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: We describe the effect of phenylephrine and ephedrine on frontal lobe oxygenation ( S ( c ) O ( 2 ) ) following anesthesia - induced hypotension .
### Output Text:
We describe the effect of <span class="chemical">phenylephrine</span> and <span class="chemical">ephedrine</span> on frontal lobe oxygenation ( S ( c ) O ( 2 ) ) following anesthesia - induced hypotension . 