MECHANICS OF RESPIRATORY MOVEMENTS 1045 



products, or the liver, a dull sound is obtained. It is easy to show that the 

 resonant area of the chest increases with each inspiration. The apices of 

 the lungs extend about one inch above the clavicle anteriorly and behind 

 reach as high as the seventh spinous process. During moderate expiration 

 the lower margin of the lungs extends in front from the upper border of the 

 sixth rib at its insertion to the sternum, and runs obliquely downwards to 

 the level of the tenth rib at the back of the chest. During the deepest 

 inspiration the lungs descend in front to the seventh intercostal space 

 and behind to the eleventh rib, while during deepest possible expiration the 

 lower margins of the lungs are elevated almost as much as they descend 

 during inspiration. In the front of the chest a triangular space can be 

 always marked out over the heart where the note obtained on per- 

 cussion is dull. This space is bounded on the right by the left border of 

 the sternum, and extends out as far as the cardiac apex, being bounded 

 above by the fourth costo-sternal articulation and below by the sixth costal 

 cartilage. 



BREATH SOUNDS. If the ear be applied to the chest wall, either 

 directly or through the medium of a stethoscope, each inspiration is found 

 to be accompanied by a fine rustling sound, the ' vesicular murmur.' It is 

 thought to be caused by the sudden dilatation of the air- vesicles during 

 inspiration or perhaps by the current of air passing from the narrow terminal 

 bronchioles into the wider infundibula. It is important to remember that 

 this sound is heard only during inspiration and over healthy lungs. On 

 listening over the larger air-passages, i.e. the larynx, trachea, and bronchi, 

 we hear a much louder sound which accompanies both expiration and 

 inspiration and may be compared to a sharp whispered hah. This is known 

 as the ' bronchial murmur.' It can be heard also at the back of the chest 

 between the scapulae at the level of the fourth dorsal vertebra, where the 

 trachea bifurcates. In all other parts of the chest the healthy lung prevents 

 the propagation of this sound to the chest wall. If, however, the lung is solid, 

 as occurs in pneumonia, it conducts the sound easily from the large air-tubes 

 to the chest wall. Bronchial breathing at any part of the chest other than 

 that immediately over the air-tubes is therefore a distinctive sign of con- 

 solidation of the lung. Absence of breath sounds at any part of the chest 

 implies either that air is not entering that part of the lung, or that the lung 

 is separated from the chest wall by effused fluid. 



INTRATHORACIC PRESSURE. Even at the end of expiration the lungs 

 are in a stretched condition. This is shown by the fact that if in an animal 

 or in the corpse an opening be made into the pleural cavity, air rushes into the 

 opening and the lungs collapse, driving a certain amount of air out through 

 the trachea. Since the lungs are always tending to collapse, it is evident that 

 they must exert a pull on the thoracic wall. This pull of the lungs gives rise 

 to a negative pressure in the pleural cavity. If we aonnect a mercurial 

 manometer with the pleural cavity, we find that the pull of the lungs amounts 

 in the corpse to 6 mm. of mercury. If the lungs are fully distended, as after 

 full inspiration, the elastic forces are more brought into play, and the negative 



