Overview of the History of the Cranial Nerves: From Galen to the 21st Century

Unveiling the vulnerability of the human abducens nerve: insights from comparative cranial base anatomy in mammals and primates

The topographic anatomy of the abducens nerve has been the subject of research for more than 150 years. Although its vulnerability was initially attributed to its length, this hypothesis has largely lost prominence. Instead, attention has shifted toward its intricate anatomical relations along the cranial base. Contrary to the extensive anatomical and neurosurgical literature on abducens nerve anatomy in humans, its complex anatomy in other species has received less emphasis. The main question addressed here is why the human abducens nerve is predisposed to injury. Specifically, we aim to perform a comparative analysis of the basicranial pathway of the abducens nerve in mammals and primates. Our hypothesis links its vulnerability to cranial base flexion, particularly around the sphenooccipital synchondrosis. We examined the abducens nerve pathway in various mammals, including primates, humans (N = 40; 60% males; 40% females), and human fetuses (N = 5; 60% males; 40% females). The findings are presented at both the macroscopic and histological levels. To associate our findings with basicranial flexion, we measured the cranial base angles in the species included in this study and compared them to data in the available literature. Our findings show that the primitive state of the abducens nerve pathway follows a nearly flat (unflexed) cranial base from the pontomedullary sulcus to the superior orbital fissure. Only the gulfar segment, where the nerve passes through Dorello's canal, demonstrates some degree of variation. We present evidence indicating that the derived state of the abducens pathway, which is most pronounced in humans from an early stage of development, is characterized by following the significantly more flexed basicranium. Overall, the present study elucidates the evolutionary basis for the vulnerability of the abducens nerve, especially within its gulfar and cavernous segments, which are situated at the main synchondroses between the anterior, middle, and posterior cranial fossae-a unique anatomical relation exclusive to the abducens nerve. The principal differences between the pathways of this nerve and those of other cranial nerves are discussed. The findings suggest that the highly flexed human cranial base plays a pivotal role in the intricate anatomical relations and resulting vulnerability of the abducens nerve.

Revisiting Galen: enduring contributions from ancient times towards modern neurosurgery

This historical vignette aims to reflect on the life of Claudius Galen and critically discuss his contributions towards modern neurosurgical practice; specifically, neuroanatomy, neurophysiology, and cranial trepanation. Born in 129 AD in the Greek city of Pergamon, Galen dedicated his early life to the study of medicine and established himself as one of the foremost physicians of the time. Through vivisections of Barbary apes, he was one of the first to provide a detailed description of cranial nerves, the ventricular system of the brain, and various deep brain structures. He made an important distinction between motor and sensory nerves and mapped out the fundamental arrangement of neuronal fibers within the spinal cord. However, his fundamental understanding of neurophysiology, as well as cerebral blood circulation, was largely flawed as it was based on speculation of inter-species comparative anatomy. On the technical side, he made a modest contribution to the practice of cranial trepanation and his writings helped establish the technique as an essential component of a surgeon's armamentarium. His work in the fields relevant to modern neurosurgery, although imperfect with the benefit of hindsight, laid the important foundation for much of the progress of neurosurgical practice in the Renaissance and beyond.

Historical awareness of the brainstem: From a subsidiary structure to a vital center

The historical trajectory of the discovery of the brainstem as a vital center has been rarely explored. Focusing on its tracts and neurons first, anatomists much later expanded their understanding of the brainstem with the discovery of groups of nuclear networks that affected major vital functions. Comparative anatomists (i.e., Edinger) postulated a primordial paleopallium that indirectly implied the centrality of the brainstem and a neopallidum with its differentiation and specialization up to Homo sapiens Methods that governed the discovery of the brainstem were (1) comparative anatomy, (2) embryonic growth, (3) vivisection, (4) brain dissection, and, much later, (5) microscopy and chemical feedback loops. This historical study traces how neuroscientists of the 18th and 19th century became increasingly aware of the vital functions performed by the brainstem. The anatomists of the 20th century found the ascending reticular formation, the respiratory center, and pressor centers-all automatic and vital functions. It took centuries for this realization to open the way to use the testable brainstem centers to establish the criteria for a neurologic determination of death. The ontogenetic conclusion is that the brainstem is the ancestor of the developed human brain; the physiologic conclusion is that the brainstem is a vital center and a structural support system and conduit. When afunctional, life ends.

Cranial Nerves: Morphology and Clinical Relevance

This Special Issue entitled "Cranial Nerves: phylogeny, ontogeny, morphology and clinical significance" has been divided into two consecutive volumes. This second volume is devoted to morphology and clinical relevance. Articles in this volume examine these topics from a macroscopic point of view and with a surgical interest. This volume includes articles on oculomotor nerves III, IV, and VI and their course in the orbit; intracranial and extracranial views of the V and VII pairs; and branching patterns of IX, X, XI, and XII pairs with medical significance. Together, these articles provide a general overview of cranial nerves' gross anatomical organization, as well as improving on the knowledge necessary for clinical approaches. Anat Rec, 302:555-557, 2019. © 2019 Wiley Periodicals, Inc.