THE RESPIRATORY SYSTEM. 265 



Methods of Recording Respiratory Movements. In order to study 

 these changes, it is desirable to obtain a graphic record of the rate 

 and depth of the respiratory movements ; and numerous methods 

 have been devised for this purpose. 



(1) In man the respiratory movements can be recorded by a stetho- 

 graph, one form of which consists of a small metal cylinder, provided 

 with a lateral opening and closed at each end by a rubber membrane ; 

 the lateral opening is connected by rubber tubing with a tambour. 

 Strings are attached to the centre of each rubber membrane, and are 

 passed round the chest and tied. Each expansion of the chest causes 

 the strings to pull upon the rubber membrane, so that the capacity of 

 the cylinder increases and the lever of the tambour falls; during 

 expiration the membranes return to their former position. The same 

 apparatus can be used to record the respiratory movements in the 

 lower animals. 



(2) Another method, used in animals, is to connect the side piece 

 of a cannula, inserted into the trachea, with a tambour ; with each 

 inspiration air is sucked out of the tambour, and the lever falls. 



(3) In rabbits, a small slip of the diaphragm on each side is inserted 

 into the xiphisternum ; and by separating the xiphisternum from the 

 sternum, this strip of muscle can be isolated without interfering with 

 its vascular or nervous connections. It contracts synchronously with 

 the rest of the diaphragm, and by connecting the xiphisternum with 

 the membrane of a tambour by means of a thread, each contraction of 

 the slip can be recorded and serves as an index of the movements of 

 the diaphragm as a whole (Head's method). 



THE CHEMICAL REGULATION OF RESPIRATION. 



The two most important changes in the composition of the blood 

 which alter the respiratory movements are: (1) variations in the 

 tension of carbonic acid, and (2) a fall in the tension of oxygen. 



(1) The Tension of Carbonic Acid. If an animal is allowed to 

 breathe air containing 2 to 3 per cent, carbonic acid, the respiratory 

 movements become much deeper, and after a short time usually more 

 frequent; such an experiment is illustrated in fig. 103. If the experi- 

 ment is made on man, it is further found that the percentage of 

 carbonic acid in the alveolar air remains constant. The immediate 

 effect of breathing air containing an excess of carbonic acid is to 

 increase the percentage in the alveolar air, thereby diminishing the 

 passage of carbonic acid by diffusion from the blood into the air in 

 the lungs. As a result, the tension of this gas in the blood rises ; this 

 increase in tension stimulates the respiratory centre to increased 



