312 RESPIRATION 



By the former method the intrathoracic pressure in man has been found 

 to be : for the normal expiratory position, 5 to 6 mm. Hg. ; for ordinary 

 inspiration,, in the neighborhood of 8 to 9 mm. Hg. ; for deepest inspira- 

 tion, 30 mm. Hg. Since the intrathoracic pressure rises immediately after 

 death, these* figures may be somewhat too low (van der Brugh). 



With the glottis open the pressure in the pleural space is never positive. 

 But when the glottis is closed and expiratory efforts are made so that the air in 

 the lungs is compressed, the intrathoracic pressure may become positive. But 

 in this case also the pressure within the lungs is greater than in the pleural 

 spaces, for even now a part of the air pressure is consumed in unfolding the 

 lungs. 



The effect of suction in the thorax on the circulation has already been 

 mentioned (pages 176 and 227). It plays an important part also in respira- 

 tion. For the work to be done by the inspiratory muscles is considerably 

 increased by this negative pressure, whereas expiration is favored by it. Thus 

 since the air pressure acting upon the inner wall of the thorax is lower than 

 the atmospheric pressure exerted upon the outer wall, every dilatation of the 

 thorax is counteracted by a force corresponding to the difference between the 

 outer and the inner pressure. If the pressure necessary to expand the lungs 

 be taken as 8 mm. Hg., with an atmospheric pressure of 760 mm. the internal 

 pressure would be only 752 mm. Hg. That is, the inspiratory effort at every 

 movable point of the thoracic wall would be opposed by a pressure of 8 mm., 

 and this resistance increases more and more as the expansion increases. It 

 is obvious without further discussion that the expiratory contraction of the 

 thoracic wall is favored by the same circumstances. 



2. INSPIRATION 



The expansion of the thorax is accomplished in two ways: by elevation 

 of the ribs and by contraction of the diaphragm. 



A. REGISTRATION OF RESPIRATORY MOVEMENTS 



Some of the methods in use are for the purpose of recording movements of 

 the thoracic wall or of the diaphragm. The former can be registered either for 

 man or animal by fastening a receiving tambour to the chest wall by means of 

 a girth of suitable form, and transmitting the pressure variations accompanying 

 the respiratory movements to a recording tambour. Fig. 121 represents a pneu- 

 mographic curve obtained in this way. 



Through a small hole in the upper part of the anterior wall of the abdomen 

 a spoon-shaped instrument may be introduced between the diaphragm and the 

 liver, and the movements of the diaphragm recorded by the movements which 

 the instrument makes (phrenograph of Rosenthal). 



By still other methods the volumes of inspired and expired air may be 

 recorded. To this end tracheotomy is performed on the animal, and the 

 trachea is connected with a receiver of suitable size (Fig. 122, B), which in its 

 turn communicates with a recording tambour of Marey, or, better still, with a 

 small spirometer (Fig. 122, A), or a similarly devised box known as an aero- 



