Clinical anatomy of the cranial nerves


We are very happy to present this Special Issue on the cranial nerves to the readers of Clinical Anatomy. The history of the exploration of the cranial nerves and elucidation of their anatomy is ancient. Galen's classification of the cranial nerves (Table 1), notably excluding the olfactory nerves, was composed of seven pairs where it is interesting to note that the sixth pair included the glossopharyngeal, vagus and accessory nerves all travelling through the jugular foramen (Clarke and Jacyna, 1987). The cranial nerves were essentially identified, numbered and characterized based on the opening through which they exited the skull base. Our knowledge of the cranial nerves became more clearly discussed and disseminated by such medieval Middle Eastern and European scholars as Rhazes, Avicenna, Jorjani, Mundinus, Benedetti, Achillini, Massa and Berengario de Carpi (Shoja et al., 2007; Shaw, 1992). The Galenic seven-pair classification of the cranial nerves remained unchanged for more than a millennium until the celebrated German physician and anatomist, Samuel Thomas von Sömmerring, in 1778, implemented a novel classification scheme that is still used today (Squire et al., 2013). Niccolo Massa, an Italian physician and anatomist, was probably the first anatomist in Europe who challenged Galen's classification scheme by enumerating the olfactory nerves as the first pair of cranial nerves. Thomas Willis is notable for his work enumerating and illustrating nine cranial nerve pairs, including the olfactory nerves as the first pair and counting the trochlear, trigeminal and abducens nerves individually (Casey, 2007; Shaw, 1992).

Table 1. Classification of the cranial nerves (adopted from Charles Singer (1926) with slight modifications).
Modern classificationGalen's classification
I. Olfactory nerveNot mentioned
II. Optic nerveFirst pair
III. Oculomotor nerveSecond pair
IV. Trochlear nerveNot mentioned.
V. Trigeminal nerveThird and fourth pairs
VI. Abducens nerveProbably united with the second pair
VII. Facial nerveFifth pair
VIII. Vestibulocochlear nerveFifth pair
IX. Glossopharyngeal nerveSixth pair
X. Vagus nerveSixth pair
XI. Accessory nerveSixth pair
XII. Hypoglossal nerveSeventh pair

In this special issue of Clinical Anatomy, we have attempted to include a wide range of relevant articles with emphasis on their originality and clinical applications of the cranial nerves. The review article by Dr. Vilensky on cranial nerve 0 or the “neglected cranial nerve” is very informative. The special issue contains a landmark study by Dr. Tubbs and his colleagues, which represents “the largest study to date documenting the presence of a cranial root of the accessory nerve.”

An article by Dr. Hendrix and colleagues (2014) on the arterial supply of the lower cranial nerves is one that needs specific mention. Skull base surgeons are often perplexed by the fact that following resection of a vestibular schwannoma, some patients develop facial nerve paralysis without their facial nerve being traumatized during the operation. The vast and complex anatomy of the cranial nerve blood supply may provide an explanation for this observation. Whether a proportion of patients with facial paralysis following tumor resection suffer from acute ischemic neuropathy secondary to arterial disruption needs further research. The first step in such research is to elucidate the anatomy of the arterial supply.

The two articles on the anastomoses between lower cranial and upper cervical nerves (see Shoja et al., 2014a, 2014b) are products of an over three year review, which attempted to further elucidate the anatomy of the neural network formed by the lower cranial and upper cervical nerves and their branches. The reviews conclude that these nerves “with their extra-axial communications can be collectively considered a plexus.” The anatomy of this craniocevical nerve plexus needs to be revisited and appreciated in the anatomical literature.

Ultimately, we hope that clinical anatomists from around the world will enjoy reading this issue and find its contents valuable in their teaching and research.