RESPIRATION 303 



across it between the inspired air and that in the alveoli. It is impossible 

 to fix any anatomic point at which the interface occurs. 



The above described mechanism for the ventilation of the alveoli in- 

 sures the maintenance of slight but constant changes in the composition 

 of the air next the alveolar epithelium. It helps to prevent sudden varia- 

 tions in the amount of gases in the blood, particularly of C0 2 . Should 

 such variations occur, irregular stimulation of the respiratory and other 

 important centers that are influenced by the amount of this gas present 

 in simple solution in the blood, would be the result. The mechanism 

 serves as a sort of mechanical buffer ~by diminishing the sudden changes 

 in gas concentration produced l)y inspiration and expiration. 



Respiratory Tracings 



The measurements of air for the determination of the foregoing val- 

 ues are made by the use of meters of various types. Sometimes, how- 

 ever, it is necessary to obtain an inscribed record of the respirations. 



Fig. 108. Pneumograph. The straps (b, b) are held around the thorax, and the tube of the 

 tambour connected by rubber tubing with a recording tambour. 



This may be either qualitative or quantitative. A. qualitative record is 

 taken by attaching some sort of receiving tambour to the thoracic wall 

 (the best type is shown in Fig. 108), and connecting this with. a record- 

 ing tambour arranged to w r rite on a blackened surface. When it is 

 desired merely to count the respirations or to observe their regularity, 

 such a tracing is all that is required, but obviously it does not tell us 

 how much air has entered and left the lungs at each respiration. To 

 obtain a quantitative tracing, we must either connect a recording instru- 

 ment with the trachea or inclose the body of the animal in what is 

 known as a body plethysmograph. In observations on laboratory an- 

 imals the best type of recording instrument to connect with the respira- 

 tory passages is the Gad or Krogh pneumograph. A body plethysmograph 

 as used in the case of man is shown in Fig. 109. All these instruments 

 must of course be calibrated, which is done by pouring a definite num- 



