206 APPLIED PHYSIOLOGY 



It is an interesting fact that whilst voluntary or 

 forcible breathing soon produces a feeling of fatigue, 

 he equally powerful breathing of hard exercise does 

 not. The physiological explanation of this apparent 

 anomaly may, perhaps, be that voluntary breathing 

 involves the use of the brain cortex, whereas the in- 

 voluntary breathing of exercise does not, and it is well 

 known that the sense of fatigue, though referred to the 

 muscles, is really experienced in the brain. 



The efferent impulses from the respiratory centre 

 travel along the vagi, the phrenics, and the intercostal 

 nerves. Those which proceed by the vagi innervate the 

 posterior crico-arytenoid muscles, and so cause the 

 glottis to open more widely during respiration. Inter- 

 ference with these fibres is the cause of the ' abductor 

 paralysis ' so common in cases in which there is pressure 

 upon the recurrent laryngeal nerve. The separate route 

 pursued by the fibres for the diaphragm and the inter- 

 costals respectively enables one or other of those portions 

 of the breathing apparatus to be paralyzed without the 

 other. Thus a patient who has sustained a crush of 

 the cord below the level of the fifth cervical roots can 

 breathe for a time by his diaphragm alone. 



If, again, the phrenic nerve be in any way injured 

 or degenerated, the patient can breathe in a sort of way, 

 at least with the intercostals, although the imperfect 

 expansion of the lower parts of his lungs is very apt to 

 lead to troublesome complications. 



Division of one vagus in man does not appear to lead 

 to bad results. On the other hand, a case is on record 

 in which the left vagus was included by mistake in a 



