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 independent groups , morphine inhibited the intestinal transit in 48 + / - 4 % and 38 + / - 4 % after acute and chronic treatment , respectively , suggesting that tolerance did not develop to the constipating effects .
### Output Text:
In independent groups , <span class="chemical">morphine</span> inhibited the intestinal transit in 48 + / - 4 % and 38 + / - 4 % after acute and chronic treatment , respectively , suggesting that tolerance did not develop to the constipating effects . 