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 cerebrospinal fluid : serum ratio of cimetidine concentrations was 0 . 24 : 1 and indicates that cimetidine passes the blood - brain barrier ; it also raises the possibility that M . S . changes are due to blockade of histamine H2 - receptors in the central nervous system .
### Output Text:
The cerebrospinal fluid : serum ratio of <span class="chemical">cimetidine</span> concentrations was 0 . 24 : 1 and indicates that <span class="chemical">cimetidine</span> passes the blood - brain barrier ; it also raises the possibility that M . S . changes are due to blockade of <span class="chemical">histamine</span> H2 - receptors in the central nervous system . 