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: Both plasma pethidine and norpethidine were elevated in the range associated with clinical manifestations of central nervous system excitation .
### Output Text:
Both plasma <span class="chemical">pethidine</span> and <span class="chemical">norpethidine</span> were elevated in the range associated with clinical manifestations of central nervous system excitation . 