THE MECHANISM OP BREATHING. 213 



thoracic walls during inspiration and expiration. A low-pitched 

 resonant sound is elicited over the lungs containing air, whereas 

 a high-pitched non-resonant or tympanitic hollow sound is heard 

 over the solid viscera and abdominal organs. In diseases where 

 changes take place in the substance of the lungs, as in tubercu- 

 losis, pneumonia, etc., alterations occur in the tone elicited on 

 percussion. These alterations are of great diagnostic import- 

 ance. In pleurisy, a condition in which the pleural surfaces are 

 roughened, a friction rub or vibration, produced by the rubbing 

 of the roughened surfaces of the pleura of the lungs on that of 

 the thorax, can be detected by placing the hand over the affected 

 area. The pain following a broken rib is caused by the irritation 

 of the pleural membrane by the broken edge of the rib. It is al- 

 leviated by making the ribs immovable by tightly strapping the 

 thorax with adhesive plaster over the region of the pain. 



RESPIRATORY SOUNDS. Accompanying inspiration a rustling 

 sound, described as a vesicular sound may be heard over most of 

 the lung area. It is produced by the dilatation of the alveoli and 

 fine bronchi. Over the larger air passages a high, sharper tone 

 is heard, called the bronchial breath sound. In diseases in which 

 the alveoli are destroyed and the lung fills up with fluid, etc., 

 the bronchial breath sounds replace the vesicular sounds. 



Effect of Respiration on the Movement of the Blood and on 

 Blood Pressure. "Within the thorax the changes in pressure 

 accompanying each respiration affect the heart and so influence 

 somewhat the movement of the blood. In thin individuals it is 

 easy to confirm this by observing the effect of breathing on the 

 blood flow through the jugular vein. At each inspiration the 

 jugular vein is seen to empty, and during expiration to fill. If 

 simultaneous records are taken of the blood pressure and re- 

 spiratory movements in ordinary breathing, it will generally be 

 observed that during inspiration there is a rise of blood pressure 

 and during expiration a fall. This phenomenon is explained as 

 follows: During inspiration the heart is better supplied with 

 blood and can fill more quickly and perfectly than during ex- 

 piration, because the decrease in the pressure in the thorax at 

 this period serves to accelerate the movement of venous blood 



