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 also evidence of a subtle direct toxic effect on the entire fibre population , causing axon shrinkage masked by readjustment of the myelin sheath .
### Output Text:
There was also evidence of a subtle direct toxic effect on the entire fibre population , causing axon shrinkage masked by readjustment of the myelin sheath . 