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 first course of high - dose cytosine arabinoside resulted in no unusual symptoms , but on day 21 of the second course of treatment , the patient complained of numbness in his right foot .
### Output Text:
The first course of high - dose <span class="chemical">cytosine arabinoside</span> resulted in no unusual symptoms , but on day 21 of the second course of treatment , the patient complained of numbness in his right foot . 