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: Additionally , levels of cocaine determined in hippocampus and cortex were not different between sensitive and resistant strains .
### Output Text:
Additionally , levels of <span class="chemical">cocaine</span> determined in hippocampus and cortex were not different between sensitive and resistant strains . 