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: And the results are consistent with the hypothesis that morphine acts by retarding the release of acetylcholine at some central cholinergic synapses .
### Output Text:
And the results are consistent with the hypothesis that <span class="chemical">morphine</span> acts by retarding the release of <span class="chemical">acetylcholine</span> at some central cholinergic synapses . 