MEDULLA O3LONGATA. 441 



reflected along the motor nerves to the respiratory muscles. These 

 muscles are then called into action, producing an expansion of 

 the chest. The impression so conveyed to the medulla is usually 

 unperceived by the consciousness. It is generally converted directly 

 into a motor impulse, without attracting our attention or giving 

 rise to any conscious sensation. Respiration, accordingly, goes on 

 perfectly well without our interference and without our knowledge. 

 The nervous impression, however, conveyed to the medulla, though 

 usually imperceptible, may be made evident at any time by volun- 

 tarily suspending the respiration. As the carbonic acid begins to 

 accumulate in the blood and in the lungs, a peculiar sensation makes 

 itself felt, which grows stronger and stronger with every moment, 

 and impels us to recommence the movements of inspiration. This 

 peculiar sensation, entirely different in character from any other, is 

 designated by the French under the name of " besoin de respirer." 

 It becomes more urgent and distressing, the longer respiration is 

 suspended, until finally the impulse to expand the chest can no 

 longer be resisted by any effort of the will. 



During ordinary respiration, therefore, each inspiratory move- 

 ment is excited by the partial vitiation of the air contained in the 

 lungs. As soon as a new supply has been inhaled, the impulse to 

 respire is satisfied, the muscles relax, and the chest collapses. In 

 a few seconds the previous condition recurs and the same move- 

 ments are repeated, producing in this way a regular alternation of 

 inspirations and expirations. 



Since the movements of respiration are performed partly by the 

 diaphragm and partly by the intercostal muscles, they will be 

 differently modified by injuries of the nervous system, according to 

 the spot at which the injury is inflicted. If the spinal cord, for 

 example, be divided or compressed in the lower part of the neck, 

 all the intercostal muscles will be necessarily paralyzed, and respi- 

 ration will then be performed entirely by the diaphragm. The 

 chest in these cases remaining motionless, and the abdomen alone 

 rising and falling with the movements of the diaphragm, such 

 respiration is called " abdominal" or " diaphragmatic" respiration. 

 It is a common symptom of fracture of the spine in the lower 

 cervical region. If the phrenic nerve, on the other hand, be 

 divided, the diaphragm will be paralyzed, and respiration will then 

 be performed altogether by the rising and falling of the ribs. It 

 is then called "thoracic" or "costal" respiration. If the injury 

 inflicted upon the spinal cord be above the origin of the second 



