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Willis worked as a physician in Westminster, London, and from 1660 until his death was Sedleian Professor of Natural Philosophy at Oxford. He was a pioneer in research into the anatomy of the brain, nervous system and muscles. The " circle of Willis", a part of the brain, was his discovery.

His anatomy of the brain and nerves, as described in his Cerebri anatomi of 1664, is so minute and elaborate, and abounds so much in new information, that it presents an enormous contrast with the vague and meagre efforts of his predecessors. This work was not the result of his own personal and unaided exertions; he acknowledged his debt to Sir Christopher Wren and Thomas Millington , and his fellow-anatomist Richard Lower.

Willis was the first natural philosopher to use the term " reflex actionA reflex action or reflex is a biological control system linking stimulus to response and mediated by a reflex arc. Reflexes can be built-in or learned''. Reaction time For a reflex, reaction time is the time from the onset of a stimulus until the organis" to describe elemental acts of the nervous system. He also wrote Pathologiae Cerebri et Nervosi Generis Specimen, 1667 (An Essay of the Pathology of the Brain and Nervous Stock) and of De Anima Brutorum, 1672 (Discourses Concerning the Souls of Brutes).

Willis was the first to number the cranial nerves in the order in which they are now usually enumerated by anatomists. His observation of the connection of the eighth pair with the slender nerve which issues from the beginning of the spinal cord is known to all. He remarked the parallel lines of the mesolobe , afterwards minutely described by Felix Vicq d'Azyr . He seems to have recognized the communication of the convoluted surface of the brain and that between the lateral cavities beneath the fornix. He described the corpora striata and optic thalami; the four orbicular eminences, with the bridge, which he first named annular protuberance; and the white mammillary eminence s, behind the infundibulum. In the cerebellumGeneral Features Location It is found at the bottom rear of the head (the hindbrain), directly above the brainstem. Role The cerebellum is involved in modulating, rather than initiating movements. It is involved in guiding movements based on sensory feedb he remarks the arborescent arrangement of the white and grey matter, and gives a good account of the internal carotids, and the communications which they make with the branches of the basilar artery.

In relation to the search for the brain correlates of the mind, Willis extended the concepts proposed by the Roman physician GalenClaudius Galenus of Pergamum ( 131- 201 AD), better known as Galen was an ancient Greek physician. His views dominated European medicine for over a thousand years. Life Galen was born in Pergamum (modern-day Bergama, Turkey) to an architect's family., that the brain was the organ responsible for the excretion of animal spirits (which was tought to originate from the cribiform plate , a bone in the base of the skull, overlying the nasal cavity). Willis proposed that the choroid plexusThe choroid plexus is the area on the ventricles of the brain where cerebrospinal fluid (CSF) is produced. Choroid plexus is present in the superior part of the inferior horn of the lateral ventricles. It follows up along this boundary, continuous with th was responsible for the absorption of cerebrospinal fluidCerebrospinal fluid CSF in short, is the clear fluid that occupies the subarachnoid space (the space between the skull and cortex of the brain). It acts as a "cushion" or buffer for the cortex. Also, CSF occupies the ventricular system of the brain and th. Later, in De Anima Brutorum, he proposed that the corpus striatum received all sensory information, while the corpus callosumThe corpus callosum is the largest white matter structure in the mammalian brain. It consists of mostly of contralateral axon projections. It appears as a wide, flat region just ventral (below) the cortex. It is missing in monotremes. The corpus callosum was associated with imagination and the cerebral cortexThe cerebral cortex is the extensive outer layer of gray matter of the cerebral hemispheres, and is involved in higher brain functions, including sensation, voluntary muscle movement, thought, reasoning, and memory. The grooves between the Gyri (known as with memory.