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: Although only few clinical descriptions of the relationship between hyperalgesia / myoclonus and high doses of morphine are available , experimental support from animal studies indicates that morphine , or its metabolites , plays a causative role for the observed behavioural syndrome .
### Output Text:
Although only few clinical descriptions of the relationship between hyperalgesia / myoclonus and high doses of <span class="chemical">morphine</span> are available , experimental support from animal studies indicates that <span class="chemical">morphine</span> , or its metabolites , plays a causative role for the observed behavioural syndrome . 