196 



APPLIED ANATOMY. 



THE LUNGS 



The lungs entirely fill the pleural sacs when completely distended, but only 

 partly so in quiet, ordinary respiration. They are encased in a bony cage that is 

 open below, on account of which, when the lungs distend, they expand mostly 

 downward. To a less extent they expand, in forced respiration, both laterally 

 and anteroposteriorly, due to the elevation of the ribs owing to the traction of the 

 muscles upon them. Ordinary breathing is performed mainly by the diaphragm. 

 It acts like the piston of a cylinder and as it descends the air is drawn into the 

 trachea and lungs. As the diaphragm falls a negative pressure is produced within 

 the chest and were it not for its bony framework, it would collapse. The framework 

 is sufficiently strong to retain its shape in spite of this pressure if the breathing is 



FIG. 215. Posterior surface relations of the lungs and pleurae. 



normal and the chest-walls are healthy. When, however, obstruction of the air- 

 passages is present, perhaps from enlarged pharyngeal or faucial tonsils or nasal 

 hypertrophies, then the deformities known as funnel-breast, pigeon-breast, etc. , 

 already described, arise. They are also produced if there is no obstruction to the 

 breathing but only a weakness in the bony thorax, as occurs in rickets. 



Two of the most common of the diseases of the lungs produce changes in the 

 shape of the thorax ; they are emphysema and phthisis. Pneumonia, though a frequent 

 enough disease, does not produce any changes, as it is too short in its duration. 



In emphysema the lungs are in a state of hyperdistention, hence they fill the 

 chest to its greatest capacity and tend to make the soft parts bulge between the ribs. 

 In phthisis the lungs are contracted, hence the intrathoracic pressure becomes a 

 negative one and the soft parts sink in between their bony support. In emphysema 

 the anteroposterior diameter increases and the chest assumes the barrel-form already 

 described. In phthisis it becomes lessened in its anteroposterior diameter and we 



