176 PEACTICAL PHYSIOLOGY 



demonstrate that the limit of resonance is increased owing to the 

 expansion of the lungs. 



Auscultation. The respiratory and cardiac sounds can be heard 

 by placing the ear against the chest, or by means of a wooden or 

 binaural stethoscope. Over the trachea, or at the level of the 7th 

 cervical spine, the harsh blowing sounds, due to inspiration and 

 expiration, are heard; these "bronchial sounds" are produced by the 

 vibration of the air at the orifices of the vocal cords and divisions 

 of the trachea and bronchi. 



Another sound, the "vesicular murmur," is heard on listening to 

 parts of the chest wall where the lung is in contact. It is a soft 

 breezy sound which increases during inspiration and dies away 

 during the first third of expiration. There are several views about the 

 causation of this sound ; it may be due to conduction of the bronchial 

 sounds. 



CHAPTER XLIV. 

 INTRA-THORACIC PRESSURE. 



Intra-thoracic Pressure. The thoracic cavity, when opened, is far 

 larger than its contents, for the lungs, owing to their elasticity, collapse 

 as soon as the intra-pulmonary and pleural pressures become equal. 

 The intra-pleural pressure is less than the atmospheric pressure by that 

 amount of the atmospheric pressure which is required to overcome the 

 elasticity of the lungs and distend these organs to the size of the 

 thoracic cavity. The intra-thoracic pressure or elastic traction exerted 

 by the lungs on the thoracic wall varies as follows : 



Normal inspiration about - 10 mm. Hg. 

 expiration - 7 



Deep inspiration - 40 



expiration - ,, 



,, inspiration with air-way closed ,, 100 



expiration +100 



The intra-tracheal pressure varies from - 1 irm. Hg. in quiet inspira- 

 tion to + 1 mm. Hg. in expiration. During forced breathing with the 

 air-way closed the intra-tracheal pressure is greater than the intra- 

 thoracic pressure by the airount of the elastic traction exerted by the 

 lungs. All the structures, e.g. heart and blood-vessel?, are affected by 

 the respiratory variations of pressure. 



