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: There was a combined mean loss of 46 % of fibers from the superior , inferior , and nasal quadrants in the ( six ) eyes of all three subjects ( mean average thickness of 55 + / - 29 microm ) .
### Output Text:
There was a combined mean loss of 46 % of fibers from the superior , inferior , and nasal quadrants in the ( six ) eyes of all three subjects ( mean average thickness of 55 + / - 29 microm ) . 