The boundaries between the adenoma and fibroma are not well marked. They are both slow-growing, non-malignant tumours, generally of hard consistence, distinctly encapsuled, and not infrequently multiple. Many tumours which are regarded as glandular by some authors are described as fibromas by others.

The Adenoma is a purely glandular tumour. It occurs either in the substance of, or in the neighbourhood of the gland. It has a fleshy feel and a lobulated outline. On section it is white in colour and frequently shows cysts, but without intracystic growth.

There is preserved in the Museum of the Western Infirmary a glandular tumour whose situation was behind the gland, which had to be cut through in order to remove it. The tumour was almost like a supernumerary mamma, and had to the naked eye and microscopically very much the structure of an inactive mamma (see Fig. 467). The tumour was removed from a young lady aged 21, and had been observed for two years, growing latterly more rapidly.

Under the microscope the adenoma presents elongated and enlarged acini and ducts (see Fig. 92, p. 234, and Fig. 467), and these commonly show dilatation, so that cysts are present in the majority of cases.

The Fibroma or Adeno-fibroma is a hard fibrous tumour, generally with a lobulated outline, and distinctly encapsuled. On section it shows a glistening, fibrous appearance, but this is rarely homogeneous. There are usually, even to the naked eye, indications of the presence of glandular structures, and these ate often dilated into cysts. The whole tumour may be a congeries of cavities, which only partly contain fluid, but are largely filled up with foliaceous or dendritic structures composed of tumour tissue projecting into them.

These tumours are sometimes the seat of calcareous deposition and of osseous formation. There may be a partial myxomatous transformation leading in this way to cysts.

Under the microscope wavy fibrous tissue predominates. There are also glandular structures visible, which are either ill-developed or contorted and dilated as in Figs. 468 and 469.