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: Hyperalgesia and myoclonus in terminal cancer patients treated with continuous intravenous morphine .
### Output Text:
Hyperalgesia and myoclonus in terminal cancer patients treated with continuous intravenous <span class="chemical">morphine</span> . 