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: Thus the migraine - inducing effect of nitroglycerin seems to depend on direct stimulation of the habitual site of pain , suggesting that the frontotemporal region is of crucial importance in the development of a migraine crisis .
### Output Text:
Thus the migraine - inducing effect of <span class="chemical">nitroglycerin</span> seems to depend on direct stimulation of the habitual site of pain , suggesting that the frontotemporal region is of crucial importance in the development of a migraine crisis . 