The Anatomical Record Shows its Nerve (S): An Exploration of New Findings in our Special Issue on the Cranial Nerves

Field: Neuroscience (Life Sciences)

For those of us that remember our religious school bible classes, we may recall that the number "12" is a holy number. From the Old to New Testaments, many examples abound. Jacob had 12 sons, each of whom represented a tribe of ancient Israel; God specified that 12 unleavened cakes of bread be placed every week in the Temple, and that the Priests had to change the bread every Sabbath (Leviticus 24); there were 12 Apostles; and Christmas gloriously has 12 days (and a really neat song with lords a'leaping and partridges in a pear tree to remind us!) There are12 Imans revered as the spiritual and political successors of the Prophet Muhammad. Beyond the Abrahamic faiths, the Babylonian world divided the stars and heavens into 12 constellations, each overseen by a god (and from which we have evolved our 12-month calendars), ancient Greek religion worshipped 12 main Olympian Gods, while Norse traditions held that the great god Odin had 12 sons. And we anatomists have … 12 Cranial Nerves! Any wonder how we ever arrived at that number? (Hint: see above.) The depths and mysteries of numerology have few limits, and for those who wish to delve into the mystical interpretation of the Bible, the ancient writings and traditions of Kabbalah can lead you on such a journey. For us anatomists, we have our own Holy touchstone: the 12 Cranial Nerves. Of course, they are not all "nerves," depending upon your definition of a nerve. While some nerves are bundled (e.g., the poor vestibular "nerve" never gets its full respect, always being subsumed under the banner of the "acoustic" or "auditory" nerve; even medical student memorization jingles forget the vestibular nerve!), others are usually totally ignored (can you say "nervus terminalis"). Yet others (olfactory, optic) are actually brain tracts, thank you very much, and probably a bit miffed about being lumped with those other "non-tracts." Our texts, syllabi, and standardized examinations (talk about a holy trinity!) mandate that we bow down to the holy 12 as they are now considered. And how our field collectively (with a lot of kicking and screaming as our ilk can disagree more often then current Red and Blue State politicians) eventually arrived at the sacred land of the 12 is a remarkable journey unto itself (see reviews by Flamm, 1967; Shaw, 1992; Davis et al., 2014; Porras-Gallo et al., 2019, this volume). From Galen's recognition of seven pairs (again with the holy numbers!) to Willis' nine, to Soemmering's 12, the path was long and winding. Putting to the side for the moment the enticing argument of what constitutes a cranial nerve and who can be admitted to the club (as a help: think the current argument over who qualifies to be a citizen of certain countries and what you need to be allowed in; now you have some idea over the complexity of the "who's a cranial nerve argument"), two brave neurobiologists, with the addition of the sharp eye and oversight dedication of our neurobiology associate editor, have undertaken the daunting task of re-examining the world of the cranial nerves. The product of their efforts is this Special Issue entitled, "Cranial Nerves: Phylogeny, Ontogeny, Morphology, and Clinical Significance," an expansive topic indeed. The triumvirate consists of Guest Editors Alino Martínez-Marcos of the Universidad Castilla-La Mancha and José Sañudo of the Universidad Complutense de Madrid with our own José Luis Trejo of the Cajal Institute in Madrid helping to steer the ship (Trejo, 2019, in press). Due to the vastness of the topic, and the extensive new findings offered, this Special Issue will be offered in two volumes, with the first focusing on "Phylogeny and Ontogeny" and the second on "Morphology and Clinical Significance" (Martínez-Marcos and Sañudo, 2019, this volume). In many ways, it is very fitting that a brave band from the realm of Spain leads this effort. Their homeland, for example, was birthplace to a foundation of biomedicine, the great Cordoban philosopher and physician Moses ben Maimon (Maimonides); and home for arguably the most influential neuroanatomist of all time, the incomparable 1906 Nobel laureate, Santiago Ramon y Cajal. Let us simply say, their quest is partly genetically driven. And like Cervantes' Don Quixote de la Mancha, this Spanish triumvirate, and the band of like-minded knights that they recruited, are trying to conquer the elusive windmills that are the cranial nerves. The Anatomical Record has long been the home for research on the cranial nerves, work that has encompassed the different areas that the Special Issue examines. Indeed, our journal's history boasts numerous studies, exploring the peripheral distribution of the nerves, their brain nuclei, peripheral ganglia, and their development and evolution, both in humans and comparatively. We will mention but a few here, focusing on those most related to Volume 1 on Cranial Nerve Phylogeny and Ontogeny. Delving deep into our vaults we find a most interesting study by Ralph Edward Sheldon in 1909 on the phylogeny and embryology of one of the most fascinating of nerves, the chorda tympani of the facial nerve (CN VII) (Sheldon, 1909). For those of you who do not spend your days gazing in fascination at cranial nerves, the "chorda"—a slender (chorda is Latin for cord) branch of CN VII that joins and is distributed by CN V—is one of the most remarkable. It is literally a window onto our embryology (the chorda is part of the second branchial arch yet communicates throughout its course with first arch nerves and derivatives) and thus serves as a view directly into vertebrate evolution. While the chorda's function in humans (carrying taste from the anterior two-thirds of the tongue and distributing pre-ganglionic parasympathetic fibers to the submandibular ganglion for continued distribution throughout the oral cavity) is largely charted today, much was unknown at the end of the 19th and early part of the 20th centuries, both regarding function and, certainly, its evolutionary path. Sheldon's love was the nervous system and he became enraptured in tracing the evolution of this nerve. While Sheldon's work—his 1909 study included—was greatly respected, there is a tragic end to his personal story, also shared in The Anatomical Record. Sheldon was, by all accounts, a young, energetic dynamo in the anatomical world in the early part of the 20th century. Educated at