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: We investigated how these two drugs act on the cardiovascular systems of 20 dogs whose hearts had been denervated by a procedure we had devised .
### Output Text:
We investigated how these two drugs act on the cardiovascular systems of 20 dogs whose hearts had been denervated by a procedure we had devised . 