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: Comparing the relative sensitivity to central depression and excitation revealed that rats were least likely to have convulsions at doses that did not first cause loss of consciousness , while cats most clearly showed marked central excitatory actions .
### Output Text:
Comparing the relative sensitivity to central depression and excitation revealed that rats were least likely to have convulsions at doses that did not first cause loss of consciousness , while cats most clearly showed marked central excitatory actions . 