Cornell, Harvard, and the University of Chicago for his PhD (under the revered comparative neurobiologist C. Judson Herrick), Sheldon impressed all that he met and moved quickly up the ladder. In 1909, he accepted a position as Assistant Professor of Anatomy at the University of Pittsburgh School of Medicine and by 1914 had risen to become Full Professor and Chair. Sheldon was a regular and enthusiastic participant at the annual meeting of The American Association of Anatomists, and was clearly headed for stardom in the anatomical world. As lovingly recorded by his Pittsburgh colleague, Robert Retzler, in a most moving memoriam in our journal (Retzler, 1919), Sheldon was felled with what was assumed to be Landry's acute ascending paralysis (thought by many now to be a virulent form of Guillan-Barre Syndrome) and passed away rapidly on July 9, 1918. The American Association of Anatomists' Meeting in 1919 was held, ironically, in Pittsburgh where he was both profusely eulogized and greatly missed. Sheldon's death was tragic on many fronts, and an enormous loss for science. A decade after Sheldon's (1909) article, Robert J. Terry of Washington University School of Medicine in St. Louis published insightful observations on the developmental relationship of the facial nerve and the otic capsule, looking at an assortment of amphibians, reptiles, and mammals (Terry, 1919). The article is interesting for two reasons beyond its scientific value: (1) The author provided an abstract in Spanish to precede the manuscript; and (2) who the author was. To the first, we are unsure as to why there was a Spanish abstract as there is no evidence of any relationship Prof Terry had with any persons or projects that would have prompted such as translated abstract. To the second, Robert Terry was the creator, and curator, of what has become regarded as one of the most important human skeletal collections in the world, now called "The Robert J. Terry Anatomical Skeletal Collection" housed at the Department of Anthropology of the National Museum of Natural History in Washington, D.C. (Trotter, 1981). This magnificent collection has been the welcoming home for countless studies on human variation, development, pathology, and forensics. (Of course, the museum cannot be visited at this writing as it is closed and its curators furloughed due to United States government partial shutdown. We are sure that the good Professor would like his Collection to be available again very soon!) As the prestige of The Anatomical Record grew in the first half of the 20th century, reports from researchers and schools from outside the United States started to find their way to our pages. For example, the outstanding Belgian-born developmental anatomist Ernest Van Campenhout—who later became Professor at Laval University in Montreal and subsequently Professor of Anatomy, of Embryology, and of Histology at the University of Louvain in his homeland—published his insightful observations on intraocular optic nerves in frog embryos in our journal in 1935 (Van Campenhout, 1935). He was a meticulous, unbelievably organized, experimental embryologist who earned his PhD in Anatomy at Yale in 1929, thus being an academic relative of one of us (JL) who also earned his doctoral degree at Mother Yale (although some years after Campenhout!). Many interesting studies on development and evolution—they are often tightly linked in comparative anatomical studies—appeared in the following years in The Anatomical Record. Included in these are: developmental and descriptive studies of the facial nerve in humans and trigeminal nerve (CN V) in baboons by noted embryologist and consummate teacher of embryology, Raymond Gasser (Gasser, 1970; Gasser and Wise, 1972) of Louisiana State University School of Medicine (Ray's southern warmth, hearty laugh, and gregariousness was a mainstay at anatomical meetings worldwide); Easter's meticulous study (Easter, 1979) of the growth and development of the trochlear nerve (CN IV) and superior oblique muscle in goldfish; Brown's (1980) study of the Nervus terminalis in embryos of insectivorous bats; Kitamura et al. detailed, anatomical studies on rabbit vagus (CN X) and spinal accessory nerves (CN XI) and their nuclei and pharyngeal constrictor muscle innervations, and implications for understanding the history of the complex (Kitamura et al., 1989, 1991, 1993); Kalmey et al.'s study on age-related size reduction in the foramina of the cribriform plate and effects upon reduction in olfactory nerve (CN I) function in the elderly (Kalmey et al., 1998); Butler's excellent review and synthesis of chordate head and brain evolution (Butler, 2002); Diaz et al.'s detailed review on the olfactory system sensu lato (Diaz et al., 2013); Gaudin et al.'s study on the ontogenetic development of the extra-bulbar olfactory pathway in xenopus (Gaudin et al., 2013; Wohlert et al., 2016) morphometric study of the axonal number differences in optic, trigeminal and vestibulocochlear nerves among seals as a means to gain insight into their functional differences and relationships; and last, but never least, a report by the great polymath, frequent contributor to The Anatomical Record, and one of our favorite Canadian anatomists (eh!), A. Wayne Vogl of beautiful University of British Columbia, on the functional anatomy of nerves innervating the grooved blubber of fin whales (Vogl et al., 2017). Usually in the late Fall of each year in most United States medical schools, the corridors reverberate with students chanting a mnemonic along the lines of, "On Old Olympus' Towering Top A Finn And German Viewed Some Hops" to help them remember the 12 cranial nerves (nowadays most mnemonics for the nerves could not be repeated in our journal). While the mnemonic still subsumes the vestibular nerve within the auditory, gives no special adoration to those nerves that are really brain tracts, ignores possible others, and so on, it does allow the next generation to at least say a welcome to the holy 12. And for the rest of their careers, most of them, like the rest of us, will be encountering, and trying to understand, the remarkable highways to and from the brain that govern much of our existence. While the quest to fully comprehend the cranial nerves may well rival Don Quixote's battle with windmills on the plains of La Mancha, it is a fascinating quest. We are most proud and honored that The Anatomical Record is the home to this collection of new explorations of one of the great wonders of biology, the cranial nerves.