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: Ten rats had arterial , central venous ( CVP ) , and subdural cannulae inserted under halothane anesthesia .
### Output Text:
Ten rats had arterial , central venous ( CVP ) , and subdural cannulae inserted under <span class="chemical">halothane</span> anesthesia . 