AN ABNORMAL SKULL FROM NEW GUINEA, WITH REMARKS ON THE STRUCTURE OF THE MANDIBLE BY J. EKBLEM GULLBERG AND A. N. BURKITT, M.B., B.Sc. From the Department of Anatomy of the University of Sydney INTRODUCTION THERE recently came into our hands, through the courtesy of Dr W. Simmons of Bexley, Sydney, N.S.W., four skulls from the mandated territory of New Guinea. One of these was so remarkable, even at first glance, that we decided to carefully describe it, as we considered it might throw some light upon the subject of bone growth. The outstanding feature of the skull is the marked thickening of the whole body of the mandible, especially in the region below the molar and premolar teeth. A feature which is even more remarkable, perhaps, though not so obvious at first glance, is the fact that the two remaining anterior teeth in the lower jaw actually have alveolar bone growing up, entering the root canal, and there expanding so as to retain these teeth in position. Further, nearly all the remaining teeth show marked shortening of their roots and corresponding shallowness of their root sockets. Nevertheless, the teeth appear perfectly healthy, in so far as any local disease is concerned. Attached to one of the zygomatic arches by some vegetable fibre, was the skull (minus mandible) of a dog. This abnormal human skull, with its re- markable mandible, is said to have come from the Madang district on the mainland of New Guinea (old German New Guinea)., No further history is available. DESCRIPTION OF THE MANDIBLE The thickening of the body of the lower jaw is most marked in the molar and premolar regions, but continues forwards to the symphysis. The alveolus is not involved to any appreciable extent. For descriptive purposes, we will distinguish a right from a left sub-molar thickening, as well as an anterior enlargement. As a result of the thickening in the molar regions, the normal bracket-like projection of the alveolar arch in the region of the third molar, on the lingual aspect of the bone, has been completely obliterated on the left side, but on the right side we can still discern some trace of the overhang (Fraser, p. 260). The obliteration is due to the addition of bone below the mylohyoid line, as shown by the presence of some shelving away on the right side (max. transverse thickness of body = 20 mm.) and the left side (max. transverse thickness of body = 28 mm.). The left side has completely lost the normal shelving away. The alveolar bone has apparently remained unaltered on both sides of the jaw. The left sub-molar thickening has an irregular 42 J. E. Gullberg and A. N. Burkitt lingual grooving partly overhung by a thin lamina of bone at one spot. This groove is apparently more or less continuous with the mylohyoid groove and extends forwards to within 1 cm. of the digastric fossa. About 6-8 definite minute foramina, presumably nutrient, are scattered around and along this groove, being possiblyassociated with the greater bulk of bone and its nutritive requirements. We have not observed them in normal mandibles. On the right side this grooving is absent, but the foramina are similarly distributed. When viewed from below both sub-molar thickenings are sharply defined anteromedially, possibly in correlation with the presence of the anterior belly of the digastric muscle (fig. 1). On the right side, the condition is more sharply defined, a definite smooth groove being present, extending back from the digastric fossa. There is a faint but definite grooving on the lower and Fig. 1. Drawing of Mandible of Skull No. 856 from below. Not to scale. outer sides of the right and left sub-molar thickenings. This groove lies anterior to the centre of the thickening and is more marked on the left side. At first we had thought this grooving was due to the external maxillary artery, but we came to the conclusion that it was probably too anterior in position and sloped forward too acutely, for it to be due to the artery. The thickening of the bone anteriorly has resulted in a "rounding off" of the chin, with dis- appearance of the mental tubercles. The greatest labio-lingual thickness of the bone here is on the left side of the chin region (22 mm.), this being in agreement with the greater thickness in the left molar region. Anteriorly on the lingual aspect, the overgrowth of bone has been largely below the spina mentalis, but there has almost certainly been some overgrowth above the spine, inasmuch as we find the bone above the spina mentalis is raised into a broad transverse elevation, in contrast to the usual flat surface Abnormal Skull 43 to be seen here. This transverse ridge leads to the formation of what may be called a negative pit below it, the spina mentalis