HYDATID TUMOURS. 137 



The question has been suggested why these 

 entozoa are usually situated in the abdominal and 

 thoracic viscera. A plausible explanation of this fact 

 may be obtained, if it is true that hydatids, as has 

 been advanced by some vs^riters, owe their origin to 

 the transformation or to the development of the 

 embryo of a taenia. This embryo, having been intro- 

 duced into the alimentary canal with the food or with 

 the drink, and being unable to Kve or to develop 

 itself in the intestine before it has undergone certain 

 metamorphoses, quits the intestine, and reaches the 

 adjacent viscera, either directly or else indiiectly, 

 through the medium of the blood-vessels. 



In some countries, as for example amongst the 

 white population of North America, vesicular worms 

 are very rare, whilst in others they are very common. 

 In Iceland hydatids prevail endemically ; Professor 

 Eschricht, of Copenhagen, states that one-sixth of the 

 inhabitants of Iceland are affected by these entozoa. 



Hydatid tumours are usually developed very 

 slowly ; their duration is almost always for several 

 years, and it is not rare to meet with cases of which 

 the earhest symptoms can be traced back for ten or 

 fifteen years, or even longer. 



When an hydatid tumour is situated in an organ 

 which is essential to life, and which cannot be dis- 

 placed and does not admit of distension, it may lead 

 to the death of the patient before it has acquired a 

 large size ; but when it is developed under other 

 conditions, it has no immediate effect. In the latter 

 case, it may continue for a long time without its 

 being perceived, and may become of considerable 

 dimensions before any marked functional derange- 



