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: Physical and quantitative sensory examination with determination of vibratory perception and thermal discrimination thresholds were performed , four to 77 months ( median 34 months ) after vincristine treatment .
### Output Text:
Physical and quantitative sensory examination with determination of vibratory perception and thermal discrimination thresholds were performed , four to 77 months ( median 34 months ) after <span class="chemical">vincristine</span> treatment . 