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: Analgesic effect of intravenous ketamine in cancer patients on morphine therapy : a randomized , controlled , double - blind , crossover , double - dose study .
### Output Text:
Analgesic effect of intravenous <span class="chemical">ketamine</span> in cancer patients on <span class="chemical">morphine</span> therapy : a randomized , controlled , double - blind , crossover , double - dose study . 