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 following were measured : spontaneous pain , pain evoked by punctate and brush stimuli ( VAS ) , and areas of brush - evoked and punctate - evoked hyperalgesia .
### Output Text:
The following were measured : spontaneous pain , pain evoked by punctate and brush stimuli ( VAS ) , and areas of brush - evoked and punctate - evoked hyperalgesia . 