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: There is evidence that cerebral reactions to drug toxicity can exhibit patterns that suggest highly selective involvement of functional subdivisions of the brain .
### Output Text:
There is evidence that cerebral reactions to drug toxicity can exhibit patterns that suggest highly selective involvement of functional subdivisions of the brain . 