RESPIRATION. 



399 



pirations per minute be known. If it be accepted that the inspired volume 

 varies from 328 to 492 c.c. and that the respiratory frequency averages 18 per 

 minute, then the total volume breathed would amount to from 8500 to 

 12,752 liters. 



The volume changes of the thorax indicated by the volumes of air en- 

 tering and leaving the lungs can be not only determined but graphically 

 represented by means of an apparatus similar in principle to the spirometer, 



FIG. 196. GAD'S, PNEUMATOGRAPH. 



devised by Gad and known as the pneumato graph or aeroplethysmograph 

 (Fig. 196). This consists of a quadrangular box with double walls, the 

 space between which is filled with water. The center of the box is an air 

 chamber. A thin walled mica box sinks into the water. Posteriorly it is 

 attached to and rotates around an axis, which permits of an elevation or 

 depression of the anterior portion. It is also carefully counterpoised. A 

 light lever attached to the mica box records its movements. The interior 



FIG. 197. DIAGRAM REPRESENTING THE VOLUME CHANGES OF THE THORAX AND LUNGS. 



(Modified from Boruttau.) 



of the box communicates by a tube with a large reservoir into which the 

 individual breathes, the object being to prevent a too rapid vitiation ot the 

 air. Inspiration causes the lever to descend, expiration to ascend. Previous 

 graduation of the apparatus is necessary to determine the volumes breathed. 

 A graphic record of the volume changes is shown in Fig. 197. 



Respiratory Sounds. On applying the ear over the trachea and bronchi 

 there is heard during both inspiration and expiration a well-defined sound, 

 which is loud, harsh, and blowing in character, and which from its situation is 