lying partly in and partly below this pit. Above, this ridge shades off into the alveolar bone, being itself 9-10 mm. in vertical extent. The digastric fossae are somewhat smooth and ill-defined. On the outer aspect of the body on both sides the linea obliqua presents a distinct projection, immediately below the third molar. This projection somewhat resembles a swelling frequently to be seen in what are apparently perfectly normal jaws. This swelling we have termed the trihedral eminence, when seen in the normal jaw. Fig. 2. Orthogonal projection of Norma Lateralis of Skull No. 856. The mental foramen on the right side is below the alveolar ridge between Pm. 1 and 2. On the left side it is approximately below Pm. 2. Both foramina lie approximately at the normal level below the alveolar margin. The alveolus and the sockets will be considered when describing the teeth. The charac- teristic features of the rami are (1) their obliquity and the slight but definite elongation of the condyloid process (fig. 2), and (2) the poor development of the muscular markings (e.g. masseteric and internal pterygoid). As regards the degree of obliquity and elongation of the ramus, we believe that, while 0 44 J. E. Gullberg and A. N. Burrkitt by no means usual, it lies within the normal range of variation. The angle of the jaw on both sides has been greatly-flattened out, and is consequently very difficult to define. This rounding off of the angle is partly due to the heaping up of the bone in the molar thickenings. Hence the mandible belongs to the form which, when seen in the normal jaw, is sometimes called "rocking" (Sullivan). Another factor concerned in the rounding off of the angle may be the apparent elongation of the ramus, similar to that which is said to occur in acromegaly (Keith). We may mention here, however, that Geddes distinctly Fig. 2a. Photograph of Norma Lateralis of Skull No. 856. states that it is the body and not the ramus of the mandible which had elongated in the remarkable and apparently typical case of acromegaly described by him. The crista endocondyloidea, the torus verticalis, and the crista endocoronoidea are all present and well defined. No trace is to be seen of the linea intermedia (or mylopharyngeal line, Lenhossek). As regards the internal structure, an examination of a cut section andof X-ray photographs, shows clearly the original outline of the mandible (fig. 4). To this original outline as much as 18 mm. of bone have been superposed in Abnormal Skull 45 some regions. This original outline forms a thick inner table of compact bone (up to 6 mm. in thickness), buried in the bone, but visible by X-ray and on section. It is separated from the outer, or surface layer of compact bone, by cancellous bone, which, however, is extremely dense, with only minute interspaces. The outer, or surface table varies from 1 to 5 mm. in thickness, being thinnest on the medial aspect. The original or inner compact layer is Fig. 3. Photograph of Norma Facialis of Skull No. 856. apparently broken through by cancellous tissue on either side, close to where it joins up with the surface compact layer. The fact that the inner table of bone agrees very closely both in shape, orientation, and absolute dimensions with a normal jaw, is the main argument in support of its representing the original outline of the jaw. On attempting to correlate our X-ray findings with those given by Walkhoff, certain discrepancies were observed. The so- 46 J. E. Gullbery and A. N. Burkitt called trajectorium dentale seems to show up very markedly, while the trajectorium basale seems to be narrower than normal as it approaches the head of the condyle. THE INTERNAL ARCHITECTURE OF THE NORMAL MANDIBLE Further study of X-ray photographs, and sections of aboriginal and other jaws, led us to the conclusion that the ramus of the jaw could best be considered as a tube extending obliquely from the condyle into the - \ body of the bone. We found, however, that Walkhoff had already elaborated this view, A I although subsequent workers had not fully realised this, nor are Walkhoff's figures by any means clear upon the subject. The upper portion of this tube extends from the head of the mandible to the mandibular foramen. Its' interior is filled with cancellous bone. In the mandible of the Australian aboriginal, the Fig. 4. Scale drawing of cut section of tube is often almost rounded on section, and left half of Mandible of No. 856 compared with a similar section of projects sometimes more markedly on the a normal European mandible (both outer side of the ramus, and sometimes more sections through the posterior root markedlyonthe inner side, havingbeentermed socket of the second molar tooth).