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: METHOD : Thirteen patients who developed optic neuropathy after being treated with ethambutol for tuberculosis of the lung or lymph node at Siriraj Hospital between 1997 and 2001 were retrospectively reviewed .
### Output Text:
METHOD : Thirteen patients who developed optic neuropathy after being treated with <span class="chemical">ethambutol</span> for tuberculosis of the lung or lymph node at Siriraj Hospital between 1997 and 2001 were retrospectively reviewed . 