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: NIN - induced midpontine activation may correspond to activation of the dorsomedial pontine nuclei and the nucleus reticularis tegmenti pontis , structures known to participate in the generation of multidirectional saccades and smooth pursuit eye movements .
### Output Text:
NIN - induced midpontine activation may correspond to activation of the dorsomedial pontine nuclei and the nucleus reticularis tegmenti pontis , structures known to participate in the generation of multidirectional saccades and smooth pursuit eye movements . 