128 ENTOZOA FOUND IN MAN. 



which are developed in parenchymatous viscera are 

 enclosed in a cyst which isolates them from the 

 surrounding tissues. This cyst is formed by the 

 areolar tissue of the viscus in which the vesicular 

 worms are contained, and does not appear to differ 

 from that which becomes developed around any 

 other foreign body in a similar situation ; it 

 presents certain differences of structure which bear 

 an evident relation to the nature of the viscus in 

 which it originates ; thus, for example, it is thick and 

 firm in the liver, but very delicate and of only slight 

 consistence in the brain. 



The hydatids which are developed in a natural 

 serous cavity have no special pouch, and the reason 

 for this is, doubtless, that they find in the membrane 

 which lines such cavities conditions of structure ana- 

 logous to those of areolar cysts. 



The walls of hydatid cysts are composed of areolar 

 tissue, which is more or less condensed, and arranged 

 into layers which may be separated into strips of 

 variable size, but not into distinct tunics. 



Besides the differences which these walls present 

 according to the various organs in which they are 

 situated, other differences may also be observed 

 which bear a relation to the age and to the natural 

 growth of the bodies which they contain. The thick- 

 ness of the walls increases in proportion to the size 

 which the tumour acquires, and, still more perhaps, 

 in proportion to the duration of the tumour. Although 

 it is thin and simply areolar at first, the cyst becomes 

 subsequently strong and dense, and, at a still later 

 period, it acquires the consistence of fibrous tissue, or 

 even of fibro-cartilage. Cretaceous deposits may be 



