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 clinically apparent occlusion may be preceded by gastrointestinal and musculoskeletal symptoms that appear to be ischemic in origin , and might serve to warn the clinician of these complications .
### Output Text:
The clinically apparent occlusion may be preceded by gastrointestinal and musculoskeletal symptoms that appear to be ischemic in origin , and might serve to warn the clinician of these complications . 