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 region of cortex that represents the low frequencies was not obviously affected by the cochlear hearing loss .
### Output Text:
The region of cortex that represents the low frequencies was not obviously affected by the cochlear hearing loss . 