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 similarities between the effects of structural lesions and pharmacological depression of the brain stem reticular formation are discussed .
### Output Text:
The similarities between the effects of structural lesions and pharmacological depression of the brain stem reticular formation are discussed . 