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 effects of digitalis glycosides on myocardial oxygen supply and demand are of particular interest in the presence of obstructive coronary artery disease , but have not been measured previously in man .
### Output Text:
The effects of <span class="chemical">digitalis glycosides</span> on myocardial <span class="chemical">oxygen</span> supply and demand are of particular interest in the presence of obstructive coronary artery disease , but have not been measured previously in man . 