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: Selective injection of iopentol , iohexol and metrizoate into the left coronary artery of the dog .
### Output Text:
Selective injection of <span class="chemical">iopentol</span> , <span class="chemical">iohexol</span> and <span class="chemical">metrizoate</span> into the left coronary artery of the dog . 