Circle of Willis is an important arterial communication that supplies the forebrain (telencephalon, diencephalon and optic vesicle) and often frequently tested in the exams. Circle of Willis receives blood from:
- Vertebrobasilar system: Basilar artery which gives off Posterio Cerebral Arteries (PCA) and Posterior communicating arteries which are the branches of PCA
- Internal Carotid Artery System: Gives off other arteries of Circle of Willis
Here, we will learn a mnemonic to draw the circle of willis and intracranial course of Internal Carotid Artery (ICA).
Step-wise instructions for drawing the circle of willis:
1. Draw a Circle – Circle of Willis is a circle of arteries.
Now write the “Willis” around this circle:
2. Write a large “W” at the inferior of circle – this represents Posterior Cerebral Arteries (PCA) which arises from a single basilar artery.
3. Write a horizontal “i” at the sides of the circle – this represents Middle Cerebral Artery (MCA) outside the circle and Internal Cerebral Artery (ICA) inside the circle.
4. Write “l l” at the superior of the circle – this represents the Anterior Cerebral Arteries (ACA) along with the part of circle between the MCA-ICA and Anterior communicating artery.
5. Write a horizontal “i” at the sides of the circle – this represents Middle Cerebral Artery (MCA) outside the circle and Internal Cerebral Artery (ICA) inside the circle.
Now, the intra-cranial course of Internal Carotid Artery:
6. Write a horizontal “S” – starting from carotid canal and ending in foramen lacerum.
7. Write another horizontal “S” beginning from the end of previous “S” –
- starting from foramen lacerum and then
- forming “S” shaped carotid siphon within cavernous sinus and then
- ending at the level of anterior clinoid process
- supraclinoid segment of ICA gives of branches to form Circle of Willis
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