In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: PURPOSE : To map and identify the pattern , in vivo , of axonal degeneration in ethambutol - induced optic neuropathy using optical coherence tomography ( OCT ) .
### Output Text:
PURPOSE : To map and identify the pattern , in vivo , of axonal degeneration in <span class="chemical">ethambutol</span> - induced optic neuropathy using optical coherence tomography ( OCT ) . 