THE RESPIRATORY SYSTEM. 485 



These infarctions are often multiple, usually circumscribed, and 

 rounded or wedge-shaped, from the size of a walnut to that of an orange. 

 They are of dark-red color, hard and uuaerated, with the air spaces 

 distended with blood, and are often surrounded by a zone of inflamma- 

 tory exudate. They may be situated in any part of the lungs, but are 

 most common in the lower lobes. At the apices of the infarction, the oc- 

 cluding thrombus or embolus may be discovered. When, as is usually 

 the case, they are near the surface of the lungs, a circumscribed inflam- 

 mation of the pleura often occurs. 



Such infarctions may be followed by death ; they may become gan- 

 grenous, or if the emboli or thrombi be not infectious, the blood may 

 become absorbed, and, especially in the smaller forms which are more 

 often due to embolism, they may be gradually changed into a smaller 

 mass of pigmeuted fibrous tissue. 



A large part of the lungs may be involved in haemorrhage due to 

 thrombosis of large trunks of the pulmonary artery. Ha?morrhagic in- 

 farctions from thrombosis or embolism are most frequent in lungs which 

 are the seat of chronic congestion. The most common source of the em- 

 boli of the pulmonary artery is the right heart or peripheral thrombi. ' 



ATELECTASIS. 



In atelectasis the walls of the air spaces lie together, either because 

 they are collapsed or compressed, or because, as in congenital atelectasis, 

 the lungs, or portions of them, have not been expanded in respiration. 



In fretal or congenital atelectasis defects in the respiratory mechanism, 

 or blocking of the air passages, may be responsible for the unaerated 

 condition which may affect only parts of a lung or a whole organ. The 

 atelectatic portions of the lungs are dark red in color and of fleshy con- 

 sistency. 2 



Atelectasis may, on the other hand, \>z acquired, either in childhood 

 or in adult life. In young children collapse of portions of the lung is 

 of frequent occurrence, through occlusion of bronchi by inflammatory 

 exudate. The region thus cut off is gradually deprived of air, so that 

 as the blood continues to circulate, it is dark red and firm in texture. 

 Compression atelectasis may be due to exudates, tumors, etc., in the pleu- 

 ral cavities. Under these conditions the portion of lung involved may 

 be paler than normal from the pressing out of the blood. In adults, 

 large or small portions of lung tissue may collapse from occlusion of 

 a bronchus by exudate or stenosis, by paralysis of the vagus, or in long- 

 continued feebleness of respiration. (Edema may be associated with 

 collapse. 



Atelectasis may resolve by an early admission of air to the collapsed 



1 For a fuller consideration of embolism and thrombosis of the pulmonary artery 

 consult Welch, in Allbutt's "System of Medicine," vol. vi., p. 261. 



3 Collapsed lungs from their red color and fleshy consistency are often spoken of as 

 "'cammed." 



