646 OCCURRENCE AND MEDICAL IMPORTANCE. 



When the Echinococci develop within the hones, the latter are at 

 first eroded in proportion to the growth of the parasite. Afterwards 

 the walls become thin, and often to such a degree that a moderate 

 blow is enough to cause a fracture. Thus Dupuytren mentions that 

 in the case of a man who had a hydatid Echinococcus in the much 

 enlarged medullary cavity of his humerus, the latter broke when he 

 tried to throw. Sometimes the Echinococcus breaks through the hard 

 coating at some point, and then expands among the surrounding soft 

 tissues. This happens not only when the parasite is developed in the 

 cancellous tissue of the bone, but also when contained within the 

 enclosed cavities, if it be but near the boundary. The cases are by no 

 means rare in which the Echinococcus of the thorax breaks through the 

 ribs and expands between the muscles, or where that found in the 

 cranial cavity breaks through the bones of the skull and expands in 

 the antrum of Highmore or nasal cavity. Sometimes, instead of seek- 

 ing an independent path, the clefts and foramina already present are 

 utilised. Those especially are traversed which lead into the orbits, so 

 that in consequence of the intrusion eyesight is often lost, exophthal- 

 mus ensues, or the eyeball is wholly destroyed. 



In considering these phenomena, we must not forget that the 

 Echinococcus in such cases is destitute of the firm connective-tissue 

 cyst, which otherwise surrounds it, and prevents it from exercising 

 a direct influence on the adjacent tissues. In this respect Echino- 

 coccus closely resembles the bladder-worms which also occur without 

 a capsule in the serous cavities. Besides, we must remember that such 

 a worm has, to an unusual degree, the power of adapting its growth 

 to its external relations, as we saw in noting that the differences in 

 form and size might be often directly referred to the unequal resis- 

 tance in the immediate environment. Where the connective-tissue 

 envelope yields to the pressure of the growing parasite, a diverti- 

 culum is formed, so that the protrusions and lobes of the irregular 

 Echinococcus are ultimately explicable by the same physical principle 

 as that according to which the external portion of the worm, when 

 in a superficial situation, projects from the surrounding parenchyma, 

 and continually grows further outwards. The point of least resis- 

 tance determines the direction of this growth, so that Echinococci 

 situated near the convex surface of the liver rise towards the thoracic 

 cavity, while those near its lower surface sink down into the abdomen. 



The pressure exerted on the bladder-wall and on the neighbouring 

 organs frequently excites a more or less marked reaction. First, 

 inflammation sets in, which is either limited to the Echinococcus- 

 cyst, or spreads to the adjacent tissues, and then results in a more 

 or less extensive adhesion of the affected parts. The Echinococcus 



