Circle of Willis – Mnemonic and Drawing

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:

  1. Vertebrobasilar system: Basilar artery which gives off Posterio Cerebral Arteries (PCA) and Posterior communicating arteries which are the branches of PCA
  2. 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).

Circle of willis mnemonic

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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