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: CONCLUSIONS : These results show selective inhibition by intrathecal adenosine of hypersensitivity , presumed to reflect central sensitization in humans after peripheral capsaicin injection .
### Output Text:
CONCLUSIONS : These results show selective inhibition by intrathecal <span class="chemical">adenosine</span> of hypersensitivity , presumed to reflect central sensitization in humans after peripheral <span class="chemical">capsaicin</span> injection . 