428 PHYSIOLOGY CHAP. 



rises again in the second. To interpret these facts it is necessary 

 to assume that the diaphragm intervenes actively only in the first 

 period of inspiration, and that the abdominal muscles intervene 

 actively only in the second period of expiration. This agrees with 

 what was stated above as to the relative inspiratory importance 

 of the external intercostal muscles, and the constantly active 

 character of the expiratory movements. 



The respiratory oscillations of pressure in the two great 

 body cavities are the more ample, or extensive, in proportion as 

 the entrance and exit of air from the pulmonary passages, which 

 tends to compensate them and to re-establish equilibrium, is more 

 difficult. This fact can be experimentally verified by recording 

 the tracings of intra-abdominal pressure in a tracheotomised animal, 

 and observing how the respiratory curves are modified when the 

 lumen of the tube attached to the tracheal cannula is constricted. 

 Fig. 191 shows that the effect consists more particularly in a con- 

 spicuous exaggeration of the inspiratory acts, which become deeper 



FIG. 191. Respiratory oscillations of intrathoracic pressure (T) and intra-abdominal pressure (A) 

 in anaesthetised and tracheotomised dog. At B the tracheal tube was constricted. (Luciani.) 



and longer. Expiratory activity is also exaggerated, but in a less 

 degree, and with reference solely to intensity and not to duration. 

 The interference of the two curves, intrathoracic and intra- 

 abdoniinal, persists. 



The determination in the different higher animals, and in man, 

 of the absolute values of the respiratory oscillations of intra- 

 thoracic and intra-abdominal pressure has not been fully worked 

 out. We have only the few data obtained on the rabbit from 

 Adauikiewicz and Jacobson, and those of Eosenthal, which show 

 that in this animal, in normal inspiration, the pressure falls 

 to - 40 mm. water ( = - 3 mm. Hg), and that in the most intense 

 inspirations, with closed trachea, the negative pressure may amount 

 to - 250 mm. water ( = - 20 mm. Hg). 



More recently certain observers (Ewald, Einthoven, Aron, van 

 der Brugh) have succeeded by means of a special apparatus (with- 

 out causing pneumothorax) in introducing into the pleural cavity 

 a cannula attached to a manometer, and thus directly measuring 

 the pressure of the pleural cavity. Einthoven and his pupil, 

 van der Brugh, found during expiration a negative pleural 



