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: Furthermore , dopamine appeared to act on systems more closely involved with the induction of ketamine catatonia rather than directly on the pituitary .
### Output Text:
Furthermore , <span class="chemical">dopamine</span> appeared to act on systems more closely involved with the induction of <span class="chemical">ketamine</span> catatonia rather than directly on the pituitary . 