306 TUMORS. 



Cysts. 



These structures, for the sake of convenience, are often classed among 

 the tumors, although in general characters, structure, and genesis they 

 are usually of entirely different nature. 



They may be divided into two classes: 



I. Cysts which develop from pre-existing cavities. 



II. Cysts which originate independently as the result of pathological 

 changes. 



I. CYSTS WHICH DEVELOP FROM PRE-EXISTING CAVITIES. 



1. Retention Cysts. These are chiefly formed by the accumulation in 

 glands or their excretory ducts of the more or less altered secretion of 

 the gland. They usually occur as the result of a hindrance to the nor- 

 mal discharge, as from inflammatory contractions, pressure, etc. The 

 contents of such cysts are usually mucous, sebaceous, serous, or of a 

 mixed character. Their walls are the more or less altered walls of the 

 original structure. To this class belong comedones, milium, atheroma, 

 chalazion, rauula, the ovula Nabothi, milk cysts, and certain serous cysts 

 of the ovaries, Fallopian tubes, gall ducts, and uriniferous tubules. 



2. Transudation Cysts. These arise usually, though not always, as 

 the result of a chronic inflammatory process in lymph spaces or serous 

 sacs, and among them are to be classed the so-called ganglia, hydrocele, 

 etc. Certain of the so-called hsematoceles, in which blood is extrava- 

 sated into closed cavities, form a variety of the cysts of this group. 



II. CYSTS WHICH ORIGINATE INDEPENDENTLY AS THE RESULT 

 OF PATHOLOGICAL CHANGES. 



1. Cysts Formed by the Softening and Disintegration of Tissue. Such 

 cysts may at first be small and have very meagre contents and no well- 

 defined wall. A wall may finally be present either as an entirely new- 

 formed structure, or the more or less modified capsule of the organ in 

 which they occur may partly or entirely form the wall. The contents of 

 such cysts are usually the more or less altered detritus of the tissue by 

 whose disintegration they are formed. Such cysts are very apt to occur 

 within true tumors, particularly those which are succulent and of rapid 

 growth, since these, as above stated, are very liable to degeneration. Old 

 abscesses may change into well-defined cysts of this kind. 



2. Cysts Formed around Foreign Bodies. The inflammatory reaction in- 

 duced by the presence of foreign bodies of various kinds, parasites, 

 masses of extravasated blood, etc., frequently results in the formation of 

 well-defined encapsulated cysts. 



3. Cysts Formed by a New Growth of Tissues in whose Spaces Various 

 Kinds of Fluid Accumulate. These spaces may or may not be lined with 

 epithelium and have something of the glandular character. Such forms 



