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 clinical and radiographic features of cerebral deep venous thrombosis in a 21 - year - old white woman are presented .
### Output Text:
The clinical and radiographic features of cerebral deep venous thrombosis in a 21 - year - old white woman are presented . 