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: This is consistent with prior histopathological studies that show predominant loss of parvo - cellular axons ( or small - caliber axons ) within the papillo - macular bundle in toxic or hereditary optic neuropathies .
### Output Text:
This is consistent with prior histopathological studies that show predominant loss of parvo - cellular axons ( or small - caliber axons ) within the papillo - macular bundle in toxic or hereditary optic neuropathies . 