These have been partly described in liver histology. The hepatic artery, and duct, and portal vein enter the liver at the transverse fissure, enclosed within Glisson's capsule, and continuously subdivide as they push their way through the parenchyma. The subdivisions of the portal vein never anastomose, but are distributed around the surfaces of the lobules, forming their boundaries.
At the periphery they break up into capillaries which enter the lobules. These are about 0.02 mm. In diameter, and form a network, the meshes of which are scarcely wider than capillaries. Within the lobule the capillaries unite to form the central vein, and these then empty into branches of the hepatic vein. The subdivisions of the hepatic vein are also devoid of anastomoses, but after traversing the posterior portion of the .liver in canals (which they embrace closely), unite to form the hepatic vein.
A peculiarity of this latter vein is the fact that its larger branches give off successively small lateral twigs, which enter the bases of the neighboring lobules, so that after dividing such a branch lengthwise it would seem to be pierced by small circular openings, which are the orifices of the lateral branches.
Not unfrequently a central (hepatic) vein will divide into two branches within a lobule, in which case the latter seems to possess two apices, which become joined together as we approach its base. The connection between the portal and hepatic veins takes place only through their capillaries.
The hepatic artery is comparatively small. It enters the liver together with the duct and portal vein, and at once breaks up into branches, which, anastomosing with each other, form a large-meshed network. The arterial branches are distributed to the vessels mentioned, which they enclose, and also to the connective tissue which surrounds the latter. The hepatic artery also gives off nutrient branches which supply its own walls, and small twigs, which, piercing the substance of the liver be tween the lobules, supply the branches of the hepatic vein.
The ultimate branches of the artery are contained within the interlobular canals, and break up into capillaries at the periphery of the lobules, which they traverse for a short distance to form a distinct network. There is no communication between the intralobular capillaries of the hepatic artery and those of the portal vein. The former seem destined to supply the adjacent vessels, and probably the small amount of iutralobular connective tissue to be spoken of hereafter.
It has been thought by some (Chroaszewski, Bindfleisch, and others) that the capillaries of the hepatic artery end midway between the interlobular and central veins, within the lobule. Beale and Kiernan have noticed that an arterial branch here and there enters a lobule ; while Theile, Davis, and others describe a capillary network about the periphery of the lobules.
Finally, branches of 0.05 to 0.1 mm. In diameter are distributed to the capsule of the liver, where they break up into capillaries, radiating in all directions and anastomosing with each other to form "a large-meshed network, which communicates with the capillaries of the phrenic, mammary, and supra renal arteries. This plexus empties into small twigs, the so called inner roots of the portal vein.
The capillaries of the liver may be injected either through the hepatic or portal vein, or both, as before stated. For injecting the hepatic artery the author prefers his cold solution of carmine-glycerine. 1 The gland to be injected must be as fresh as possible. If, for example, a dog be selected for this purpose, the abdomen should be opened and the animal allowed to bleed to death by section of the vena cava. Now introduce into the hepatic artery the canula of a syringe tilled with the carmine-glycerine, secure it in place and inject.
Harden the organ in alcohol, cut sections, and mount in balsam. The liver will not be uniformly injected, and only those portions can be utilized in which the injected mass seems to be widely diffused.
If, in addition to the artery, the hepatic vein be injected with a blue colored mass, beautiful results may be obtained.
Sections in which the lobules are cut transversely show the 1 See chapter on the Kidney.
Central veins occupying the position of the axes of the lobules and the capillaries, pursuing a radial course and anastomosing with each other by transverse communications.
Since the capillaries successively divide from the centre toward the periphery, it follows that they are much less numerous at the former than at the latter point. A section through the long axis of an acinus will show that the central vein is divided lengthwise, and that the capillaries are given off from it almost at right angles to its course.
Nearer the summit of the lobule, however, the central vein is seen to break up into diverging capillaries. If the section has been made to one side of the central vein, but yet parallel with its axis, many capillaries will be cut across, more or less transversely, and will then appear as small, circular, or oval rings.
