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 narrow gap between convulsant and lethal doses of local anesthetics indicates that untreated convulsions present much more of a threat to life than heretofore appreciated .
### Output Text:
The narrow gap between convulsant and lethal doses of local anesthetics indicates that untreated convulsions present much more of a threat to life than heretofore appreciated . 