which this is effected. At the closed extremity of the tube is a special provision for the separation of water, which, as it passes down the central cavity, readily dissolves away the soluble matters which the cells have separated from the blood. The solution passes down the tube, and without undergoing any further change, is very soon removed altogether. So very important is the rapid removal of the urinary constituents from the organism, that in those animals in which a sufficient quantity of water for their complete solution cannot be afforded, there exists a most perfect and beautiful arrangement by which this object is attained, but in a totally different manner. Not only is there a cavity throughout the entire length of the tube, but the cells around it are furnished with vibratile cilia, by the action of which even solid particles are whirled down the tube with a velocity greater than that of the circulation of the blood in the capillaries.

In the liver, on the other hand, we have an organ not only for the separation of substances very recently introduced into the organism from the portal blood, but for the slow elaboration and conversion of these into very different products which serve important ulterior purposes in the economy, and which are principally re-absorbed into the blood, while at the same time, it is probable that a change occurs in other constituents of the blood in its passage through the liver. A network of delicate tubes containing large secreting cells in their interior, arranged irregularly and nearly filling the cavity, without leaving any well-defined channel for the passage of the secretion, is the essential apparatus for the formation of the proper secretion of the liver. The secretion, when formed, may remain for some time in the tubes, but slowly makes its way in an opposite direction to that in which the blood flows, through very narrow channels, either between the cells themselves, or between these and the walls of the tube, until it arrives at the narrow tubular duct. Still passing slowly along these delicate tubes, it becomes exposed to the influence of arterial blood, while much of its water, probably holding in solution substances capable of permeating the basement membrane in this direction, is reabsorbed by the veins and lymphatics, which form an intimate plexus round the ducts. It has been shown how effectual is the provision for the concentration of the bile throughout every part

of the ducts from their commencement to their termination. Unlike the urine, the bile undergoes important alterations after it has been set free by the cells in its course along the ducts, and it is probable that but a small proportion of the total quantity formed in the liver is entirely removed from the organism without undergoing many other changes previously.

Position of the Liver as a secreting organ.-The liver, then, is to be classed with the true glands. In all classes of animals it consists of a formative portion and of an efferent duct, the epithelium in these two situations presenting wide and characteristic differences. It contains both absolutely and relatively a greater proportion of true secreting structure than any other organ. Its secretion is of the most complex chemical composition, and differs more widely from any known constituents of the blood than any other secreted product; so also it seems to serve a greater number of important purposes in the economy, while from the most recent experiments it appears that the amount of bile separated by the liver in twenty-four hours is larger than that of any other liquid secretion, and at the same time it contains a higher per-centage of solid matter.

The early appearance of the liver,-its large size both in the embryo and in the adult, its almost universal presence and general similarity of structure in all classes of animals, the complex and highly concentrated nature of its secretion and its large quantity, justify us in regarding the liver as the most important glandular organ in the body. The large size and vast number of its cells, their close proximity to the blood, and the beautiful arrangement of the other anatomical elements of which it is composed, render it, I think, the most perfect example of gland structure with which we are acquainted.

SUMMARY.

The livers of all vertebrate animals are penetrated in every part by two sets of channels, which alternate with each other. One series, portal canals, contains a branch of the portal vein, hepatic artery, and hepatic duct, interlobular; and the other series, hepatic venous canals, is occupied by a single branch of the hepatic vein, intralobular.

Division of the organ into Lobules.

The vessels and ducts ramifying in the portal canals are ultimately distributed in such a manner that they serve to divide the organ into little masses, and thus map out spaces, or lobules, each of which contains all the structural elements of the organ, and may be regarded as an elementary liver.

In the intervals between the fissures by which the portal vein, artery, and duct are conducted to the lobule, its capillary vessels and its secreting structure are continuous with those of adjacent lobules.

The size and form of the lobules differ much in different animals; but their essential structure is the same in all, except in the pig, in the Polar bear, according to Müller, and in the Octodon Cummingii (one of the rodents), according to Hyrtl.

