The assignment of numbers to the cranial nerves may be traced back to Galen (circa 130-201 ad) and perhaps even to Marinus of Tyre (circa 100 ad).[1 Galen recognized 11 of the 12 structures that we now designate as cranial nerves, lumping several of these together such that there were seven paired cranial nerves. Galen combined what we now know as the ninth through eleventh cranial nerves, calling these structures the sixth cranial nerve. In his system, the hypoglossal nerve was the seventh and final cranial nerve. The Galenic system for numbering the cranial nerves persisted for one and a half millennia. [1 It was still used by Andreas Vesalius, the Italian anatomist, in his text De Fabrics in 1543.y ,  Thomas Willis, the English physician and anatomist, numbered the cranial nerves at eight rather than seven. In his celebrated text of 1664, Cerebri Anatome,  he provided the first thorough description of the spinal accessory nerve. It is said that he used the word "accessory" because the nerve was accessory to the vagus nerve.y ,  , [5 Samuel von Soemmering, the Prussian artist, inventor, paleontologist, and clinical investigator, has been credited with the first modern enumeration of the 12 cranial nerves in 1778. 
The spinal accessory nerve (cranial nerve [CN] XI) and hypoglossal nerve (CN XII) share a number of features. These nerves, along with cranial nerves IV and VI, are pure motor nerves, meaning that they subserve only motor functions, and they do not carry sensory or autonomic fibers. y Nuclei of the spinal accessory and hypoglossal nerves lie in the medulla, along with the nuclei of the glossopharyngeal (CN IX) and vagus nerves (CN X). M
This chapter focuses on those disturbances of the spinal accessory and hypoglossal nerves that result in weakness rather than overactivity of innervated muscles, because the latter is discussed in other chapters on involuntary movements (see ChapterJ^ and Chapters,! ).
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