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: Adverse effects of topical papaverine on auditory nerve function .
### Output Text:
Adverse effects of topical <span class="chemical">papaverine</span> on auditory nerve function . 