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: These results , in combination with previous work , suggest that muscle rigidity , a clinically relevant side - effect of parenteral narcotic administration , may be partly mediated via serotonergic pathways .
### Output Text:
These results , in combination with previous work , suggest that muscle rigidity , a clinically relevant side - effect of parenteral narcotic administration , may be partly mediated via serotonergic pathways . 