The present study describes how historical anatomical drawings can be incorporated in anatomical teaching. High resolution digital information from historical drawings is nowadays freely available from different sources such as Internet, e-readers, electronic apps, etc. Historical anatomical awareness is of educational importance and putting ancient information into lectures or strategies to study human anatomy including the dissecting room, may be very appropriate and attractive. In our current courses, historical drawings (two skulls sectioned) by Leonardo da Vinci (1452–1519) were included to demonstrate how ancient drawings can be used for lectures, demonstrations, and discussions on the human skeleton, especially, the human skull and dental formula. This “historical approach” of teaching was practiced in lectures, presented to individual students, small classes in the dissecting room, and to teachers of the UMCG School of Dentistry.
The drawings of Leonardo da Vinci turned out to be really helpful for our approach, because the author was not only an artist but also a scientist as well. The author attended many dissections and prepared more than 200 detailed anatomical drawings, based on his observations. In the “head and neck” region, the author described the “dental formula,” and distinguished the molars from the premolars. In addition, the author described the maxillary antrum, more than 150 years before Nathanial Highmore (1651, see Ring, 1985)!
"A SKULL SECTIONED"
The Royal Collection at Windsor Castle, England, possesses a 19.0 cm × 13.7 cm drawing (drawing RL 19058v) made by Leonardo da Vinci. The pen and ink drawing over traces of black chalk shows the anterior view of two halves of the human skull. The drawing was probably acquired for the Royal Collection by Charles II in 1690 (see “Ten drawings bij Leonardo da Vinci,” 2008).
The drawing was made by Leonardo in 1489, in the so-called “early period.” In this detailed drawing, the extraordinary gifted Leonardo illustrated his skills to display precise anatomical structures of the skull and its separate components. In this period, Leonardo's drawings were purely descriptive (Keele, 1979).
ANATOMY OF THE SKULL
Drawing RL 19058v. By sagittal cross-dissection, this skull is divided into two parts. The left part is intact and shows precise external structures such as sutures and the point pterion on the lateral surface. The set of teeth on the left displays a correct occlusion. The right part is further dissected by a carefully chosen frontal cut showing detailed information with both sinus maxillaris and sinus frontalis exposed.
Note I. Directly below the drawing of the skull, in the center of the page:
The cavity of the eye socket and the cavity in the bone that supports the cheek and that of the nose and mouth are of equal depth and terminate in a perpendicular line below the ‘senso commune’ the seat of the soul. And each of these cavities is as deep as the third part of a man's face, that is from the chin to the hair (Keele and Pedretti, 1979).
As an extension of the frontal cut, mentioned before, the anterior part of the mandibula was removed and teeth exposed. In this article also, it is evident, how carefully the section plane was chosen; exactly at the level of the foramen mentale, displaying compact and spongious bony components, including a cross-section through the foramen mentale and its connection with the canalis mandibulae, through the compact bone of the lateral wall of the corpus mandibulae (Keele, 1979, Fig. 1B). Anatomically, the location of the foramen mentale is rather variable (Lanz and Wachsmuth, 2004). Its most common position is on a vertical line passing through the second mandibular premolar (Berkovitz et al., 2002). In an early study, almost 70% of the foramina were observed at the apical level of the second premolar (Matsuda, 1927; Lanz and Wachsmuth, 2004). In more recent radiologic observations, however, the main position, in about 70% of the cases studied, was described to be located “between the two premolars” (Fishel et al., 1976). As observed in a few exceptional cases, the position of the foramen mentale may vary even between the first premolar and distal to the second premolar (Lanz and Wachsmuth, 2004, Table 46, p. 221). Furthermore, Leonardo's drawing shows that the lateral wall of maxilla is partly removed, probably to study the relationship of molar dental radices/apices to the floor of the sinus maxillaris in situ (Fig. 1B).
