592 



METABOLISM 



vibration, and the opening be sufficiently large to allow a good current of air to pass. 

 It should collapse instantly and be air-tight when the current of air is reversed. The 

 back lash, or lag of closure, of these valves is extremely small, and they will open or 

 close with a pressure of air not exceeding the pressure changes in normal respiration. 

 When not in use, the valves should be kept in glycrine water on ice. Valves prepared 

 in this way have been in use a month without loss of efficiency. They are, however, 

 made with so great ease that new valves are provided for each subject, and they are 

 therefore especially adapted to ward work (Fig. 178). 



The valves are inserted in reverse order into a supporting metal T-piece, and the 

 joints made air-tight by tape. The stem of the T is connected with the mouthpiece. 



Fig. 180. The Douglas bag method for determining the respiratory exchange. The arrange- 

 ment of mouthpiece, valves, and connecting tubes shown here has been found to be more con- 

 venient than that recommended by Douglas. 



Through a rubber tube of about % inch bore, the expired air is collected in the spirom- 

 cter, or Douglas Bag. 



3. THE TISSOT SPIROMETER is pictured in Fig. 179. We have found the 100-liter 

 s'ze to be very serviceable in the clinic. This instrument is mounted on a platform 

 having rubber wheels, and can be moved about the wards with ease. The bell of the 

 spirometer is made of aluminum and is suspended in a water-bath between the double 

 Avails of a hollow cylinder made of galvanized iron. The height of the bell is 72 cm. and 

 the diameter 42 cm. An opening at the bottom of the cylinder connects through a 

 three-way stopcock with the rubber tube leading from the expiratory valve of the 

 mouthpiece (see Fig. 177). The bell is counterpoised by means of a weight. In the 

 original Tissot spirometer an automatic adjustment permitted water in amount equal 



