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: Enhancement of morphine antinociception by dextromethorphan was seen in both males and females in the acute pain models , with the magnitude of this effect being greater in males .
### Output Text:
Enhancement of <span class="chemical">morphine</span> antinociception by <span class="chemical">dextromethorphan</span> was seen in both males and females in the acute pain models , with the magnitude of this effect being greater in males . 