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: The author postulates that in cases of ethambutol associated chiasmopathy , ethambutol may initially affect the optic nerves and subsequently progress to involve the optic chiasm .
### Output Text:
The author postulates that in cases of <span class="chemical">ethambutol</span> associated chiasmopathy , <span class="chemical">ethambutol</span> may initially affect the optic nerves and subsequently progress to involve the optic chiasm . 