RESPIRATION. 395 



During Inspiration. 



1. Enlargement of the thoracic diameters by muscle action. 



2. Increase in the negativity of the in tra- 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 periods of inspira- 

 tion 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 more than 

 a few millimeters. In labored respirations these movements are naturally 

 increased in extent. The irregular movements of the membranes occasioned 

 by the unskilful use of the laryngoscope, especially with nervous patients, 

 are not to be regarded as strictly physiologic. The respiratory space in 

 quiet breathing is an isosceles triangle, with a length of 20 mm. and a width 

 at the base of 15.5 mm. with an area of 155 mm. 



Respiratory Types. Observation of the respiratory movements in the 

 two sexes shows that while the enlargement of the thoracic cavity is accom- 

 plished both by the descent of the diaphragm (as shown by the protrusion of 

 the abdomen) and the elevation of the thoracic walls, the former movement 



