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: The present studies explored the duration of the loss of righting reflex induced by sub - effective doses of ketamine and morphine , administered simultaneously .
### Output Text:
The present studies explored the duration of the loss of righting reflex induced by sub - effective doses of <span class="chemical">ketamine</span> and <span class="chemical">morphine</span> , administered simultaneously . 