SG CYSTS. 



b. Exudation-cijsts. These, too, arc secreting cysts. Their 

 mode of origin, however, is diametrically opposite to that of 

 retention-cysts. The shut sac is not in this, as in the former 

 case, produced by the isolation of a caecal recess ; it is pre- 

 formed. The foundation for exudation-cysts is Laid in mucous 

 bursce, in the sheaths of tendons, in serous sacs, in the ventricles 

 of the brain and cord. Moreover, the accumulation of fluid is 

 due, not to the continuance of the normal secretion, but to an 

 exudation of serum from the blood, transcending the normal 

 standard, and containing salts, albumen, fibrinogen, and extrac- 

 tive matters in the most variable proportions. Exudation-cysts 

 have but little to do with morbid growth. AVe shall come across 

 them again under the most diverse names — most of which are 

 derived from the watery nature of their contents — as dropsies, 

 hygromata, hydatids, &c., of the different organs of the body. 

 Of greater moment for our present subject are the — 



e. Extravasation-cysts, A parenchymatous ha?morrhage may 

 readily serve as the starting-point for the development of a cyst. 

 It were difficult indeed to give the name of a cyst to a shapeless 

 clot of extravasated blood not smoothly and .sharply marked oft* 

 from the neighbouring parenchyma, but leaving it irregularly 

 torn and broken up. The extravasation may in some cases, how- 

 ever, present the characters of a cyst from the very first, sc. 

 when the blood has been poured out between two surfaces which 

 are naturally smooth {e.g. between periosteum and bone, between 

 cartilage and perichondrium), and continues in a fluid state ; or 

 it may be converted into a cyst by the production of a layer of 

 connective tissue from the adjoining parenchyma on the one 

 hand, while on the other the blood itself undergoes a series of 

 metamorphoses, ending in its being almost wholly absorbed and 

 replaced by a clear fluid. 



d. Cysts due to softening. In studying the various '^ conditions 

 of involution " to which the tissues are liable, we came across 

 more than one process which led to softening — to the formation 

 of a morbid fluid. I allude more particularly to fatty degenera- 

 tion and mucous softening, the former of which, under certain 

 conditions, may yield a milky emulsion, the latter a fluid clear 

 as water, containing mucus and albumen. Should the escape or 

 the absorption of these fluid products of softening be hindered, 

 their local accumulation, without farther change, may offer all 



