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 protective role of betaine on myocardial damage was further confirmed by histopathological examination .
### Output Text:
The protective role of <span class="chemical">betaine</span> on myocardial damage was further confirmed by histopathological examination . 