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: Intraperitoneal administration of atropine , but not intraperitoneal or intracerebroventricular injection of hexamethonium , decreased the effect of a single dose of morphine .
### Output Text:
Intraperitoneal administration of <span class="chemical">atropine</span> , but not intraperitoneal or intracerebroventricular injection of <span class="chemical">hexamethonium</span> , decreased the effect of a single dose of <span class="chemical">morphine</span> . 