NERVOUS MECHANISM OF RESPIRATION. 339 



RESPIRATORY SOUNDS. 



As the streams of air enter the air-passages and lungs they 

 produce sounds which are of the greatest importance to the phy- 

 sician, owing to the manner in which they are altered by disease. 



A sound called "bronchial breathing" is produced in the large 

 bronchi and trachea, and is like the noise of air blowing through 

 a tube. This can normally be heard over the trachea, or at the 

 back between the shoulder-blades over the entrance of the large 

 bronchi into the root of the lung. 



Another sound called " vesicular " can be heard all over the 

 chest, being most distinct where the lung is most superficial, and 

 where other sounds are absent, as in the sub-axillary region. It 

 is a gentle rustling sound caused by the air passing into the 

 iufundibuli. It varies much with the force of respiration and 

 many other circumstances. In children up to ten or twelve years 

 of age it is remarkably sharp and loud, and is called "puerile 

 breathing." 



NERVOUS MECHANISM OF RESPIRATION. 



The movements of respiration go on rhythmically without any 

 voluntary effort, and even when we are quite awake they occur 

 almost without our being conscious of them, and repeated varia- 

 tions take place in the rate, depth, and general type of our respi- 

 rations without our knowledge. Indeed, if this self-regulating 

 arrangement did not exist we should have to devote much of our 

 attention to adapting our respiratory movements to the ever- 

 changing requirements of the gas-interchange of the blood. 



Like all other groups of skeletal muscles, those which act on 

 the thorax are regulated by nerves and work together in harmony. 

 These coordinated movements are so far under the control of the 

 will that any of the groups of muscles may be employed sepa- 

 rately, or in conjunction. 



But the respiratory differ from the other skeletal muscles, in 

 that they undergo rhythmical coordinated contractions which are 

 not directed by our will, and can be influenced by it only to a 

 certain extent, for they cannot be made to cease altogether. 



