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: It is speculated that the induction of muscular rigidity by fentanyl may involve the coerulospinal noradrenergic fibers to the spinal motoneurons .
### Output Text:
It is speculated that the induction of muscular rigidity by <span class="chemical">fentanyl</span> may involve the coerulospinal noradrenergic fibers to the spinal motoneurons . 