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: Central nervous system toxicity following the administration of levobupivacaine for lumbar plexus block : A report of two cases .
### Output Text:
Central nervous system toxicity following the administration of <span class="chemical">levobupivacaine</span> for lumbar plexus block : A report of two cases . 