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 decision whether to continue or discontinue the procedure depends on the urgency of the surgery and severity of masseter muscle rigidity .
### Output Text:
The decision whether to continue or discontinue the procedure depends on the urgency of the surgery and severity of masseter muscle rigidity . 