Paper - The fontanella metopica and its remnants in an adult skull (1918)
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Schultz AH. The fontanella metopica and its remnants in an adult skull. (1918) Amer. J Anat. 23: 259.
Modern Notes: skull
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The fontanella metopica and its remnants in an adult skull
Adolf H. Schultz
Carnegie Institution of Washington
It is not uncommon to find in the skull of a newborn infant a small fontanelle between the two frontalia in their nasal third. This is usually called fontanella metopica (f. medio-frontalis, fonticulus interfrontalis inferior) (fig. 1). A considerable number of skulls, both of children and of adults, showing short, irregular, transverse or V shaped sutures or fissures in the midline of the frontal bone above the level of the superciliary ridges have been described in the literature and interpreted as remnants of the fontanella metopica. The author has found in the skull of an adult an abnormal suture, which is comparable to those above mentioned, but which is more extensive than in any of the cases previously described; accordingly its publication appears justifiable. This specimen (fig. 2) belongs to the Anatomical Department of the Johns Hopkins Medical School and was kindly placed at my disposal by Dr. W. H. Lewis.
The skull is that of an American negro, fifty-five years of age. It might be mentioned that the skin over the frontal region was absolutely normal, therefore any external factor, whether accidental or surgical (trepanation) can be excluded as the cause of the anomaly. The greatest length of the skull is 193 mm., the greatest breadth 148 mm., the basion-bregma height 128 mm. and the horizontal circumference 553 mm. The weight of the skull, including the mandible, is 985 grams, a figure, which is close to the upper limit of variation of weight for the human skull. This is an indication of the thickness of the bones of the skull, which is characteristic of the negro. Most of the sutures are obliterated, on the inner surface more than on the outer. This is also true of the internasal suture, although its course can still be recognized (the suture was retouched in fig. 2), and therefore the right nasal bone is found at its upper end to extend far into the left. On each side, at the incisura parietalis, there is a Wormian bone. The lambdoid suture is rich in Wormian bones. It is noteworthy that there is present on both sides of the mandible a well pronounced processus anguli mandibulae (apophysis lemurica) , which points downward and outward and shows rough outhnes for muscle-insertion. The latter are likewise present on the thick zygomatic arch. Attention may also be called at this point to the prominent processus marginalis on the posterior border of the malar bone. The processus anguli mandibulae assumes in our case special interest, in as much as Herpin ('07) reported that this anomaly is rare in the negro and when present is poorly developed.
Fig. 1 Frontal view of the skull of a male negro fetus with a fontanella metopica.
Fig. 2 Frontal view of the skull of a negro with an abnormal suture on the frontal bone.
The abnormal suture on the frontal bone, which is situated not exactly median but somewhat to the right, consists on its outer surface of a transverse, irregular, dentate part, 15 mm. long, and of two lateral, ascending limbs, which diverge upward and have a length of 9 and of 13 mm. on the right and on the left respectively. The distance between the upper ends of these diverging limbs is 23 mm. The middle of the transverse part is situated 25 mm. above the nasion and 15 mm. below the line connecting the two tubera frontalia. If, as according to Schwalbe ('01) the length of the frontal arc is represented as 100, then the transverse portion of the suture lies 20.3 above the nasion. It is of interest to compare the position of the abnormal suture on the frontal bone in the author's case with those reported by Schwalbe ('01), Fischer ('02) and Davida ('14). Table 1 is a compilation of the tables of the two first mentioned authors with the corresponding measurements of Davida's case and of that herein described. The figures show that the suture or fissure is always situated below the level of the tubera frontalia, and with only two exceptions always in the upper half of the nasal third of the nasion-bregma arc. In the twelve European skulls the average relative distance between the nasion and the suture is 18.8 mm., and the distance between the intertuberal Hne and the suture 18.3 mm. The average of the two negro skulls of Fischer and that of the author's case is, for the corresponding measurements, 23.2 and 12.6 mm. respectively. Therefore the suture in the negro seems to be relatively higher above the nasion and closer to the intertuberal line than in the white. The exact determination of the position of this abnormal suture is also of importance in the explanation of its origin, as will be seen later.
