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: Selectivity for the cerebrovascular bed makes nimodipine potentially useful in the treatment of subarachnoid hemorrhage , migraine headache , dementia , and stroke .
### Output Text:
Selectivity for the cerebrovascular bed makes <span class="chemical">nimodipine</span> potentially useful in the treatment of subarachnoid hemorrhage , migraine headache , dementia , and stroke . 