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 present experiments were designed to investigate the role of the midline cells of the B3 serotonin neurons in the medulla , coinciding with the raphe magnus .
### Output Text:
The present experiments were designed to investigate the role of the midline cells of the B3 <span class="chemical">serotonin</span> neurons in the medulla , coinciding with the raphe magnus . 