THE ORGANS OF RESPIRATION. 147 



Hydatids developed witliin the cavity of the 

 thorax are not very common ; they usually exist in 

 the pulmonary tissue. Those which are developed 

 between the pleura and the ribs, or in the mediastina, 

 are very rare ; and it would be impossible to deter- 

 mine their position during the lifetime of the patient, 

 unless the tumours had acquired large proportions. 



Hydatids of the lungs usually exist in the lower 

 lobes, and especially upon the right side ; very few 

 cases are recorded in which the cysts were developed 

 in the upper or middle portions of the lungs. 



It is a rare occurrence to find two cysts in the 

 same lung, but the cases in which a cyst was 

 present in each lung are more common. When 

 hydatids exist in the chest, the liver is often similarly 

 affected. 



In the greater proportion of cases the intra- 

 thoracic cyst contains a single hydatid which occupies 

 its whole cavity. This cyst has thin, smooth waUs, 

 at any rate until it has undergone some changes of 

 structure ; sometimes it is of enormous size, and 

 fills up the whole of one side of the chest. The 

 affected side is then notably enlarged, the intercostal 

 spaces are deeper than in the normal state, and the 

 lung, which is folded upon itself and flattened, or 

 reduced to a thin layer, is pressed back towards its 

 root, or the upper part of the chest, or against the 

 vertebral column, and the heart, which is changed in 

 position according to the side from which the pres- 

 sure of the hydatid is exerted, may be forced either 

 towards the left or the right axilla, or towards the 

 epigastrium. The fiver may also be displaced, and 

 pushed lower down into the abdominal cavity. 



L 2 



