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 timing of papaverine application and ongoing operative events was reviewed relative to changes in neurophysiological recordings .
### Output Text:
The timing of <span class="chemical">papaverine</span> application and ongoing operative events was reviewed relative to changes in neurophysiological recordings . 