The Collaborative International Dictionary of English v.0.48:

Inogen \In"o*gen\, n. [Gr. 'i`s, 'ino`s, a muscle + -gen.]
   (Physiol.)
   A complex nitrogenous substance, which, by Hermann's
   hypothesis, is continually decomposed and reproduced in the
   muscles, during their life.
   [1913 Webster]