The connective tissue of the liver
Glisson's capsule is formed of longitudinal bundles of connective tissue which are loosely interwoven. It serves to bind together the hepatic artery, portal vein, and hepatic duct, and also fills out the small spaces left between the ramifications of these vessels. Sections from a liver hardened in chromic acid or alcohol, and immersed in a dilute solution of caustic potassa, or simply pencilled, show the connective tissue well. About the hepatic vein it is thin and dense, and firmly united to the glandular structure, so that when cut transversely these vessels appear to gape. In the camel the connective tissue is greatly developed, 1 even more so than in the hog. The interlobular septa are very dense and fibrillated ; in the interior of the lobule the connective tissue has a lamellar structure.
According to Ewald and Kuehne, minute bundles of fibrous tissue extend beyond this interlobular connective tissue, and piercing the lobules eventually surround the central veins.
Liver Cells
The liver-cells are found lying within the meshes of the capillary network of the lobules. If we bear in mind the shape of the intralobular capillary reticulum, the arrangement of the hepatic cells will be readily understood.
The meshes of the capillary network have about the same diameter as the capillaries themselves. Hence it follows that the cells which occupy these meshes must also have the appear ance of a reticulum. But inasmuch as the vascular meshes contain two or three liver-cells, it is evident that two neighboring capillaries must be separated from each other by at least one liver-cell. Hence, in sections where the capillaries are cut transversely, their circular openings will be surrounded by a ring of liver-cells, or a circle of capillaries will enclose a mass of glandular substance. In sections which cut the central vein transversely the radiating capillaries will enclose radiating rows of liver-cells. These are either joined to one another by the intervention of other liver-cells, or they are separated from one another by transverse capillary branches.
On the other hand, in sections where the central vein is cut lengthwise, the (nearly) parallel intralobular capillaries will ap pear separated from one another by correspond ing rows of liver-cells. The glandular substance of the liver would then be composed of small, solid columns or rows of cells united to each other by other cells, thus forming one connected mass, and containing within its meshes the cap illary network. In the fresh state the liver-cells appear as spherical or egg-shaped bodies, usu ally presenting facets. They are somewhat flattened by being pressed against one another.
The hepatic cells are about 0.0130.02 mm. in diameter, and possess one or two nuclei, 1 which are gener ally spherical, although they occasionally appear to be flattened ; the diameter is 0.0060.007 mm. The liver-cells do not possess any membrana propria, but a hardened boundary layer seems to exist in its place. It is probable also that the 1 Occasionally three or five nuclei, especially in young subjects (Beale).
Sections of a dog's liver, immersed for a short time in dilute osmic acid, will occasionally exhibit a brown or black tracing between adjoining cells. Pres sure on the cover glass will part them and leave the darkened material free in the field of vision. I have satisfied myself that this tracing is not of a nerve, biliary duct, or connective-tissue fibril ; it is either a portion of the boundary layer of a liver-cell, or, as I suppose, a colloid substance between two cells.
This appearance, however, is not constant.
The protoplasm is of a dark brownish or greenish color. It is viscid, and contains numerous granules of small size, in addition to smaller or larger fat-droplets. 1 In livers hardened by chromic acid or alcohol, the shrinkage of the cells causes them to appear polyhedral, and they also seem much darker than in the fresh state. If the portal or hepatic vein has been injected, the cells will show distinct indentations produced by the dis tended capillaries.
When liver-cells are treated with diluted acetic acid, their protoplasm becomes pale, while their nuclei are rendered more conspicuous. In a dilute solution of caustic potassa the cells swell up, become rounded, and are finally dissolved. With water they also swell up, become paler and more rounded, and at length disintegrate. In the fresh state, by the addition of an indifferent fluid (i per cent, solution of chloride of sodium, or iodized serum), the liver-cells are said to show protoplasmic movements. The granular substance of the liver-cells has been shown (by Schiff, in frogs, and by Nasse, in certain mammalia) to consist of an animal amylum, which is converted into sugar through the agency of a peculiar ferment.
The fat-droplets may be either small in number and size or quite numerous and large. Not infrequently they coalesce to form larger fat-globules. In the so-called fatty infiltration they are very large, and compose the greater part of the cells. The nuclei are granular, and where two or more of them occupy the same cell, they may apparently be united to each other.
