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: Effect of ketamine on the SA node of the dog heart was studied in vivo using a selective perfusion technique of the SA node artery .
### Output Text:
Effect of <span class="chemical">ketamine</span> on the SA node of the dog heart was studied in vivo using a selective perfusion technique of the SA node artery . 