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: These findings show a significant interaction between morphine and metamizol in chronically treated rats , suggesting that this combination could be useful for the treatment of chronic pain .
### Output Text:
These findings show a significant interaction between <span class="chemical">morphine</span> and <span class="chemical">metamizol</span> in chronically treated rats , suggesting that this combination could be useful for the treatment of chronic pain . 