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: Aventyl ( nortriptyline ) and Elavil ( amitriptyline ) each produced left bundle branch block in a 73 year old woman .
### Output Text:
<span class="chemical">Aventyl</span> ( <span class="chemical">nortriptyline</span> ) and <span class="chemical">Elavil</span> ( <span class="chemical">amitriptyline</span> ) each produced left bundle branch block in a 73 year old woman . 