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: This work evaluates the antinociceptive and constipating effects of the combination of 3 . 2 mg / kg s . c . morphine with 177 . 8 mg / kg s . c . metamizol in acutely and chronically treated ( once a day for 12 days ) rats .
### Output Text:
This work evaluates the antinociceptive and constipating effects of the combination of 3 . 2 mg / kg s . c . <span class="chemical">morphine</span> with 177 . 8 mg / kg s . c . <span class="chemical">metamizol</span> in acutely and chronically treated ( once a day for 12 days ) rats . 