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in any direction would likewise be proportional to the force of elastic recoil. 

 Therefore the intra-thoracic pressure would vary from a uniform decrease 

 and increase just as the curve of Fig. 191 varies from uniform straight lines. 

 The changes in intra-thoracic pressure are graphically represented in Fig. 

 191, B. 



The intra-thoracic pressure and its variations influence favorably the 

 flow of lymph through the thoracic duct (see page 225), as well as the flow 

 of blood from the extra-thoracic veins into the intra-thoracic veins, the right 

 side of the heart, and the cardio-pulmonary vessels. (See paragraphs at the 

 end of this chapter.) 



The succession of events in the thorax at the time of a respiratory act 

 may be summarized as follows: 



During Inspiration. 



1. Enlargement of the thoracic diameters by muscle action. 



2. Increase in the negativity of the intra-thoracic pressure. 



3. Expansion of intra-pulmonic (alveolar) air. 



4. Expansion of the lungs. 



5. Lowering of the intra-pulmonic air pressure below the atmospheric 



air pressure. 



6. Inflow of atmospheric air, in consequence of its higher pressure, until 



the intra-pulmonic air pressure rises to that of the atmosphere. 

 During Expiration. 



1. Diminution of the thoracic diameters by the action of elastic forces. 



2. Decrease in the negativity of the intra-thoracic pressure. 



3. Recoil of the lungs. 



4. Compression of the intra-pulmonic (alveolar) air. 



5. Rise of intra-pulmonic air pressure above the atmospheric air 



pressure. 



6. Outflow of intra-pulmonic air, in consequence of its higher pres- 



sure, until the intra-pulmonic air pressure falls to that of the 

 atmosphere. 



Respiratory Movements of the Upper Air-passages. The resistance 

 to the entrance of air into and through the respiratory tract is much dimin- 

 ished by respiratory movements of the nares and larynx which are associated 

 and occur synchronously with the movement of the thorax. 



The nares at each inspiration are dilated by the outward movement of 

 their alae or wings, the result of muscle activity. At each expiration they 

 are diminished by the return of their cartilages through the play of elastic 

 forces. The larynx, as shown by observation with the laryngoscope, exhibits 

 corresponding movements of the vocal membranes. Their introduction at 

 this level naturally narrows the tract, and would interfere with both the 

 entrance and the exit of air were they not kept widely asunder during the 

 time they are not required for purposes of phonation. This is accomplished 

 by the tonic contraction of the posterior crico-arytenoid muscles, which are 

 entirely respiratory in function. 



It is not infrequently stated that these membranes exhibit considerable 

 oscillations, outward and inward, corresponding to the movements of 

 inspiration and expiration. The statements of the majority of laryngologists 

 do not favor this view. During tranquil breathing the membranes are 

 widely separated and almost stationary, seldom moving in either direction 



