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: Intramedullary administration of clonidine , but not saline vehicle , caused a dose - dependent decrease in both the mean arterial pressure and the heart rate .
### Output Text:
Intramedullary administration of <span class="chemical">clonidine</span> , but not saline vehicle , caused a dose - dependent decrease in both the mean arterial pressure and the heart rate . 