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: In conclusion , the evaluation of time - to - onset of symptoms and the severity of neurodegenerative processes induced by the tacrine - lithium model of epilepsy in the rat , could be used to evaluate preliminarily the capability of a drug delivery system to trespass ( or not ) the BBB in vivo .
### Output Text:
In conclusion , the evaluation of time - to - onset of symptoms and the severity of neurodegenerative processes induced by the <span class="chemical">tacrine</span> - <span class="chemical">lithium</span> model of epilepsy in the rat , could be used to evaluate preliminarily the capability of a drug delivery system to trespass ( or not ) the BBB in vivo . 