CYSTS. 83 



a. Retention-cysts. — These cysts originate, as their name im- 

 plies, in the retention of secretions : we may add that the secre- 

 tions are invariably such as would naturally be voided on the 

 free surface of the body, whether skin or mucous membrane, 

 and then removed or otherwise disposed of. The presence of a 

 recess or diverticulum, of a cavity open at one end and closed at 

 tlie other, is necessary for the production of a retention-cyst. 

 This cavity is shut off by some accident, and converted into a 

 •cyst by the retention of the matters which continue to be unin- 

 terruptedly secreted, and which therefore accumulate at their 

 place of origin. Such conditions are obviously most common in 

 the physiological recesses of the skin and mucous membranes, in 

 the ducts and terminal cffica of the tubular and acinous glands ; 

 that similar conditions may also occur elsewhere will be shown 

 farther on. 



On inquiring into the causes which may lead to closure of the 

 efferent duct of a gland, we are met by various possibilities. 

 The simplest, yet least common mode, is that by plugging of 

 the duct (ohturatio). Solid bodies, whose size and weight do 

 not allow them to pass through the ducts, nearly always originate 

 as precipitates, or concretions from the fluid secretion itself, e.^. 

 biliary, urinary, and salivary calculi. More rarely the efferent 

 ducts of a gland are stopped up by solid matters of extraneous 

 origin, e.cf. hydatid vesicles. Again, a duct may be obstructed 

 ])y mutual adhesion of its walls (obliteratio). Causes which pre- 

 dispose to this mode of obstruction are external pressure, and a 

 raw, ulcerated condition of the surface, particularly when this is 

 followed by cicatrisation. When incomplete in degree it is known 

 as stricture (stenosis, strictura). Finally, pressure from without 

 (compressio) is sufficient of itself to narrow, pucker, and constrict 

 the duct. This last mode of closure, which usually becomes 

 complicated at a later period with '' obliteration," is of prime 

 importance as regards new growths; for not only are such 

 growths in the neighbourhood of the urinary passages, the pan- 

 creatic and choledoch ducts, capable of closing these great 

 efferent canals by compression and constriction, giving rise 

 thereby to colossal dilatation of that part of the mucous tube 

 which is above the point of obstruction, but the same process 

 is repeated on a small scale whenever a new growth involves 

 the gland-substance itself. Single uriniferous tubes, lacteal 



