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: Downbeat nystagmus is often associated with structural lesions at the craniocervical junction , but has occasionally been reported as a manifestation of metabolic imbalance or drug intoxication .
### Output Text:
Downbeat nystagmus is often associated with structural lesions at the craniocervical junction , but has occasionally been reported as a manifestation of metabolic imbalance or drug intoxication . 