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: Noxious chemical stimulation of rat facial mucosa increases intracranial blood flow through a trigemino - parasympathetic reflex - - an experimental model for vascular dysfunctions in cluster headache .
### Output Text:
Noxious chemical stimulation of rat facial mucosa increases intracranial blood flow through a trigemino - parasympathetic reflex - - an experimental model for vascular dysfunctions in cluster headache . 