The female breast is one of the most frequent seats of tumours, and they present considerable variety in form.

As the two constituents of the breast are the proper glandular structures and the connective-tissue stroma, so tumours are derived from one or other of these, and belong to the connective-tissue or epithelial forms. But the connective-tissue tumours do not originate in a particular small piece of tissue, and grow so as to displace the remaining structures; on the contrary, they usually involve a considerable portion of the gland and sometimes its whole extent. Hence they enclose more or less the glandular elements, and it may be impossible to say whether these are increased so as to form an integral part of the tumour. Moreover, these glandular elements are subject to great alterations, consisting of contortions and dilatations, which also give their character to the tumours, producing fissures and cysts in them. Again, the tumour tissue frequently grows into the cysts, producing the so-called intracystic growths, which sometimes assume a papillomatous character.

It will be inferred that the distinction between connective-tissue and glandular tumours is frequently difficult, and that dilatation of the gland structures, implying, as it does, new-formation of these, gives a partly glandular character to a large proportion of the tumours.

The atypical mammary tumours show a great preponderance in frequency, especially the cancers. According to statistics collected by Gross, there were in 973 tumours, 832 cancers (85'5 per cent.), and 77 sarcomas.

Amongst the typical tumours, the lipoma, chondroma, and myoma are excessively rare, and require little notice.

Lipoma occurs doubtfully as a solid tumour in the midst of the gland. The Lipoma capsulare is also rare. In it the adipose tissue around the gland undergoes great increase, while the gland itself atrophies. Such tumours may assume enormous dimensions, weighing up to 100 pounds.

Chondroma rarely occurs as an independent tumour, but cartilage is sometimes found in sarcomas and cancers. Bone sometimes exists in ribromas, causing a partial conversion of them.

Myoma (leiomyoma) sometimes develops in connection with the nipple. Billroth observed a case in which striated muscle existed in an adeno-sarcoma.