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 pharmacological characteristics of vitamin E and the degenerative changes brought about by its deficiency , especially in the muscles , are illustrated .
### Output Text:
The pharmacological characteristics of <span class="chemical">vitamin E</span> and the degenerative changes brought about by its deficiency , especially in the muscles , are illustrated . 