1 According to Kupffer and Klein the substance of the cells is composed of a honeycombed network, i.e., an intracellular reticulum. Klein says the nucleus is limited by a thin membrane, and includes an intranuclear network, containing occasionally one or two nucleoli. The intranuclear network is in continuity with the ictracellular one, and the network of contiguous cells are in connection with one another (Klein and Smith : Atlas of Histology).
When two nuclei are placed in contact, there may be an appearance of division, but the actual process is not easy to see. Thin liver sections may be stained either in carmine fluid or hsematoxylon, and preserved in glycerine or balsam.
The larger bile-ducts
If, for the sake of convenience, we imagine that the hepatic duct enters the liver to be distributed to its substance, we may describe it as giving off two primary branches at the transverse fissure, one passing to the right lobe, the other to the left. As these branches continue their course, following the subdivisions of the hepatic artery and portal vein, they also undergo successive divisions, and at length enter the interlobular canals. In this position their diameter varies between 0.02 and 0.03 mm.
The primary branches do not, however, pass unchanged into the liver tissue. They ramify even before entering the gland, but such vessels are distributed only to the under surface (Henle). Other biliary ducts, given off in the trans verse fissure, form a network on the upper surface, as may be demonstrated by injecting the hepatic duct with carmine-glycerine. The branches of these networks then enter the liver tissue and ramify throughout it, following the subdivisions of the hepatic artery and portal vein.
As the divisions of the hepatic duct diminish in size, the thickness of their walls undergoes proportionate diminution.
The trunk of the hepatic duct comprises an internal layer measuring 0.15 mm. In thickness, and an external layer of 0.2 - 0.3 mm. Both of these coats are composed, according to Henle, of interlacing connective-tissue bundles, in which elastic fibres are freely intermixed. These ducts have an internal lining of cylindrical epithelium, which is 0.05 mm. In height.
Even where the branches measure only 0.2 mm. In diameter they have cylindrical epithelium surrounded by a single layer of connective tissue longitudinally disposed, in which there are also muscle-corpuscles, distinguished by their long, rod shaped nuclei (Heidenhain). The most minute biliary pas sages consist of a structureless membrana propria, which is lined with flattened cylindrical epithelia.
Glands of the ducts
In the trunk of the hepatic duct and its subdivisions, down to those branches of which the diameter is not less than 0.5 mm., the mucous membrane is provided with numerous irregular excavations, measuring 0.15 0.3 mm.
In their long diameter. In this trunk there occur also a great number of pores or orifices, which, on examination, prove to be the mouths of the passages leading from simple and com pound gland -like bodies, the so-called glands of the bile-ducts.
The simple glands consist merely of single vesicles, or alveoli, with afferent passages, all of which are embedded in the mucous membrane ; or of two or more vesicles with a single pas sage. The compound glands are formed by the union of two or more simple ones, which have a common passage. The}^ are quite large, and their expanded portions lie on the outer surface of the hepatic duct. When filled by injection with gelatine they are visible to the naked eye. The passages pierce the walls of the duct at an acute angle, pursuing a course within its walls, nearly parallel to the duct itself ; the opening into the mucous membrane is therefore quite a distance from the gland-vesicles. According to Henle, these compound glands are not found in the larger branches of the hepatic duct, but they occur frequently in the network of bile-ducts situated in the transverse fissure. Allusion has already been made to them. The vesicles measure 0.04 mm. In diameter, and, like the excavations in the larger branches of the duct, are lined with a cylindrical epithelium, in no way differing from that of the duct itself ; the afferent passages also possess the same kind of epithelium.
Structures allied to these excavations and glands occur in small number, in the bile-ducts ' which are found in the ligamentum triangulare and on the diaphragm, where they appear as villous prominences on the ductwalls.
According to Theile, Weber, and others, these bile-ducts represent the last vestiges of an atrophied liver substance, the existence of which dates back to infancy, or perhaps to fetal life.
The excavations in the larger branches are either simple diverticula of the internal walls, or the openings of lateral bile ducts ; the punctate pores are the orifices of the outlet pas sages of duct-glands.
