298 PULMONARY MUCOUS TISSUE. 



that, as with the arteries, this form is never completely 

 attained during life. If the thoracic cage of a dead animal 

 be opened, the lung is seen in the form of a spongy mass, 

 lying firmly retracted towards the vertebral column ; 

 this is not, however, the natural form of the lung: the 

 muscular tissue in a corpse has lost its elasticity, and the 

 elastic tissue alone remains in a physiological state. If, again, 

 we open the thoracic cage of a living rabbit, we find that the 

 lung immediately retracts towards the vertebral column, in a 

 much more remarkable degree than in the dead body ; it is 

 reduced to a small substance containing little or no air or 

 blood, a compact parenchyma, hepatized, we might say. 

 Should an abundant effusion, filling one of the pleural 

 cavities, oblige the corresponding lung to retract on itself, we 

 shall find that it retracts as in the foregoing experiment. In 

 the case of the lung of a fetus which has not breathed, strong 

 points of resemblance to those here mentioned may be 

 observed. 



The natural form of the lung is thus that of a sponge, a 

 bladder with numerous partitions, firmly retracted against 

 the vertebral column ; but, from the first inspiration of the 

 foetus, at birth, this form is prevented ; the thorax dilates, 

 and, by means of the pleural cavity, forces the lung, as we 

 have already seen, to develop in a cavity represented in the 

 diagram as a cone. From that moment, on account of the 

 rigidity of the ribs, the lung can never (unless in the case of 

 perforation or effusion of the pleura) attain its natural form, 

 although it is always approaching it, exactly as we saw in the 

 case of the arteries. 



Inspiration, as we have studied it, may be considered as a 

 fresh violeiice done to the lung, opposing to a greater degree 

 its natural form. 1 



From this point of view it is easy to comprehend the 

 mechanism of expiration: as soon as contraction of the 

 inspiratory muscles ceases, the pulmonary elasticity which 

 till then has been opposed, re-asserts itself; the lung retracts 

 on itself, drawing with it, on account of the pleural vacuum, 

 the wall of the thorax. It thus appears that, contrary to 

 what takes place in inspiration, the lung is active, and the 

 wall of the thorax passive ; but, in reality, both these organs 

 are passive. The diaphragm will act in the same way ; if 



1 See L. Oger, " Considerations Physiologiques sur la Forme 

 Naturelle de Certains Organes. These de Strasbourg, 1870, No. 283. 



