xii MECHANICS OF EESPIEATION 405 



smooth. The smooth muscle cells found along the air-passages, 

 and most abundantly in the smallest bronchi, are able, according 

 to the degree of their tonus, to alter the lumen of the canal as 

 required. 



III. The lungs of the foetus are void of air (atelectatic), and 

 therefore sink (in the test known as docimasia hydrostatica) when 

 immersed in water. The first respiratory movements which 

 occur after birth expand the alveoli of the lungs and fill them 

 with air, and lungs removed from any individual who has already 

 breathed float on the surface of water in docimasia hydrostatica, 

 because the elastic retraction they undergo when the thorax is 

 opened is not strong enough to expel all the air from the alveoli. 

 This is because the capillary bronchi or bronchioles (which, as we 

 have seen, contain no cartilaginous platelets such as give a certain 

 rigidity to the larger bronchi) collapse, and hinder the complete 

 expulsion of the alveolar air. The lungs of a new-born infant, 

 which has already breathed, may after a certain time become 

 atelectatic again by the reabsorption of the air which they contain. 

 From the same cause the lungs of a man or other animal become 

 atelectatic in a few hours, if the pleural cavity has been opened on 

 one side. 



The form and volume of the atelectatic foetal lungs correspond 

 exactly with the form and volume of the thoracic cavity in which 

 they are enclosed. They are in perfect elastic equilibrium, since 

 on opening the thorax they do not retract, and if a mercury or 

 water manometer is connected with the trachea so as to measure 

 the intrathoracic pressure (Bonders), it is found to be equal to 

 atmospheric pressure, the manometer showing no movement on 

 opening of the thoracic wall (Bernstein). 



These conditions gradually alter during extra-uterine life. As 

 the thorax grows more rapidly than the lungs, these are thrown 

 into a state of increasing elastic tension, in proportion as the 

 difference between the capacity of the thorax and the volume of 

 the lungs increases, when they are respectively in the normal 

 position of equilibrium. This can easily be determined by Bonders' 

 method on the bodies of persons of different ages, provided they 

 did not die of pulmonary diseases. If a manometer is connected 

 with the trachea of a dead new-born infant, that had breathed, the 

 lungs do not retract, and the column of mercury does not rise, on 

 opening both pleural cavities; the same conditions, therefore, 

 persist after birth as in the foetal atelectatic lung (Hermann). 

 When, on the contrary, the same experiment is repeated on the 

 body of an adult, the lungs retract more or less according to age, 

 by elastic recoil, till they are in equilibrium with a column of 

 mercury of 5-7*5 mm. Accordingly, in the cadaveric, or the 

 passive respiratory position of the adult thorax, it is the atmo- 

 spheric pressure which acts within the pulmonary air-passages, 



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