The whole surface of the liver may show bulging, rounded masses, and the morbid growth may have involved the capsule and caused adhesion to the back of the diaphragm (Bächstädt). The cut surface of the neoplasm is smooth, elastic, yellowish and circular or oval in outline. It may have a variable consistency—friable or tough, according to the activity of growth and the relative abundance of cells and stroma. The portal glands are hypertrophied and thrombosis of the portal vein is not uncommon.
Microscopic examination of the dark red scrapings shows numerous blood globules, intermixed with the round or spindle shaped cells and nuclei of the tumor. Sections of the tumor show these cells surrounded by a comparatively sparse fibrillated stroma. The round cells may vary from .005 to .05 m.m. They contain one or more rather large nuclei and a number of refrangent nucleoli. The nuclei are often set free by the bursting of the cells in the scrapings. They become much more clearly defined when treated with a weak solution of acetic acid. Small grayish areas in the mass of the tumor represent the original structure of the liver, the cells of which have become swollen and fatty.
A liquid effusion more or less deeply tinged with red is usually found in the abdominal cavity.
Symptoms are those of a wasting disease, with some icterus, sometimes digestive disorder, and a marked enlargement of the liver. The last feature can be easily diagnosed by palpation and percussion. If an examination through the rectum detects the enlargement and irregular rounded swellings of the surface of the liver or spleen, or the existence of rounded tumors in the mesentery or sublumbar region, this will be corroborative. The precise nature of the neoplasms can only be ascertained after death.
Melanoma. Melanosis of the liver is comparatively frequent, especially in gray horses, and above all when they are aging and passing from dark gray to white. In many cases a more certain diagnosis can be made than in sarcoma for the reason that primary melanotic neoplasms are especially likely to occur on or near the naturally dark portions of the skin, as beneath the tail, around the anus or vulva, in the perineum, sheath, eyelids, axilla, etc. The extent of the disease is likely to be striking, the liver, next to the spleen, being the greatest internal centre for melanosis. The whole organ may be infiltrated so that in the end its outer surface is completely hidden by melanotic deposit. The surface deposits tend to project in more or less rounded, smooth masses of varying size according to the age of the deposit and the rapidity of its growth. Individual deposits may vary in size from a pea to a mass of 40 or 50 lbs. They are moderately firm, and resistant, and maintain a globular or ovoid outline. The color of the melanotic deposits is a deep black with a violet or bluish tint. If the pigmentary deposit is in its early stage it may be of a dark gray. The deposits are firmer than the intervening liver tissue and rarely soften or suppurate.
Melanosis in the horse is not always the malignant disease that it shows itself to be in man, and extensive deposits may take place externally and considerable formations in the liver and other internal organs without serious impairment of the general health. It is only in very advanced conditions of melanosis of the liver that appreciable hepatic disorder is observed. If, however, there is marked enlargement of the liver, in a white or gray horse, which shows melanotic tumors on the surface, hepatic melanosis may be inferred.
Lymphadenoma. Adenoid Tumor. Lienaux describes cases of this kind in which the liver was mottled by white points which presented the microscopical character of adenoid tissue, cells enclosing a follicle and a rich investing network of capillaries.
Angioma. These are rare in the horse’s liver, but have been described by Blanc and Trasbot as multiple, spongy tumors on the anterior of the middle lobe, and to a less extent in the right and left, of a blackish brown color, soft and fluctuating. The largest mass was the size of an apple, and on section they were found to be composed of vascular or erectile tissue. The tendency is to rupture and extensive extravasation of blood (30 to 40 lbs.) into the peritoneum.
Carcinoma. Epithelioma. These forms of malignant disease are not uncommon in the liver as secondary deposits, the primary lesions being found in the spleen, stomach, intestine, or pancreas, or more distant still, in the lungs. The grafting or colonization of the cancer in the liver depends on the transmission of its elements through the vena portæ in the one case, and through the pulmonary veins, the left heart and hepatic artery in the other.
Lesions. The liver may be greatly enlarged, weighing twenty-seven pounds (Benjamin) to forty-three pounds (Chauveau), hard, firm, and studded with firm nodules of varying sizes. These stand out from the surface, giving an irregular nodular appearance, and are scattered through its substance where, on section, they appear as gray or white fibrous, resistant, spheroidal masses shading off to a reddish tinge in their outer layers. Microscopically these consist of a more or less abundant fibrous stroma, enclosing, communicating alveoli filled with cells of various shapes and sizes, with large nuclei (often multiple) which stain deeply in pigments. The relative amount of fibrous stroma and cells determines the consistency of the mass, and whether it approximates to the hard cancer or the soft. In the horse’s liver they are usually hard, and, on scraping off the cut surface, yield only a limited quantity of cancer juice. In the epithelial form, which embraces nearly all that have originated from primary malignant growth in the walls of the intestine, the epithelioid cells, flattened, cubical, polyhedral, etc., are arranged in spheroidal masses or cylindrical extensions, which infiltrate the tissues more or less. These seem in some cases to commence in the radical bile ducts (Martin), and in others in the minor coats of the larger biliary ducts.