218 A MANUAL OF PHYSIOLOGY . 



tions in a single circumference, of the boundary of the thoracic 

 cavity. In man changes in the circumference of the thorax at any 

 level can be recorded by means of a tambour adjusted to the chest 

 (Figs. 99 and 128), and in communication with another, which is 

 provided with a writing lever (Figs! 86 and 131). Or an elastic tube, 

 with a spiral spring in its lumen, may be fastened around the thorax 

 or abdomen and connected with a piston-recorder (a small cylinder 

 in which works a piston carrying a writing-point) (Fitz). 



(2) By recording the changes of pressure produced in the air- 

 passages by the respiratory movements. This can be done by con- 

 necting a cannula in the trachea of an animal with a recording 

 tambour in the manner described in the Practical Exercises, p. 288, 

 The variations of pressure may be measured by connecting a mano- 

 meter with the trachea, or in man with the nostril. 



(3) By writing off the changes of pressure which occur in the 

 thoracic cavity during respiration. For this purpose a trocar is 

 introduced through an intercostal space into one of the pleural sacs, 

 without the admission of air, or into the pericardium, and then con- 

 nected with a manometer or other recording apparatus. Or a tube, 

 similar in construction to a cardiac sound (p. 89), may be pushed 



FIG. 100. RESPIRATORY TRACING FROM MAN (MAREY). 

 Down stroke, inspiration ; up stroke, expiration. 



down the oesophagus. The variations in the intrathoracic pressure 

 are transmitted to the air in the elastic bag, and thence to a tambour. 

 (4) In the rabbit the part of the diaphragm attached to the ensif orm 

 cartilage may be isolated from the rest and its contractions recorded 

 by a lever (Head). For some purposes this is the best method. 



When the respiratory movements are studied in any of these 

 ways, it is found that there is practically no pause between 

 the end of inspiration and the beginning of expiration. Nor, 

 although the chest collapses more gradually than it expands, 

 is there any distinct interval in ordinary breathing between the 

 end of expiration and the beginning of the succeeding inspira- 

 tion. When, however, the respiration is unusually slow, an actual 

 pause (expiratory pause) may occur at this point. Expiration 

 takes somewhat longer time than inspiration, the ratio varying 

 from 7 : 6 to 3 : 2, according to age, sex, and other circumstances. 



The frequency of respiration is by no means constant even 

 in health. All kinds of influences affect it. It is difficult even 

 to direct the attention to the respiratory act without bringing 



