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: Both obstructive ( P less than 0 . 05 ) and central apnoea ( P less than 0 . 05 ) occurred more frequently in patients who had a morphine infusion .
### Output Text:
Both obstructive ( P less than 0 . 05 ) and central apnoea ( P less than 0 . 05 ) occurred more frequently in patients who had a <span class="chemical">morphine</span> infusion . 