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: Neither verapamil nor diltiazem affected atrial conduction in the presence of autonomic blockade .
### Output Text:
Neither <span class="chemical">verapamil</span> nor <span class="chemical">diltiazem</span> affected atrial conduction in the presence of autonomic blockade . 