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: Under barbiturate anesthesia , virtually all of the myocardial contractile indices were depressed significantly in barium - exposed rats relative to the corresponding control - fed rats .
### Output Text:
Under <span class="chemical">barbiturate</span> anesthesia , virtually all of the myocardial contractile indices were depressed significantly in <span class="chemical">barium</span> - exposed rats relative to the corresponding control - fed rats . 