38 BULLETIN 1449, U. S. DEPARTMENT OF AGRICULTURE 
fluid, or the fluid may be thick and viscid. Neither the walls nor 
the interior contain any dermal structure. 
CYSTS 
Cysts (not dermoid) are circumscribed, encapsulated, or walled- 
in cavities containing an abnormal accumulation of fluid or semi- 
fluid substance and not provided with an outlet. The term is often 
very loosely used by some writers. Cysts are generally classed with 
neoplasms, but this is more for convenience and not by reason of 
structural or etiological similarity, as cysts stand midway between 
neoplasms and dermoid cysts. Cysts are described as simple or uni- 
locular when the cyst wall is passive, serving only to retain the 
contents. When several cysts occur together and are identical in 
structure, arising from the same cause, they are called multiple cysts, 
or when the cysts spring from the inside wall of a cyst they are 
spoken of as multilocular cysts. 
According to the method of formation, cysts may be classified as 
retention cysts, exudation cysts, extravasation cysts, softening cysts, 
and parasitic cysts. 
Retention cysts arise from the accumulation of secretion when 
the duct of a gland has been occluded, preventing the escape of the 
secretion. The most common cysts are the sebaceous cysts of the 
skin, called wens, the mucous salivary gland cj^sts of the tongue, 
called ranula, the pancreatic cysts, the galactoceles (milk cysts) in 
the mammary glands, etc. The cyst contents are derived from the 
functional activity of the glands and are eventually altered by ab- 
sorption of some of the fluid and by the subsequent degenerative 
changes that affect the fluid as well as the lining and the walls of 
the cyst. These degenerative changes may bring about an irritation 
that gives rise to inflammation. 
Exudation or distention cysts resemble so closely the retention 
cysts that many writers make no distinction between them. They 
are cysts which occur in closed cavities not supplied b}^ an excre- 
tory duct, as in hydrocele that may be found in the tunica vaginalis 
testis, or cysts in the ovaries. The distention of enlarged bursse 
occurring in the elbow and the hock in horses as pouchlike dilations 
and the cysts found in the course of tendons are retention cysts and 
are known as windgalls. The dilated spaces forming cysts in the 
thyroid gland and the pituitary gland may be classed with the 
retention or with the exudation cysts. 
Extravasation or hemorrhagic cysts are the result of blood escaped 
from a vessel into a tissue or an organ. They are known as hema- 
tocele or sanguineous cysts. They become eventually surrounded by 
a capsule, which varies in thickness in different organs. Such cysts 
are common in domestic animals and are generally the result of 
traumatism, or they may result from rupture due to disease in the 
walls of a blood vessel. 
Softening cysts are pathological cavities which result from disin- 
tegration of solid tissues by retrograde changes and liquefaction 
necrosis. Such cysts may be found in rapidly growing neoplasms, 
especially in sarcomas and carcinomas in which myxomatous degen- 
eration has taken place. Colloid and mucoid degeneration finally 
terminates in the formation of softening cysts. Some writers also 
