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: On the contrary , though not clinically apparent , pethidine potentially causes inhibitory impacts on the CNS and impairs normal cerebellar and oculomotor function in the short term .
### Output Text:
On the contrary , though not clinically apparent , <span class="chemical">pethidine</span> potentially causes inhibitory impacts on the CNS and impairs normal cerebellar and oculomotor function in the short term . 