A cyst may be defined as a tumor containing one or more cavities filled with fluid or semifluid contents. This specifies nothing with regard to the location nor the character of the cyst wall nor the nature of the fluid contents. Following Sutton, I divide cysts into four groups:

Retention Cysts.

—These imply a previously existing cavity whose outlet is obstructed and whose contents consequently accumulate, often to such a degree that the original character of both containing wall and contained fluid is entirely altered. When this occurs in glands or gland ducts there is usually complete atrophy of gland tissue, providing sufficient time have elapsed. Such cysts are due either to permanent or temporary arrest of flow. In hydronephrosis, for example, there is obstruction of the renal outlet and dilatation of its pelvis, with partial or complete atrophy of the kidney structure, until a cyst of enormous size may be present. When a similar condition obtains in the uterus, as by obstruction of the cervix, perhaps due to injury done during labor, we have a condition known as hydrometra, seen occasionally in women, often in the lower animals, and particularly in those having a bicornate uterus, causing a condition often mistaken for an enormously dilated Fallopian tube. Similarly, when the common bile-duct is obstructed, which may be due to impacted gallstones, to inflammatory lesions or tumors, there may be such backing up of bile in the gall-bladder as to produce the condition known as hydrocholecyst.

Under any of these circumstances pyogenic bacteria may produce infection which will be more or less promptly followed by suppuration; and then, instead of hydronephrosis, ydrometra, hydrosalpinx, etc., we have pyonephrosis, pyometra, and pyosalpinx.

Tubulo Cysts.

—These are cystic dilatations of certain functionless ducts and obsolete canals which no longer serve a useful purpose. They comprise:

1. Cysts of the Vitello-intestinal Duct.

—Cysts originating from this functionless duct occupy the umbilical region, sometimes projecting externally, sometimes internally. They are usually lined with mucous membrane furnished with villi and columnar epithelium. Such a cyst may be confounded with an umbilical hernia. These cysts occasionally open at the umbilicus and discharge irritating material, sometimes fecal matter. Cystic dilatation of the portion of the duct originally connected with the ileum is also sometimes seen.

2. Allantoic Cysts.