318 CIRCLE OF WILLIS. 



formed by the anterior communicating branch, anterior cerebrals, and 

 internal carotid arteries in front, and by the posterior communicating, 

 posterior cerebrals, and basilar artery behind. 



Fig.lig.* 



The THYROID AXIS is a short trunk which divides almost immedi- 



* The circle of Willis. The arteries have references only on one side, on 

 account of their symmetrical distribution. 1. The vertebral arteries. 2. The 

 two anterior spinal branches uniting to form a single vessel. 3. One of the 

 posterior spinal arteries. 4. The posterior meningeal. 5. The inferior cere- 

 bellar. 6. The basilar artery giving off its transverse branches to either side. 

 7. The superior cerebellar artery. 8. The posterior cerebral. Q. The posterior 

 communicating branch of the internal carotid. 10. The internal carotid artery, 

 shewing the curvatures it makes within the skull. 1 1 . The ophthalmic artery 

 divided across. 12. The middle cerebral artery. 13. The anterior cerebral 

 arteries connected by, 14. The anterior communicating artery. 



