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: The incidence of postoperative respiratory apnoea was compared between five patients receiving a continuous i . v . infusion of morphine ( mean 73 . 6 mg ) and five patients receiving a continuous extradural infusion of 0 . 25 % bupivacaine ( mean 192 mg ) in the 24 - h period following upper abdominal surgery .
### Output Text:
The incidence of postoperative respiratory apnoea was compared between five patients receiving a continuous i . v . infusion of <span class="chemical">morphine</span> ( mean 73 . 6 mg ) and five patients receiving a continuous extradural infusion of 0 . 25 % <span class="chemical">bupivacaine</span> ( mean 192 mg ) in the 24 - h period following upper abdominal surgery . 