Spontaneous mammary tumors occur in middle aged or senile dogs and are of three principal pathological types; a diffuse epithelial and connective tissue overgrowth often with associated cartilage and bone; solid masses of epithelial cells; intracystic papillomatous tumors. The metastases in this series were always composed of papillary carcinomatous cysts; this tissue in transplants to abdominal connective tissue induced epithelial osteogenesis. All of the dogs with mammary tumors had lipid-rich suprarenal cortical tumors.

The tumors frequently secrete a dilute milk with citric acid content considerably increased above serum levels; large numbers of colostrum corpuscles are present in this fluid. After weaning, normal parturient dogs undergo a complete cessation of mammary secretion; lactation persists considerably longer in dogs with mammary tumor than in normal dogs. Absence of suckling does not eliminate lactation in pseudopregnancy.

The failure of the breast to lactate under the stimulus of prolactin signifies a lack of physiological maturity. The breasts of mature dogs have two types of response to ovariectomy; either great physiological involution results or lactation, actual or potential, is retained in areas for at least 3 to 6 months. Those in which excision of the ovaries does not cause prompt and great involution of the breast include all dogs with mammary tumors, all with pseudopregnancy, and certain dogs, presumably normal which we are unable to characterize further at this time. Excision of the suprarenal glands in addition to the ovaries usually did not completely eliminate lactation or the lactational potential.

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