TABLE 1 Position of the transverse abnormal suture on the frontal hone of adults
OF THE SUTURE
OF THE SUTURE
IN PER CENT OF
50 y. 55 y.
Upon examining the inner surface of the skull, the suture is likewise found to be extensive (fig. 3). Incidentally it might be stated that the crista frontalis interna is only moderately developed, as was the case in the skull with the same anomaly described by Rauber ('03). In two of Schwalbe's cases the crista frontalis was examined, and found in both to be broad and blunt. For the most part the abnormal suture on the inner surface communicates with that on the outer surface, often allowing the passage of a fine bristle. The transverse portion presents itself on the inner surface as eight short perpendicular adjacent fissures, with a total width of 9 mm. and located 19 mm. above the foramen coecum. The lateral hmbs of the suture, which also diverge upward, are straight regular fissures, in contrast to those on the tabula externa. The right limb is 13, the left 19 mm. in length and they are 16 mm. apart at their upper ends. The bony part included by the suture is narrower but higher on its inner side than on the outer. On a horizontal section through the frontal bone, at a level somewhat above the transverse portion of the abnormal suture, a trapezium is formed by the lateral limbs, with its shorter base directed inward. This wedge-shaped piece of bone is plainly shown in Rauber's section of a similar case.
Fig. 3 Frontal bone of the skull in tigure 2 seen from the inside (upper part sawed off).
Schwalbe directed attention to the fact that adult skulls showing remnants of a fontanella metopica present an unusually large interorbital breadth. Table 2 is a compilation of tables by Schwalbe and Fischer with Rauber's case and that of the author, in which the interorbital breadths and the interorbital indices are given. The interorbital index represents the relation between the interorbital breadth and the internal biorbital breadth; the technique of these measurements may be found in Schwalbe's studies on Pithecanthropus erectus ('99). In this same work are pubhshed similar measurements of a considerable number of normal skulls of most heterogeneous races. According to these measurements, the interorbital breadth varies between 18 and 31 mm. with an average of 24.2 mm., the interorbital index lies between 20 and 30.1 with an average of 24.3. A comparison of these figures with table 2 shows that both the absolute and relative interorbital breadths of skulls showing remnants of a metopical fontanelle are much above the average.
Inlcrorhital breadth and interorhital index on adult skulls ivith the abnormal suture or fissure on the frontal bone
European European European European European
& d' d" d'
& d" d' 9 9 d' d" & d"
m m .
28.5 32.0 27.5 28.0 31.0
31.0 29.0 29.0 24.0 30.0 26.0 37.0 30.5 32.0
26.6 30.2 27.5 26.4 29.8
30.1 26.9 26.6 25.8 29.4 26.3 33.3
For the determination of the relative frequency of the anomaly in the two sexes, the material at our disposal has been much enlarged through the published cases of transverse fissures in the frontal bone of children and newborns. In Schwalbe's cases sex is stated in 9 juvenile and in 5 adult; all were male except one newborn. Fischer found in 1 newborn and in 7 adults, in which sex was known, that the female sex was represented twice.
Adding to these the case of Rauber and that of the author, both of which were males, the total of males is 21, of females 3. The anomaly, therefore, would appear to be of much greater frequency in males. This same preponderance has been found by the author ('16) in another anomaly, namely, the persistent canalis cranio pharyngeus, and this relatively greater frequency has been likewise shown in respect to other anomalies. From this it would seem probable that anomalies are more common in the male, but whether this is a rule for progressive or for atavistic anomalies, or for both, can only be determined when care is taken by investigators to always mention the sex in reporting anomalies.
