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 increase of activation in the region of the cavernous sinus however , suggests that this structure is more likely to be involved in trigeminal transmitted pain as such , rather than in a specific type of headache as was suggested for cluster headache .
### Output Text:
The increase of activation in the region of the cavernous sinus however , suggests that this structure is more likely to be involved in trigeminal transmitted pain as such , rather than in a specific type of headache as was suggested for cluster headache . 