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: Additionally , the protective effect of verapamil on this ulcer model was evaluated .
### Output Text:
Additionally , the protective effect of <span class="chemical">verapamil</span> on this ulcer model was evaluated . 