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: Systemic cocaine ( 10 mg / kg ) significantly increased the locomotor activity of rats .
### Output Text:
Systemic <span class="chemical">cocaine</span> ( 10 mg / kg ) significantly increased the locomotor activity of rats . 