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: Median convulsant ( CD50 ) and median lethal ( LD50 ) doses of three representative local anesthetics were determined in adult mice to evaluate the threat to life of local anesthetic - induced convulsions .
### Output Text:
Median convulsant ( CD50 ) and median lethal ( LD50 ) doses of three representative local anesthetics were determined in adult mice to evaluate the threat to life of local anesthetic - induced convulsions . 