RESPIRATION. 387 



The amount of this intra-thoracic pressure can be approximately deter- 

 mined in several ways. Thus, if shortly after death a mercurial manometer 

 be inserted air-tight into the trachea of a human being and the thorax opened, 

 the lungs will recoil and compress their contained air. The mercurial 

 manometer will at once show an excess of pressure in the trachea of about 

 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 

 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. 189.) 



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 

 contain fluid. The lungs therefore are devoid of air (atelectatic) , and, 

 having a specific gravity greater than water, readily sink when placed in this 

 fluid. The capacity of the thorax does not exceed the volume of the lungs. 

 With the first inspiration, however, the thoracic walls take a new position. 

 The air at once rushes into the lungs and distends them. But as the 

 capacity of the thorax even at the end of the expiration is now greater than 

 the volume which the lungs would assume unless distended, there at once 

 arises the elastic recoil in the opposite direction, the condition which gives rise 

 to the negativity of the pressure in the thoracic cavity. It is also probable 

 that as the child develops, the thorax grows more rapidly than the lungs, 

 giving rise to a condition which would increase and accentuate the elastic 

 tension and thus increase the negativity of the intra-thoracic pressure. 



THE DYNAMIC CONDITION. 



In the dynamic condition, the thorax and its contained organs undergo a 

 series of movements in consequence of which the relations of the static con- 