Short transverse sutures or fissures occurring in the lower third of the frontal arc in adults have always been interpreted by the various authors as remnants of the fontanella metopica, but the origin of the latter has been explained in widely different ways. The metopic fontanelle was first described by Gerdy in 1837. He was followed by Hamy and the Italian scientists Maggi, Riccardi, Staderini and Zanotti. Of these, Hamy ('72) sees in the metopical fontanelle a divergence of the lines of ossification of the tubera frontalia. Maggi ('94, '98, '99) interprets the fontanella metopica as a product of the approximation of the four frontalia media. These assumptions are based upon his isolated comparative anatomical observations. Zanotti ('01) explains the medio-frontal fontanelle as the last trace of the foramen, which corresponds to the location of the paraphysis in primitive vertebrates; in other words, a foramen frontale for the paraphysis similar to the foramen parietale for the epiphysis. Both Maggi and Zanotti to a certain extent place atavistic interpretations upon the fontanelle, but these must be considered as extremely hypothetical.
Bolk ('11) w^as led to believe that the fontanella metopica arises at the site of the primitive or primary nasofrontal suture. This opinion was based upon observations on monkeys, in which the nasal bones have become shortened, that is the supramaxillary portion of the nasalia is displaced by a medial growth of the frontalia, by which process a secondary naso-frontal suture, situated closer to the apertura nasalis, is formed. This theory does not explain in a satisfactory manner the extremely rare occurrence of a true metopic fontanelle in monkeys, together with the relatively frequent appearance of incomplete nasal reduction. On the other hand, the relative frequency of the metopic fontanelle in man according to Schwalbe is 15.2 per cent in children up to 1| years, whereas high reaching nasal bones, such as are found in monkeys, have never been described in the human skull. Moreover, it must be borne in mind that the remnants of the fontanella metopica are often situated in the adult high above the nasion. As shown in table 1 , the lowest point of the remnants of the fontanelle is located as much as 27.5 per cent of the frontal arc above the nasion, its middle point, being even higher. If the fontanelle reallj^ corresponds to the original uppermost end of the nasalia, then the latter must have extended between the frontalia high above the orbits and the supercihary ridges. Bolk assumes that the supranasal portion of the frontal suture (supranasal field or triangle) — a frequent finding in adults — is the result of the reduction of the nasaha. However, this supranasal suture reaches as a rule only shghtly above the glabella and not, as Bolk supposes, to the level at which the fontanella metopica occurs.
Rauber ('06) describes the skull of a child with two fontanelles at the frontal suture (fonticulus interfrontalis superior et inferior) which in his opinion had become separated from the frontal arm of the anterior fontanelle. The fonticulus interfrontahs inferior corresponds to the metopic fontanelle, and as a factor in its remaining patent Rauber considers it possible that the site of the anterior neuropore of the medullary canal of vertebrates exerts its influence under special circumstances, even to the ossification of the skull.
Schwalbe ('01) in contrast to the explanations offered by previous authors, considers it possible that the metopic fontanelle is to be conceived as a progressive variation, which bears a relation to the greater development of the frontal lobe of the cerebrum. The adult skull described in this paper would seem to support this theory inasmuch as its capacity was 1520 cc. and its smallest frontal width was 109 mm. Both these measurements are rather large for the negro; on the other hand Fischer's cases showed the metopic fontanelle to be present in two idiots, one of them a microcephahis with a skull capacity of only 704 cc. Schwalbe in his explanation makes use of the hypothetical supposition that the tubera frontalia might consist of two adjacent ossification centers, which usually join immediately, but in exceptional cases remain separate, later forming two independent systems of lines of ossification. The divergence of these lines forms the metopic fontanelle, which in children is situated on a plane with the tubera frontalia. Schwalbe emphasizes the fact that the metopic fontanelle and its derivatives are always found at a definite location, while the fontanelles and fontanelle bones which are found at times in the upper portion of the frontal suture have a more variable situation and are to be included in the great fontanelle. Schwalbe cites among other the cases described by Staderini, in which the fontanella metopica is connected with the great fontanelle by a wide space. In spite of this, however, he makes a distinction between the two above mentioned fontanelles, which rests purely upon the situation of the metopic fontanelle. According to Schwalbe in children up to 13 months the latter varies in respect to the lower end of the fontanelle from 5.6 to 17.8, in respect to its middle point from 11.2 to 22 per cent of the frontal arc above the nasion. Fischer described the skulls of two children in which interfrontal fontanelle bones are divided in two and in three partfe respectively. In one of these the middle point of the fontanelle bone was situated 30.6 in the other 50 per cent of the frontal arc above the nasion.
