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 extent of ' flattened ' affect with pergolide was related to the degree of learning inhibition .
### Output Text:
The extent of ' flattened ' affect with <span class="chemical">pergolide</span> was related to the degree of learning inhibition . 