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: PURPOSE : This paper reviews the effect of chronic lithium therapy on serum calcium level and parathyroid glands , its pathogenesis , and treatment options .
### Output Text:
PURPOSE : This paper reviews the effect of chronic <span class="chemical">lithium</span> therapy on serum <span class="chemical">calcium</span> level and parathyroid glands , its pathogenesis , and treatment options . 