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: An experimental model was developed in the rat to measure changes in lacrimation and intracranial blood flow following noxious chemical stimulation of facial mucosa .
### Output Text:
An experimental model was developed in the rat to measure changes in lacrimation and intracranial blood flow following noxious chemical stimulation of facial mucosa . 