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: Other known reasons for seizures were ruled out and the convulsions stopped a few hours after cessation of morphine and did not reoccur in the subsequent 8 months .
### Output Text:
Other known reasons for seizures were ruled out and the convulsions stopped a few hours after cessation of <span class="chemical">morphine</span> and did not reoccur in the subsequent 8 months . 