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Medical Terminology Daily (MTD) is a blog sponsored by Clinical Anatomy Associates, Inc. as a service to the medical community, medical students, and the medical industry. We will post a workweek daily medical or surgical term, its meaning and usage, as well as biographical notes on anatomists, surgeons, and researchers through the ages. Be warned that some of the images used depict human anatomical specimens.

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A Moment in History

Thomas Willis, MD
Thomas Willis
(1621-1675)

An English physician and anatomist, Willis was born on his parents' farm in Great Bedwyn, Wiltshire, where his father held the stewardship of the Manor. He was a kinsman of the Willys baronets of Fen Ditton, Cambridgeshire. He graduated M.A. from Christ Church, Oxford in 1642. In the Civil War years he was a royalist, and was dispossessed of the family farm at North Hinksey by Parliamentary forces. In the 1640's Willis was one of the royal physicians to Charles I of England. He obtained his medical degree in 1646.

Thomas Willis might well be one of the greatest physicians of the 17th century.He is one of the founders of the Royal Society of London. He is remembered by his many publications, especially "Cerebri Anatome: Cui accessit Nervorum Descriptio et Usu", where he describes the arterial anastomoses at the base of the brain. This work is also the first detailed description of the vasculature of the brain. Willis described nine cranial nerves.

He is considered as the father of Neurology as a discipline. He used the term "neurology" for the first time in 1664. He described several neurological conditions

The Arterial Circle of Willis is a famous eponymous structure found at the base of the brain. It represents an anastomotic roundabout that connects the right and left sides as well as the carotid and vertebral arterial territories that supply the brain. Named after Thomas Willis, this structure was known well before him, but it was Willis who described its function.  If you click on the image or here, you will be redirected to a detailed description of this structure.

Sources:

1. "The legendary contributions of Thomas Willis (1621-1675): the arterial circle and beyond" Rengachary SS et al J Neurosurg. 2008 Oct;109(4):765-75
2. "Thomas Willis, a pioneer in translational research in anatomy (on the 350th anniversary of Cerebri anatome)" Arraez-AybarJournal of Anatomy, 03/2015, Volume 226, Issue 3
3. " The naming of the cranial nerves: A historical review" Davis, M Clinical Anatomy, 01/2014, Volume 27, Issue 1
4. "Observations on the history of the circle of Willis". Meyer A, Hieros, R.Med Hist 6:119–130, 1962


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Arterial circle of Willis

UPDATED: The arterial "circle of Willis" is a roundabout of arteries found at the base of the brain, allowing for collateral circulation at this level. This arterial circle has been described by many anatomists, but it was Thomas Willis (1621 - 1675) who described it in most detail, and he was the first to understand its function.

The circle of Willis receives blood from the two main paired arteries that provide blood supply to the head and brain: the carotid arteries anteriorly, and the vertebral arteries posteriorly.

This arterial circle is formed by the anastomosis of several arteries, paired and unpaired:

Anterior cerebral arteries: These paired arteries are one of the terminal branches of the internal carotid arteries. They provide blood supply to the medial aspect and part of the lateral aspect of frontal and parietal lobes of the brain

• Anterior communicating artery: A single unpaired small artery communicating both anterior cerebral arteries and providing potential collateral circulation between them

• Internal carotid arteries: These two bilateral arteries are one of the branches of the carotid artery found at the root of the neck. Its two main terminal branches are the anterior cerebral arteries and the middle cerebral arteries

Arterial circle of Willis (en.wiklipedia,org)
Image courtesy of www.wikipedia.org 

• Posterior cerebral arteries: These two arteries are formed by the bifurcation of the basilar artery, which itself is formed by the junction of the right and left vertebral arteries. The posterior cerebral arteries provide blood supply to the occipital lobe and part of the temporal lobe of the brain

Posterior communicating arteries: These paired arteries provide communication between the carotid and vertebral arterial territories 

Middle cerebral arteries: Although not technically part of the arterial circle of Willis, these paired arteries are one of the two terminal branches of the internal carotid arteries. The middle cerebral artery travels deep in the lateral sulcus (Sylvian fissure) of the brain and provides blood supply to the lateral aspect of the brain including the frontal, parietal, occipital, temporal, and insular lobes

The arterial circle of Willis provides all of the arterial blood to the brain. Cerebral blood flow in humans averages 55 mL per 100 g of brain tissue per minute. This is a about over 742.5 mL/min for the average 1350 g brain. Depending on the situation the brain will use between 15 to 20 percent of the total cardiac output, although by weight the brain is only about two to three percent of the average body weight. Incredibly, the brain uses more oxygen that most organs averaging close to 25% of the total oxygen needs of the body!

The importance of the arterial circle of Willis is that beyond this point the arterial supply to the brain becomes terminal, that is, there are little or no anastomoses between the bifurcating branches exposing the brain to ischemia and necrosis should there be an arterial stenosis or stricture. The circle of Willis is an area prone to aneurysms, with over 27,000 cases yearly in the US.

For an image of the vascular territories of the brain, click here.

Clinical anatomy, pathology, and surgery of the brain and spinal cord are some of the lecture topics developed and delivered by Clinical Anatomy Associates, Inc.

Thanks to Jackie Miranda-Klein for making me review this post and update it!... and congratulations to Jackie for starting her Physician Assistant Master's degree at Kettering College. Dr. Miranda.