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: BACKGROUND : Posteroventral medial pallidotomy sometimes produces striking improvement in patients with advanced Parkinson ' s disease , but the studies to date have involved small numbers of patients and short - term follow - up .
### Output Text:
BACKGROUND : Posteroventral medial pallidotomy sometimes produces striking improvement in patients with advanced Parkinson ' s disease , but the studies to date have involved small numbers of patients and short - term follow - up . 