Capillary bile-ducts
When the larger bile-ducts have at length reached the interlobular canals, in conjunction with the branches of the portal vein and hepatic artery, they send capillary branches within the sub stance of the lobule, and thus form an intralobular network.
These capillary ducts are of extreme delicacy, measuring only from 0.001 to 0.0012 mm.
In order to demonstrate them fully they should be filled by natural injection. The substance to be employed for this purpose is a solution of pure indigo-carmine. The animal serving for injection (rabbit or dog) should be secured in the manner described in the chapter on the Kidney, where all the necessary manipulations are fully detailed. The best results are obtained by injecting a cold, saturated solution of indigo carmine into the external jugular vein, directing the stream toward the periphery (brain) ; 5 or 10 cms. Are to be injected at intervals of thirty to forty minutes, and the injection continued until from 25 to 50 cms. Have been used, the amount varying according to the size of the animal. It takes a longer time for the elimination of indigo-carmine through the capillary bile-ducts than for the same process by way of the renal tubules, and a larger amount of solution will therefore have to be em ployed. As soon as large quantities of the indigo solution have been injected into the jugular vein, the animal becomes unconscious and there is a decrease of temperature ; hence, it should be covered over with layers of cotton-batting. After a variable time (three to twelve hours) the animal is killed in the following manner : The abdomen is opened and the canula of a large syringe filled with absolute alcohol secured in the lumen of the portal vein ; the inferior vena cava is then cut across above the entrance of the hepatic vein, and the piston of the syringe pushed home. The liver, which before was of a uniform blue color, now presents a marbled appearance, not unlike that of malachite.
Or, the portal vein may be injected with the writer's carmine glycerine, the vena cava having been divided as above. In either case the liver is to be removed at once and placed in a vessel containing absolute alcohol, and while immersed in that fluid cut into small fragments. Sections may then be made in a few hours.
The arrangement of the bile-capillaries differs in different animals. In the rabbit, for instance, they lie between the adjoining surfaces of two contiguous cells, and rarely in the canals formed by the edges of three or more cells (Hering '). So that while the blood-capillaries occupy the canals previously described, the bile-capillaries form an independent network be tween the boundary surfaces of the liver-cells. In cross sections they may be seen, appearing as small, circular.
The narrow, reticulated bile-capillaries are shaded with longitudinal, the broader blood-capillaries with transverse lines. Within the boundary line or septum of two contiguous cells the cross-section of a bile-capillary is seen as a dark spot or point. The liver celU contain one or two nuclei.
According to Hering, both in rabbits' and dogs' livers the blood-capillaries are separated from the bile-capillaries by the intervention of at least one liver cell. Livers in which the bile-capillaries have been injected by the natural method with indigo-carmine do not always demonstrate this. And here it may be remarked, that in artificial or forced injections of the bile-capillaries they are always distended beyond their natural diameters.
In an article on the liver by Dr. W. Gdavis, in the Amer. Jour. Med. Sci., Vol. LXXVIII., the distention of the capillaries is excessive.
By conjoined natural injection of the bile-capillaries and artificial injection of the portal system with carmine-glycerine by the methods above detailed, very gratifying results are obtained. Care must be taken, however, not to use too much force during the process of injection, and only such portions of the liver should be chosen for sections as show, by their red color, a perfect filling of the portal branches.
While the elimination of the indigo-carmine is taking place within the liver of the living animal, the bile-capillaries probably contain the salt in a soluble form. The addition of absolute alcohol at once precipitates this coloring reagent in the form of exceedingly fine stellate crystals, or as finely granular matter, which may in some measure account for the angular character of the biliary capillaries, as seen in such specimens. Gentle curves never appear. The constringing action of the alcohol on the liver-cells has unquestionably some effect, and therefore modifies the normal appearance.
Natural injections further show the great preponderance of the biliary over the blood-capillaries. In the liver of a dog, for instance, each liver-cell seems suspended within two or three (rarely four) bile-capillaries.
The capillaries are filled with red blood-corpuscles (Indicated by colorless rings) and a few leucocytes. The cross section of a bile-capillary is shown within the boundary line of any two contiguous cells. A similar cross section is shown in the canal formed by three adjoining liver-cells.
