Macklin and Macklin 359 



The patient, for instance, might have been lifting a heavy weight, or straining 

 at stool, or violently coughing, even some hours before the fust symptoms ap- 

 peared. It takes time for the air to work its way up the sheaths of the vessels to 

 the hilum of the lung, and more time for it to gather in the mediastinum in 

 sufficient quantity and under sufficient pressure to produce symptoms. Again, 

 prolonged pressure upon one area of the thorax may induce relative collapse 

 of the lung, as in lying on one side too long, or in sitting crouched over the 

 wheel of a car in a long drive, with possible loss of thoracic-muscle tone.*^ 

 Thus in every such case the stage may have been set for local or general 

 alveolar overstrain and a resulting PIE. As physicians become more cognizant 

 of the conditions which usher in PIE they will incpiire more fully into the 

 happenings of the preceding 24 or more hours, and will, we feel confident, 

 find more often a satisfactory explanation for PIE. Things like PIE never 

 "just happen"; there is always a cause. Factor "B" should not be forgotten. 

 There may be doubt that it alone could induce PIE, but in combination with 

 Factor "A" and elevated intra-alveolar pressure it is looked upon as an adju- 

 vant. There is a report of pneumothorax in a patient with a pulmonary em- 

 bolus,-* and it is possible that Factor "B" was here one of the causes of a PIE. 



Rupture Sites 



A final word is in order on the sites of air leakage and the nature of the tissue 

 in which the ruptures occur. The lung has been referred to as an air "container" 

 which is efficient under ordinary circumstances. It may be regarded as a special 

 part of the organism built around a little of the gaseous environment, from 

 which it appropriates energizing material, and into which it eliminates certain 

 catabolites. By means of its bellows action the lung is enabled to take in por- 

 tions of this oxygenated gas as needed, and through its specialized structure 

 to subdivide these into minute parcels in order that the surface may be greatly 

 increased. The inner surface of the container, to which this air surface is 

 applied, is similarly widespread, and is essentially a great capillary net or 

 curtain, enormously reduplicated. The ultimate functional anatomical units 

 are the alveoli, which are small contiguous diverticulae from the terminals 

 of the respiratory part of the airway, known as the alveolar sacs. Each alveolus 

 is like a little box with one side left out. Opposite to this opening, or "mouth," 

 is the base. 



Now there are tioo categories of base. Many bases are partitions between 

 the alveoli of two separate but approximated sacs. Their capillaries function 

 on each of their two sides. Air, in breaking through such a "partitional" base, 

 would merely pass into an adjoining alveolus. An analogous transit of air 

 apparently occurs normally, through the pores. The other type of base does 

 not act as a partition between alveoli; on the contrary, it rests on connective 

 tissue, as already pointed out. Such bases unite to form tubes around blood 

 and air vessels, and layers flanking interlobular septa or underlying the pleura, 

 as already described. Their capillaries, of course, as far as external respiration 



