RESPIRATION. 129 



The Thorax. The thorax in which the respiratory organs are lodged, is 

 of a conic shape, having its apex directed upward, its base downward. Its 

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

 num, 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 them- 

 selves 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 diminished 

 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 condition 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 pre- 

 viously 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 pres- 

 sures being neutralized, there is nothing to prevent the elastic tissue from re- 

 coiling, driving the air out, and collapsing. The elastic tension of the lungs 

 can be readily measured in man after death by inseriting a manometer into 

 the trachea. Upon opening the thorax and allowing the tissue to recoil, the 

 air passes upon the mercury and elevates it, the extent to which it is raised 

 being the index of the pressure. Hutchinson calculated the pressure to be 

 one half pound to the square inch of lung surface. 

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