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: Effect of methoxamine on maximum urethral pressure in women with genuine stress incontinence : a placebo - controlled , double - blind crossover study .
### Output Text:
Effect of <span class="chemical">methoxamine</span> on maximum urethral pressure in women with genuine stress incontinence : a placebo - controlled , double - blind crossover study . 