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: TREATMENT : Despite stopping ethambutol on diagnosis , visual function continued to deteriorate for a few months .
### Output Text:
TREATMENT : Despite stopping <span class="chemical">ethambutol</span> on diagnosis , visual function continued to deteriorate for a few months . 