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: Scatter diagrams of the g ratio ( quotient fibre diameter / axon diameter ) define regenerating fibres as a distinct population , distinguishable from the surviving fibres by reduced sheath thickness and reduced axon calibre .
### Output Text:
Scatter diagrams of the g ratio ( quotient fibre diameter / axon diameter ) define regenerating fibres as a distinct population , distinguishable from the surviving fibres by reduced sheath thickness and reduced axon calibre . 