, near their origin, as has been described; but, more rarely, different branches of the duct communicate with each other near the point where they join the cellcontaining network. In most animals the latter communications are very scarce. In injecting one branch of the duct the injection will often pass out from another. Kiernan observed, that, if the left duct were injected with size or mercury, the injection returned by the right duct. This probably depends upon the intimate communications between these ducts in the transverse fissure of the liver, rather than upon the existence of anastomoses between the small branches near their points of distribution.

The anastomosis of small interlobular branches coming from opposite points, according to my observation, is very rare, as proved from the careful examination of well-injected specimens of the livers of different animals. Indeed, I have only been able to satisfy myself of its existence in one instance.

With regard to the communications of the larger trunks, and of the smaller branches, with the ducts from which they come off, it may be remarked, that not only do the right and left hepatic ducts anastomose, by the intervention of small, tortuous branches in the transverse fissure of the human liver, which form an intimate network, first described by Weber; but many of the branches which

* In the following pages the word "duct" is used to denote the tubes which carry off the secretion, in contradistinction to the secretory tubes, or cell-containing network" in which the secretion is formed.