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: Pentobarbital , scopolamine and MK - 801 protected the brain damage by pilocarpine , though in the MK - 801 - treated group , the pyramidal cells of hippocampus appeared darker than normal .
### Output Text:
<span class="chemical">Pentobarbital</span> , <span class="chemical">scopolamine</span> and <span class="chemical">MK - 801</span> protected the brain damage by <span class="chemical">pilocarpine</span> , though in the <span class="chemical">MK - 801</span> - treated group , the pyramidal cells of hippocampus appeared darker than normal . 