In these exceptional cases the liver is divided into a number of distinct and separate lobules, each provided with a capsule of its own, just as the kidney of the porpoise and of the seal is divided into a number of separate renules.

In the pig each lobule is provided with a separate fibrous capsule of its own, and is, therefore, completely isolated from its neighbours. Branches of the portal vein, artery, and duct run between them, and give off branches to contiguous lobules. In the intervals between the fibrous capsules areolar tissue can frequently be demonstrated.

In all cases, upon a section, the lobule is seen to be bounded externally by branches of the vein, artery, and duct, and in the centre is situated a small branch of the hepatic vein.

Areolar Tissue in Portal Canals.

In the liver of the human subject, and in that of vertebrate animals generally, with the exceptions above mentioned, the lobules are not separated from each other by any fibrous partition, and there is no areolar or fibrous tissue, or prolongation of Glisson's capsule between them, or in their interior.

The vessels at their entrance into the liver, and as they run for some distance in the larger portal canals, are surrounded with much areolar tissue; but the disposition of this texture about the vessels of the liver is very similar to its arrangement about the larger vessels distributed to other organs.

Intimate Structure of the Lobule.

The lobule itself is composed of a solid capillary network, and of another network composed of a very delicate tubular membrane, in which the liver cells are contained.

These networks mutually intertwine with each other.

The capillary network is directly continuous with the smallest interlobular branches of the portal vein, distributed upon the circumference of the lobule on the one hand, and with the small intralobular branch of the hepatic vein arising in its centre upon the other. The vessels of the network converge toward the centre of the lobule.

Small branches of the artery open into the venous capillaries of the lobule, near its circumference, and the diameter of these small branches is considerably less than that of the venous capillaries into which they open; the former not more than the 1-4000th of an inch in diameter, the latter about the 1-1600th.

In all cases, the blood, enriched with constituents recently absorbed from the intestine, flows with a gradually increasing rapidity from the circumference of the lobule towards its centre, while the bile flows in a precisely opposite direction.

Of the Liver Cells and of the Tubular Network in which

they lie.

The liver-cells lie within a tubular network of basement membrane, which separates them from the walls of the capillaries. In many cases, however, these thin membranous tubes cannot be separated, and are, no doubt, incorporated with each other.

The cells are not attached to the basement membrane of the tube, but lie in its cavity. Among them free oil globules and

granular matter are often found. Usually, there is only room for one row of cells, but sometimes two or more lie across the tube. In the embryos of mammalia, in young animals generally, and in fishes, there is room for several rows to lie transversely across the tubes of the cell-containing network.

The cells near the margin of the lobule take the most active part in the formation of bile. The secretion passes along the tubes in the slight interstices between the cells and the basement membrane, and coloured fluid can be forced along these same interstices in a direction the opposite to that in which the bile flows during life, and, therefore, at a great disadvantage. The amount of space is in great measure determined by the quantity of blood in the vessels, and it is liable to great alteration.

The cell-containing network is directly continuous with the most minute ducts, which ramify at the circumference of the lobule, and it terminates in the centre by loops, which lie close to the intralobular vein.

Of the Finest Ducts.

The tubes of the cell-containing network are many times wider than the narrow thin-walled ducts with which they are directly continuous.

The smallest ducts are lined with a very delicate layer of epithelium, composed of flattened cells of a circular form, contrasting remarkably with the large secreting cells, which are not arranged any definite manner within the tubes of the network.

in

The tubes of the cell-containing network are about 1-1000th of an inch in diameter, or more, but the finest ducts are commonly not more than 1-3000th, and they are often seen even less.

The smallest ducts in some animals branch very freely, and the branches communicate with each other at intervals. In others they pursue a long course without branching, and in the pig they form an intimate network upon the surface of the lobule. In fatty livers of the pig, however, this ductal network often contains livercells loaded with oil globules.