Historical Observations on the Cranial Nerves

EUGENE S. FLAMM, M.D. Department of Neurosurgery, New York University Medical Center, New York, N. Y.

NTEREST in the cranial nerves has existed Bauhin 3 illustrated or discussed the olfactory since the earliest medical writings. Al- tracts and often referred to them as the I though no mention of nerves as such oc- mammillary processes. Although they were curred in the Edwin Smith Papyrus, observa- aware of their role in olfaction, they did not tions of the brain and some functional local- consider them to be true cranial nerves. ization had already been made. 6 Descriptions Leonardo da Vinci 42 and Eustaehius 13 showed of individual cranial nerves had begun by the the olfactory bulbs, but they left no text to time of Herophilus, ~9 and in the first century accompany their illustrations. Willis in 1664 A.D., Marinus listed seven pairs of cranial was the first to list the olfactory tracts as the nerves. 4~ The prominent position of these first pair of cranial nerves; ~6 all classifica- nerves led to frequent commentaries on them tions prior to this began with the optic by many anatomists. During the years, nerves. Willis' designation has persisted even systems of numbering the nerves changed, though he described tracts as nerves. Be- but seldom were new anatomical observa- cause of this it is now necessary to list the tions nmde. nervus tcrminalis without a number. This paper compares the descriptions of the cranial nerves by prominent and some Optic Nerves less well-known anatomists in an attempt to The optic nerves offer several interesting present the background of our current classi- points in the history of anatomical descrip- fication of 1s pairs of cranial nerves. The tion. It is understandable why many writers discussion deals with the intracranial course, commented on these easily visible structures. points of exit, and eponymic designations of Galen was well aware of the difference in the nerves and not with their nuclear origins texture of the optic nerves compared to the or peripheral distributions. Particular atten- other cranial nerves; he also noted that the tion is given to European anatomical writ- inner portion of the nerve was softer than ings. No attempt has been made to examine the outer part. He discussed the joining of the the anatomical works of the Arabian writers, nerves in the form of the Greek letter chi (X) as they did not directly influence Western but was quite explicit about the absence of anatomists except through the preservation any intermingling of the left- and right-sided of Galen's writings. fibers34 Olfactory Tracts According to Cogan, the first illustration of the optic chiasm was made by Leonardo da The olfactory tracts have always been Vinci in 1505. s These drawings were not seen mentioned in any description of the cranial by other 16th century anatomists as they re- nerves. Galen described them in Book XIV of mained unpublished until the end of the 19th On Anatomical Procedures but excluded them century. Garrison has shown an earlier illus- from his numbered pairs of cranial nervesY tration of the chiasm in an Arabian manu- He argued that they were softer than the scriptY ~ other nerves and often contained a cavity. The first 50 years of the 16th century He did not specify that this cavity existed marked a period of great productivity in only in lower mammals, but the distinction of anatomical illustration. The chiasm was the olfactory tracts from actual cranial shown by many anatomists including Ve- nerves was certainly a valid one. This differ- salius, 49 Eustachiusy and Estienne22 Ve- ence was commented upon by anatomists salius actually reported a case in which no well into the 17th century. Vesalius, 4~ chiasm was present and the optic nerves were Colombo, I~ Eustachius, is Fallopius, 16 and separate. D~ Winslow as late as 1782 also re- Received for publication March 1, 1967. ferred to a similar case) 0 Vesalius did state ~85 286 Eugene S. FlamE that he could find no cavity within the optic not distinguish the trochlear nerve; a nerve nerves; ~c~ this represented a departure from which could be the abducens is labelled "d" Galen's writings. It was not until the 18th in his illustration and is described as a nerve century that the truc structural relationship which inserts in the inferior maxillary mus- of the fibers within the ehiasm was es- cles. a This nerve is not listed in the nun> tablished. bered pairs. Ophthalmology in the 17th and 18th We now turn to the question of Barto- centuries was hardly a medical specialty as lomeo Eustachi and his role in the descrip- we understand it today. Throughout Wes- tion of the cranial nerves. In 1714 Lancisi tern Europe and England, diseases of the published the anatomical plates of Eusta- eye, both real and imaginary, were treated by chius, la These plates were engraved in 155~ in itinerant oculists who, for the most part, Rome, but only the first eight were published came close to being prototypes of the medical during Eustachius' lifetime? 4 Eustachius' quack. From this group, however, ophthal- position in 16th century anatomy was there- mology eventually developed, and the spe- fore delayed for over 150 years. Unfortu- ciality is indebted to these men for their nately, he did not write any text to ac- early observations24 company his illustrations; we must therefore John Taylor, or Chevalier Taylor as he infer his observations from the drawings titled himself, was one of this group who themselves. The text published with the treated some of the more illustrious person- 18th century editions was by Lancisi and ages of the day. Among his patients were reflected the writings of 17th century anato- George II of England and George Frederick mists, particularly Willis, in regard to the Handel2 In 1788 he published a book on the cranial nerves. Tabula XVIII of Eustachius anatomy of the eye, with copper plates that shows the base of the brain; a distinct clearly showed the decussation of the nasal oculomotor, trochlear, and abducens nerve fibers at the chiasnl (Fig. 1). He quite cor- can be seen in the main illustration as well as rectly interpreted the role of the chiasm in in the four detailed inserts (Fig. 8). the intogration of binocular vision. 4s These In 1561 in Padua, Fallopius published observations in themselves must certainly Observationes Anatomicae which contains challenge the charge of charlatan and quack much significant information on the crani~[ in his ease. A more thorough discussion of nerves35 Although this book is not illus- the early writings on the optic nerves is given trated, the text is explicit enough to allow us by Polyak. 4a to follow closely Fallopius' classification. His second pair of nerves was clearly the ocuh)- Oculomotor, Trochlear, and motor; 17 he described the course of this nerve Abducens Nerves into the orbit and its division into a superior The nerves concerned with eye movements and inferior branch. The former went to the have been described under different headings muscle which "opens the eye" (the levator and with varying degrees of accuracy. palpebrae superioris) and to the muscle Galen's second pair was composed of "hard which "draws the eye directly upward" (the nerves, whose origin springs from the anterior rectus superioris). Fallopius described the parts of the brain. ''2a He felt that this nerve inferior branch as going to the muscles that divided and reached each of the extraoeular draw the eye inward, downward, and out- muscles. It was not until the 16th century ward. This last action was performed by the that the innervation of the intraorbital nms- "sixth muscle" which is the inferior oblique. cles was clarified. In summation he stated that this nerve went None of the pre-Vesalian anatomists pre- to four muscles of the eye as well as to the sented anything resembling an accurate muscle that elevated the eyelid. Fallopius' picture of the base of the brain or the origins fourth pair of cranial nerves entered the orbit of the cranial nerves. Vesalius' own plates of and became entirely "embedded in the mus- the inferior aspect of the brain omitted a cle which draws the eye straight towards the structure as obvious as the pons and showed outer corner"--surely the rectus lateralis3 s the cranial nerves with only minor variations In closing his discussion of the cranial nerves, of the Galenie system (Fig. s176 Vesalius did Fallopius added one other pair and noted