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: In contrast , both methscopolamine and neostigmine , which do not penetrate the blood - brain barrier , had no effect on the hyperactivity produced by morphine .
### Output Text:
In contrast , both <span class="chemical">methscopolamine</span> and <span class="chemical">neostigmine</span> , which do not penetrate the blood - brain barrier , had no effect on the hyperactivity produced by <span class="chemical">morphine</span> . 