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: To this end , persistent hyperalgesia was induced by administration of capsaicin in the tail of gonadally intact F344 rats , following which the tail was immersed in a mildly noxious thermal stimulus , and tail - withdrawal latencies measured .
### Output Text:
To this end , persistent hyperalgesia was induced by administration of <span class="chemical">capsaicin</span> in the tail of gonadally intact F344 rats , following which the tail was immersed in a mildly noxious thermal stimulus , and tail - withdrawal latencies measured . 