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: The results suggest that rigidity , which is assumed to be due to an action of morphine in the striatum , can be antagonized by another process leading to dopaminergic activation in the striatum .
### Output Text:
The results suggest that rigidity , which is assumed to be due to an action of <span class="chemical">morphine</span> in the striatum , can be antagonized by another process leading to dopaminergic activation in the striatum . 