In both orbitae, the fissurae orbitales superior and inferior were delineated and the canalis opticus was positioned medially to these fissures. In the lateral wall of the orbitae, the foramen zygomaticoorbitale can be observed. The left orbita shows the fissura orbitalis superior and the foramen infraorbitale. In the floor of the sectioned orbita and in its medial wall, the canalis infraorbitalis and the ductus nasolacrimalis, the latter opening into the meatus nasi inferior underneath the concha nasalis inferior, can be observed, respectively. On both sides, the foramen zygomaticofaciale in the os zygomaticum is drawn. In close proximity to the nasal septum, the opening of the canalis incisivus running into the foramen incisivum is bilaterally present.
In another drawing (RL 19057r; Leonardo da Vinci, 1977) Leonardo studied a cross-section through the canalis incisivus and showed the direct relationship between the nasal and oral cavities (Fig. 2A). In the same drawing, the author paid attention to the foramen mandibulae, the canalis palatinus major (some faint stripes) and the canalis opticus, as indicated by pointers (Fig. 2A and 2B). In our reconstruction of the pathways, the authors observed that the pointer in the foramen mandibulae connects to the neurocranium through the foramen jugulare and not through the foramen ovale as might be expected with our present knowledge about the normal trajectory of the nervus mandibularis. In the lateral wall of the cavitas nasi, the openings of the ductus nasolacrimalis and the paranasal sinuses have been indicated.
In humans, all teeth differ morphologically. They have a so-called heterodont dentition (Berkovitz et al., 2002). The number of teeth of each type is written into the dental formula for two quadrants, with the upper (maxillary) and lower (mandibular) teeth shown at separate levels . Accordingly, the total number of teeth in the permanent dentition is 32. In each set (from left to right), incisors are indicated first, canines second, premolars third, and finally the total number of molars (Berkovitz et al., 2002).
On the left side of drawing 1A, adjacent to the ramus mandibulae, Leonardo sketched the arrangement and morphology of the teeth as present in a right upper quadrant of the mouth. From right to left: an incisor (4), a canine (2), a premolar (4), and a molar (6). Directly above the drawing, the author wrote the number of these elements of the complete dental formula of one half of the mouth (from right to left: 4246). As such, as far as authors know, Leonardo was the first to describe the human dental formula.
The drawing properly depicts the roots of the tooth types. Based on the roots of the molar (on the left), it can be reconstructed that all elements are meant to originate from the upper half of the mouth (maxilla). Consequently the elements are positioned upside down.
Note II. Below the drawings of the teeth, on the left hand side:
The six upper molars have three roots each, of which two roots are on the outer side of the jaw and one on the inner, and the two last of these are cut at twenty-four years of age or thereabouts. Then come the four premolars with two roots each, one on the inner side, and the other on the outer side. Then follow the two master-teeth [canines] with only one root, and in front of them are four teeth which cut [incisors] and have only one root. The lower jaw also has sixteen teeth like the upper, but its molars have only two roots; the other teeth are like those in the upper jaw. In animals two teeth fasten on the prey, four cut it up and six grind it up (Keele and Pedretti, 1979).
The molar can be classified as a first right maxillary molar (mesiodistal view), with the palatal root on the right, and both buccal roots (mesiobuccal and distobuccal) on the left. The fact that Leonardo commented on this drawing with "The six upper molars have three roots each, of which two roots are on the outer side of the jaw and one on the inner, …" (see Note II) gives evidence that the author dissected the lateral wall of the sinus maxillaris to determine the position of the roots of the maxillary molars. Remarkable, however, is the “premolar” drawn with four distinct radices, or, depending on interpretation: two flat radices and lack of cusps (second element from the left in the sketch). It is generally accepted that permanent elements in human have no more than three radices (Malagnino et al., 1997; Maggiore et al., 1998; Berkovitz et al., 2002; Putz and Pabst, 2006). The maxillary first premolar is the only premolar that has two roots (one buccal and one lingual). Single root or three rooted (less than 1%) belong to variants and anomalies (Kartal et al., 1998; Cleghorn et al., 2007a, b, 2008). The maxillary second premolars usually show a single root (Berkovitz et al., 2002). According to Weine (1996) 60% of maxillary second premolars have one root, while Vertucci (1978) reported that 97% of mandibular second premolars and 94% of mandibular first premolars are single-rooted.
