RESPIRATION. 61 



The thoracic cavity in which the respiratory organs are lodged is of a 

 conical shape, having its apex directed upward, its base downward. Its 

 framework is formed posteriorly by the spinal column, anteriorly by the 

 sternum, and laterally by the ribs and costal cartilages. Between and over 

 the ribs lie muscles, fascia and skin ; above the thorax is completely closed 

 by the structures passing into it and by the cervical fascia and skin ; below 

 it is closed by the diaphragm. It is therefore an air-tight cavity. 



The Pleura. Each lung is surrounded by a closed serous membrane, 

 the pleura, one layer of which, the visceral, is reflected over the lung, the 

 other, the parietal, reflected over the wall of the thorax ; between the two 

 layers is a small amount of fluid which prevents friction during the play of 

 the lungs in respiration. 



Owing to the elastic tissue which is present in the lungs, they are very 

 readily distensible, so much so, indeed, that the pressure of the air inside 

 the trachea and lungs is sufficient to distend them until they completely 

 fill all parts of the thoracic cavity not occupied by the heart and great 

 vessels. The elastic tissue endows them not only with distensibility, 

 but also with the power of elastic recoil, by which they are enabled 

 to accommodate themselves to all variations in the size of the thoracic 

 cavity. 



When the chest walls recede, the air within the lungs expands and 

 presses them against the ribs; when the chest walls contract, the air 

 being driven out, the elastic tissue recoils and the lungs return to their 

 original condition. The movements of the lungs are therefore entirely 

 passive. 



As the capacity of the chest in a state of rest is greater than the volume 

 of the lungs after they are collapsed, it is quite evident that in the living 

 condition the lungs are distended and in a state of elastic tension, which 

 is greater or less in proportion as the thoracic cavity is increased or dimin- 

 ished in size. The elastic tissue, always on the stretch, is endeavoring to 

 pull the visceral layer of the pleura away from the parietal layer, but is 

 antagonized by the pressure of the air within the air passages. This con- 

 dition of things persists as long as the thoracic cavity remains air tight ; but 

 if an opening be made in the thoracic wall, the pressure of the external air 

 which was previously supported by the practically rigid walls of the thorax 

 now presses upon the lung with as much force as the air within the lung. 

 The two pressures being neutralized, there is nothing to prevent the elastic 

 tissue from recoiling, driving the air out and collapsing. The elastic ten- 

 sion of the lungs can be readily measured in man after death by inserting 



