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: Comparison of the respiratory effects of i . v . infusions of morphine and regional analgesia by extradural block .
### Output Text:
Comparison of the respiratory effects of i . v . infusions of <span class="chemical">morphine</span> and regional analgesia by extradural block . 