HYDATIDS. 131 



envelopes the hydatid vesicle, or else presses it 

 against one side of the pouch. The liquid contained 

 in the hydatid usually remains limpid, but it dimi- 

 nishes in quantity, and the vesicle collapses ; at the 

 same time, the cyst contracts, and thus contributes 

 to the further effacement of the cavity. 



In the course of time the secreted matter thickens, 

 becomes harder, and assumes the appearance of 

 painter's putty, and occasionally that of chalk ; the 

 hydatid is reduced to a few membranous shreds and 

 finally disappears ; the only traces of echinococci, 

 which are also destroyed, are their hooks. 



In other cases, the hydatid tumour undergoes 

 changes which are different in appearance, although 

 of a similar character ; the substance which fills up 

 the cyst is fluid, and resembles pus or softened 

 tubercle. The matter contained within the cyst may 

 also have a reddish, yellow, or greenish tint, in con- 

 sequence of its admixture with the animal fluids, 

 such as blood or bile. 



The knowledge of the chemical composition of 

 the hydatid membrane is comparatively unimportant, 

 but such is not the case with respect to the com- 

 position of the liquids, or of the matters which they 

 enclose. 



The hydatid hquid contains only traces of albu- 

 men, and no phosphates ; but it holds in solution a 

 considerable amount of chloride of sodium, the 

 crystals of which become evident under the micro- 

 scope when a drop of the fluid has been placed upon 

 a piece of glass, and allowed to evaporate. Its 

 specific gravity isfirom I'OOS to 1-013 ; and it is either 



K 2 



