810 SPECIAL PHYSIOLOGY. 



Through simple membranous tubes, however firm, the free and instant 

 entrance of the air necessary to proper inspiration, would have been 

 impracticable; and, moreover, such tubes would have quickly collapsed 

 during expiration. At the larynx, the narrow triangular glottis has 

 musculo-mernbranous margins; but the state of this aperture is regu- 

 lated by the nervous system, which exercises a special control over it, 

 and though it may be voluntarily or involuntarily closed, it is habitu- 

 ally open. 



Mechanical obstructions in any part of the air-passages, by excluding 

 the air, may prove fatal. The forcible closure of the mouth and nose 

 for criminal purposes, the wilful filling of the fauces with a hand- 

 kerchief or cloth, the compression of the windpipe, the accidental im- 

 paction of pieces of food or of some foreign body in the glottis, closure 

 of this aperture from spasm, or from swelling of the surrounding mu- 

 cous membrane, a condition known as oedema of the glottis, the lodg- 

 ment of masses of food, too large to be swallowed, in the oesophagus, 

 and, lastly, the introduction of fluid in any quantity, as in drowning, 

 operate in this way. 



When the thoracic walls are so injured that an opening exists 

 through them into the pleural chamber, their expansion is no longer 

 followed by the proper inflation of the lungs; but the air passing in 

 through the artificial opening, to supply the threatened vacuum, the 

 lung is subjected to equal atmospheric pressure both on its pleural sur- 

 face and within its air-passages and air-cells, and owing to the elasti- 

 city of its component structures, it collapses to a greater or less extent. 

 If the opening be oblique or valved, as in certain punctured or gunshot 

 wounds, some air may still enter the lung by the trachea. As the two 

 pleurae form distinct chambers, when one only is punctured, the cor- 

 responding lung alone becomes collapsed, and though respiration is 

 embarrassed, death does not necessarily ensue. If, however, both 

 pleurae are simultaneously wounded, both lungs collapse, and death 

 follows from asphyxia or suffocation. 



Expiration. The movement by which the air, having entered the 

 lungs by an inspiratory effort, is again driven from them, is more pas- 

 sive in its character than that of the inspiration, depending less upon 

 muscular action, but more on the relaxation of the inspiratory muscles, 

 and on the elastic resilience of the organs and tissues concerned. 



As the muscles of inspiration cease to act, the tendinous part of the 

 diaphragm, the chief of those muscles, ascends into the thorax, fol- 

 lowed by the abdominal viscera, which are supported in their upward 

 movement by the co-operation of the muscles of the abdominal walls. 

 At the same time the ribs and the sternum, which were elevated, de- 

 scend and fall back, whilst the effects of the rotation of the ribs are 

 counteracted by the elastic recoil of the costal cartilages. Lastly, the 

 elasticity of the lungs themselves plays a most important part, acting 

 like an extended spring let loose, and serving to expel the air from the 

 air-tubes and air-cells. The longitudinal and circular fibres of the 

 bronchi and bronchia shorten and narrow those tubes; the elastic 

 walls of the air-cells diminish their size, and the interlobular, and 



