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: Contrary to an earlier report by Coxon , scoline pain occurs in African negroes .
### Output Text:
Contrary to an earlier report by Coxon , <span class="chemical">scoline</span> pain occurs in African negroes . 