It is evident that the position of the metopic fontanelle is not as definite as claimed by Schwalbe, who makes the following statement :
In the rare cases in which two or even three groups of Wormian bones occur in the frontal suture, only the lowest corresponds to the normal medio-frontal fontanelle; those situated near the parietal bones, however, are to be considered as Wormian bones in an abnormally wide suture (hydrocephalus). The latter may even represent the anterior end of the large fontanelle, which has extended abnormally far into the frontal region. It sometimes occurs that the anterior end remains open for a longer period than that portion lying directly posteriorly; therefore the anterior end may become separated as a secondary fontanelle.
This distinction of Schwalbe seems somewhat arbitrary, inasmuch as all transitions can be observed in juvenile skulls. On the basis of original observations the author is convinced that the metopic fontanelle is derived from the bregmatic fontanelle, and at some time has become separated from it. Figure 4 gives the best proof. An interfrontal suture wide at its upper part, that is, a very long arm of the great fontanelle, as shown in numbers 1, 2, 3 and 4 in figure 4 is not of rare occurrence. Among 35 skulls of infants up to a few months' old the frontal arm of the great fontanelle was found to extend six times to within 10 to 17 mm. of the nasion. In three other cases the great fontanelle reached within 22 mm. of the nasion. This prolonged arm of the great fontanelle is an extreme variation, and is not necessarily a result of hydrocephalus. In the skull of a year old hydrocephalic negro, the author found the great fontanelle reaching to within 16 mm. of the nasion; in contrast to the cases in figure 4, however, it was, even at its lower end, 17 mm. wide; in the middle of the frontal arc 29 nun. and at its upper end 35 mm. It is striking that the lowest portions of the frontal bones always approximate each other and indeed to a height which is considered typical for the position of the metopic fontanelle, that is, to a point to which the frontal arm of the great fontanella may extend uninterrupted or constricted. As a designation for the lowest portion of such long bregmatic fontanelles extending into the nasal third of the frontal arc, the name fontanella metopica may well be retained. However, no fundamental difference is to be made between the two mentioned fontanelles. It is more frequent for the lower end alone to remain patent in children and to be recognizable in adults. The constriction of different portions of the frontal arm of the great fontanelle results from locally decreased or increased growth of the lines of ossification, and may occur in any situation, but appears to be most common between the two tubera frontalia. Double constriction to form secondary fontanelles has also been described (Rauber '06). This identity of the metopic and the great fontanelle is also demonstrated by the position of the fontanelle bones, which occur anywhere from the bregma to the upper portion of the nasal third of the frontal arc (Hartmann 1869, Barclay-Smith '09 and '10, GulUver 1890). Whether the above described case of partial persistence of the metopic fontanelle in an adult was associated with a fontanelle bone can not be determined with certainty, but seems probable, especially upon examining the inner surface.
Fig. 4 Normae frontales of frontal bones of juvenile skulls with a long arm of the bregmatic fontanelle, which has been constricted in the lower cases to form a metopic fontanelle.
Fig. 5 Norma verticalis of a skull of erethizon dorsatus with fontanelle bones.
Before any definite statements can be made as to the cause of the occurrence and partial persistence of a long frontal arm of the great fontanelle, more material must be available, and attention must be paid to correlations, especially in the frontal region. The author hopes by this contribution to stimulate interest in this anomaly in order that further cases may be reported. Observations on the occurrence of fontanelle structures in the frontal bones of mammals have been reported in a limited number, and further cases would be of great value. Among 10 skulls of erethizon dorsatus, which the author collected recently, 3 cases presented paired symmetrical fontanelle bones extending far between the frontalia. Figure 5 shows one of these cases.
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