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: Animals were administered nicotine , carbachol , or neostigmine via timed tail vein infusion , and the latencies to onset of tremor and clonus were recorded and converted to threshold dose .
### Output Text:
Animals were administered <span class="chemical">nicotine</span> , <span class="chemical">carbachol</span> , or <span class="chemical">neostigmine</span> via timed tail vein infusion , and the latencies to onset of tremor and clonus were recorded and converted to threshold dose . 