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 patients of the study and control group ( without digoxin ) were of similar cardiac functional capacity and age ( 25 - 40 years ) and were randomly selected from the rheumatic heart disease patients .
### Output Text:
The patients of the study and control group ( without <span class="chemical">digoxin</span> ) were of similar cardiac functional capacity and age ( 25 - 40 years ) and were randomly selected from the rheumatic heart disease patients . 