Four-rooted maxillary (second) molars are very rare (Libfeld and Rotstein, 1989; Peikoff et al., 1996; Malagnino et al., 1997; Maggiore et al., 1998). Maxillary deciduous and permanent molars have three roots in contrast to the two roots (mesial and distal) found in the mandibular deciduous and permanent molars (Berkovitz et al., 2002). After consulting a forensic physical anthropologist, an orthodontist and after questioning dentists from the University of Groningen/UMCG dental school, the author learned that the element concerned in all likelihood has to be classified as a permanent molar with characteristic heavily worn occlusal surface. However, if authors take into account that the discernment of the four roots is ambiguous, then a maxillary premolar with heavily worn occlusal surface may be considered also. The sectioned-skull shows the mesial surface of maxillary first right premolar and mandibular first right premolar (Fig. 1B). When comparing the premolar in the formula with the premolars depicted in the dissected skull, it can be concluded that Leonardo made an ambiguous sketch of this premolar. In his sketch of the dental formula, Leonardo further showed the labial surface of the canine and an incisor. In the left margin and under the drawing, in mirror-writing, Leonardo discusses in a correct way the form and number of teeth in the human jaw (see note II). Keele and Pedretti, 1979 commented: “The study of the teeth clarifies their shapes for all to see, although it fails to show the correct roots and bicuspid surfaces of the premolars.” Based on the observations described above, it has to be questioned whether work in his early period on both skulls sectioned (drawings RL 19057r and RL 19058v) was purely descriptive (Keele, 1979) or was also explorative.
It can be concluded that this fine drawing “a skull sectioned” of the human skull by Leonardo da Vinci hides more detailed information than reported earlier; a dissected lateral wall of the maxilla, probably to study dental roots. With respect to the dental formula Leonardo deserves the credits to be the first to describe a correct dental formula (6424). However, taking into account, the morphological features of the separately depicted elements of the maxilla, it can be suggested that Leonardo sketched a permanent mandibular molar on the position of two maxillary premolars (Fig. 1C). Premolars with such morphological features do not exist in humans.
Apparently, da Vinci's drawing was anatomically not correct, which keeps us guessing about the reason. “Solutions” like the possibility that the element was an ambiguously drawn maxillary premolar, or a “dislocated” mandibular molar cannot be fully excluded. The fact that the author did not make any comment on that peculiarity is most remarkable. Leonardo could have had sufficient knowledge of the precise morphology of maxillary and mandibular premolars, since the author presented these elements correctly in the left side of the dissected skull. The fact that author also had access to premolars in situ, justifies the assumption that something “might have gone wrong” in this part of the drawing.
The present study shows how detailed historical anatomical information, either correct or not, can be integrated in anatomical lectures, anatomical dissection and discussions on anatomical details, and it shows also how anatomy can be learned from historical studies. The authors experienced that the combination of historical and modern anatomy is a powerful teaching tool that makes anatomy lively and keeps it attractive in an artful way.
The authors acknowledge the generous help of Karen Lawson, Picture Library Supervisor, The Royal Collection, and Lauren Porter, Print Room Assistant to the Royal Collection at Winsor for supplying text and high resolution pictures of drawings RL 19058v and RL 19057r. The authors would like to thank Prof. Dr. George J. Maat, forensic physical anthropologist (LUMC) and Dr. Gerard J. Pruim, orthodontist (UMCG), as well as some of the anonymous reviewers for their valuable suggestions and expert comments to improve the manuscript.