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: Intracerebroventricular injection of atropine , hexamethonium , and naloxone also decreased catalepsy induced by morphine plus nicotine .
### Output Text:
Intracerebroventricular injection of <span class="chemical">atropine</span> , <span class="chemical">hexamethonium</span> , and <span class="chemical">naloxone</span> also decreased catalepsy induced by <span class="chemical">morphine</span> plus <span class="chemical">nicotine</span> . 