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: Changes in heart size during long - term timolol treatment after myocardial infarction .
### Output Text:
Changes in heart size during long - term <span class="chemical">timolol</span> treatment after myocardial infarction . 