226 MECHANISM OF RESPIRATION. 



the child, the lungs float. Hence, this hydrostatic test is largely used in 

 medico-legal cases, as a test of the child having breathed. If a healthy 

 lung be squeezed between the fingers, it emits a peculiar and character- 

 istic fine crackling sound, owing to the air within the air-cells. A 

 similar sound is heard on cutting the vesicular tissue of the lung. The 

 colour of the lungs varies much ; in a young child it is rose-pink, but 

 afterwards it becomes darker, especially in persons living in towns or 

 a smoky atmosphere, owing to the deposition of granules of carbon. 

 In coal-miners the lungs may become quite black.] 



107, Mechanism of Respiration. 



The mechanism of respiration consists in an alternate dilatation and 

 contraction of the chest. The dilatation is called inspiration, the con- 

 traction expiration. As the whole external surfaces of both elastic lungs 

 are applied directly, and in an air-tight manner by their smooth moist 

 pleural investment to the inner wall of the chest, which is covered by 

 the parietal pleura, it is clear, that the lungs must be distended with 

 every dilatation of the chest, and diminished by every contraction 

 thereof. These movements of the lungs, therefore, are entirely passive, 

 and are dependent on the thoracic movements (Galen). 



On account of their complete elasticity and their great extensibility, 

 the lungs are able to accommodate themselves to any variation in the size 

 of the thoracic cavity, without the two layers of the pleura becoming 

 separated from each other. As the capacity of the non-distended chest 

 is greater than the volume of the collapsed lungs after their removal 

 from the body, it is clear that the lungs even in their natural position 

 within the chest are distended, i.e., they are in a certain state of 

 ELASTIC TENSION ( 60). The tension is greater the more distended 

 the thoracic cavity, and vice versd. As soon as the pleural cavity is 

 opened by perforation from without, the lungs, in virtue of their 

 elasticity, collapse, and a space filled with air is formed between the 

 surface of the lungs and the inner surface of the thoracic wall (pneumo- 

 thorax). The lungs so affected are rendered useless for respiration; 

 hence a double pneumo-thorax causes death. 



It is also clear that, if the pulmonary pleura be perforated from within the lung, 

 air will pass from the respiratory passages into the pleural sac, and also give rise 

 to pneumo-thorax. 



[Not unfrequently the surgeon is called on to open the chest, say by removing a 

 portion of a rib to allow of the free exit of pus from the pleural cavity. If this 

 be done with proper precautions, and if the external wound be allowed to heal, 

 after a time the air in the pleural cavity becomes absorbed, the collapsed lung 

 tends to regain, its priginal form, and again becomes functionally active.] 



