39 6 TEXT-BOOK 'OF PHYSIOLOGY 



6 mm. This was taken by Bonders as a measure of the force with which 

 the lungs endeavor to recoil. The intra-thoracic pressure would be, there- 

 fore, atmospheric pressure, 760 mm., less 6 mm., or 754 mm. Hg. Another 

 method is to insert a rubber catheter through a small opening in an intercostal 

 space into the thoracic cavity. The air which enters through the open ex- 

 tremities of the catheter and leads to a collapse of the lungs may be subse- 

 quently aspirated, when the lung returns to its normal position. The 

 catheter is then placed in connection with a water manometer. On 

 establishing a communication between them, by the turning of a stopcock, 

 the water will rise in the proximal and fall in the distal limb of the manometer, 

 indicating a pressure in the thorax negative to that in the lung. The differ- 

 ence in the level of the water in the two limbs of the manometer, expressed in 

 millimeters of mercury, would also represent the force with which the elastic 



A S V..... 



FIG. 1 88. SECTION OF THORAX WITH THE LUNGS, HEART, AND PRINCIPAL VESSELS. 5. 

 Catheter introduced into the pleural space and connected with a manometer. (After Moral and 

 Doyen.) 



tissue strives to recoil, and the extent to which it opposes the atmospheric 

 pressure. This subtracted from the atmospheric pressure would give the 

 intra-thoracic pressure. In the living dog this latter is less than the former, 

 to the extent of from 3.5 to 5.5 mm. For the same reason the superior surface 

 of the diaphragm also experiences a pressure less than that of the atmosphere. 

 Owing to the soft and yielding character of the abdominal walls the atmospheric 

 pressure is transmitted through the abdominal organs to the inferior surface 

 of the diaphragm. The pressure being greater from below than above, the 

 diaphragm is forced upward until it assumes the dome-like appearance it 

 usually presents. (These relations are shown in Fig. 188.) 



The cause of the negativity of the intra-thoracic pressure is connected 

 with the change in the relation of the lungs to the thorax attending the first 

 inspiration. Previous to birth the walls of the alveoli and bronchioles are 

 collapsed and in apposition. The larger bronchial tubes in all probability 



