542 



RESPIRATORY CURVES. 



[BOOK ii. 



vessels, and arranged so that while one end of the slip is securely fixed 

 to the chest wall as a fixed point, the other end can by a thread be 

 brought to bear on a lever. The slip, even when thus arranged, 

 ^appears to contract rhythmically in complete unison with the con- 

 tractions of the whole rest of the diaphragm ; it serves so to speak as a 

 sample of the diaphragm ; and hence its contractions like those of the 

 whole diaphragm may be taken as a record of respiratory movements. 

 The record has to be corrected for variations in the position of the fixed 

 point. 



329. In these various ways curves are obtained, which, 

 while differing in detail, exhibit the same general features, and 

 more or less resemble the curve shewn in Fig. 72. 



Fm. 72. TRACING OF THORACIC RESPIRATORY MOVEMENTS OBTAINED BY 

 MEANS OF MAREY'S PNEUMOGRAPH. 



A whole respiratory phase is comprised between a and a; inspiration, during which 

 the lever descends, extending from a to b, and expiration from b to a. The 

 undulations at c are caused by the heart's beat. 



As the figure shews, inspiration begins somewhat suddenly and 

 advances rapidly, being followed immediately by expiration, which 

 is carried out at first rapidly, but afterwards more and more 

 slowly. Such pauses as are seen usually occur between the end of 

 expiration and the beginning of inspiration. In normal breathing, 

 hardly any such pause exists, but in cases where the respiration 

 becomes infrequent, pauses of considerable length may be observed. 

 As we shall see in detail hereafter, the several parts of the whole 

 act vary much, under various circumstances, in relation to each 

 other. Sometimes expiration, sometimes inspiration is prolonged ; 

 and either inspiration or expiration may be slow or rapid in its 

 development. At times the chest may remain for a while at the 

 height of inspiration, thus making a pause between inspiration and 

 expiration. 



In what may be considered as normal breathing, the respiratory 

 act is repeated about 17 times a minute, the duration of the 

 inspiration as compared with that of the expiration (and such 

 pause as may exist) being about as ten to twelve ; but the rate 

 varies very largely ; and in this as in the volume of each breath it 



