170 ENTOZOA FOUND IN MAN. 



It is smooth upon its internal surface, at least in 

 its earliest stages, and consists of a delicate mem- 

 brane, which is distinct from the surrounding bony- 

 tissue ; upon the bony tissue may be observed digital 

 depressions, which are caused by the j)ressure of the 

 hydatids, and are analogous in their appearance to 

 the little depressions which are seen on the internal 

 surface of the cranium. 



The cyst undergoes modifications in form which 

 bear a relation to the obstacles which are opposed to 

 its growth in various directions ; and it also under- 

 goes modifications of structure which are similar to 

 those observed in hydatid cysts in other parts of the 

 body. The bony walls which enclose it acquire, at 

 first, an increase in size proportional to that of the 

 hydatid pouch ; they are distended, and become 

 thinner, so that the portion of the bone in which the 

 hydatids are contained forms a swelling of uniform 

 dimensions ; at a later period, those portions of the 

 bone which are thinnest are absorbed, and the cyst 

 comes into contact with the soft parts which it 

 encroaches upon as its development continues, and 

 finally the neighbouring organs are seriously affected, 

 owing to their compression or displacement. 



The hydatids which are situated in the cranial 

 bones give rise to the same compHcations as are 

 produced by cystic tumours developed in the brain ; 

 hydatids contained in the walls of the orbit cause 

 protrusion of the eyeball, and loss of vision ; and 

 those which are developed in the long bones may 

 eventually invade an articular cavity and produce 

 severe arthritis. 



Hydatids in bones are usually painless at the 



