BY H. S. HALCRO WARDLAW. 807 



great as in experiments "b" and "c", although the movements 

 of the chest were very much smaller than in those experiments. 

 It is evident, then, that neither the extent, nor the frequency of 

 the respiratory movements in a given time, has any effect on the 

 respiratory exchange under the conditions of these experiments, 

 in which the renewal of the air in the lungs was prevented. 

 There still remain to be considered, however, the variations of 

 pressure of the air in the lungs, which accompany the respiratory 

 movements. 



Effect of Variations of Pressure. 



To enable the pressure in the lungs to be measured, the mouth- 

 piece, through which the expirations were made, was provided 

 with a small, lateral opening near its end. This opening lies 

 inside the mouth of the subject when the mouthpiece is in posi- 

 tion. Another small hole was drilled through the wall of the 

 mouthpiece at a position lying outside of the mouth of the sub- 

 ject. These two small holes were connected together by a very 

 narrow brass tube lying inside the bore of the mouthpiece, and 

 soldered in place. The end of the narrow tube, which is to lie 

 inside of the mouth of the subject, terminates flush with the 

 wall of the mouthpiece. The end of the tube lying outside of 

 the mouth of the subject projects through the wall of the mouth- 

 piece and is connected with a mercury-manometer. When in 

 position during an experiment, the mouthpiece is held firmly 

 between the lips, the end being tightly closed by the tongue. 

 The nose of the subject is held at the same time. Pressures 

 existing in the mouth are then registered by the manometer. 

 When the pharynx is kept open, the pressure in the mouth will 

 be practically equal to that in the lungs, if no sudden variations 

 of pressure occur. The maximal differences of pressure between 

 the air in the lungs and the atmosphere, which the present sub- 

 ject is able to maintain for about 20 seconds, are approximately 

 plus and minus 30 mni.Hg. When these diflferences are greater 

 than about lOmm.Hg, it is found impossible to keep them abso- 

 lutely steady. These variations, which cannot be avoided, lie 

 within a range of about 2 mm.Hg from the average pressure. 



