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: There also were no significant differences between groups when expressing diameters as the sum of the precommunicating segment of the anterior cerebral + sphenoidal segment of the middle cerebral + supraclinoid internal carotid artery + basilar artery divided by the diameter of the petrous internal carotid artery ( p greater than 0 . 05 , unpaired t - tests ) .
### Output Text:
There also were no significant differences between groups when expressing diameters as the sum of the precommunicating segment of the anterior cerebral + sphenoidal segment of the middle cerebral + supraclinoid internal carotid artery + basilar artery divided by the diameter of the petrous internal carotid artery ( p greater than 0 . 05 , unpaired t - tests ) . 