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: Lesions of the substantia nigra are therefore associated with erectile dysfunction in rats and may serve as a model to study erectile dysfunction in Parkinson ' s disease .
### Output Text:
Lesions of the substantia nigra are therefore associated with erectile dysfunction in rats and may serve as a model to study erectile dysfunction in Parkinson ' s disease . 