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: Most electrophysiological responses were usually just within normal limits ; two patients had subnormal Arden electro - oculography indices ; and one patient showed an abnormally delayed photopic b wave .
### Output Text:
Most electrophysiological responses were usually just within normal limits ; two patients had subnormal Arden electro - oculography indices ; and one patient showed an abnormally delayed photopic b wave . 