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: IMPLICATIONS : This case documents a patient who developed dizziness with downbeating nystagmus while receiving a relatively large dose of IV patient - controlled analgesia morphine .
### Output Text:
IMPLICATIONS : This case documents a patient who developed dizziness with downbeating nystagmus while receiving a relatively large dose of IV patient - controlled analgesia <span class="chemical">morphine</span> . 