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 contractile element shortening velocity of the cardiac muscle fibers was significantly slower in both groups of barium - treated rats relative to the control groups , irrespective of the anesthetic regimen .
### Output Text:
The contractile element shortening velocity of the cardiac muscle fibers was significantly slower in both groups of <span class="chemical">barium</span> - treated rats relative to the control groups , irrespective of the anesthetic regimen . 