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 outcome of pallidotomy in which the lesion involves the ventral and dorsal GPi could be the net effect of alteration in the activity of pathways which mediate different symptoms , and hence could be variable .
### Output Text:
The outcome of pallidotomy in which the lesion involves the ventral and dorsal GPi could be the net effect of alteration in the activity of pathways which mediate different symptoms , and hence could be variable . 