T H E INFERIOR FRONTAL TRIANGLE-A NEW CRANIAL TRIANGLE

CRANIOMETRlC STTJDIES, No. 33 JOHN CAMERON

ONE FIQUBE

This is the first of a series of studies that will deal with a group of five new cranial triangles devised by the author. It will therefore be necessary to direct attention at the very outset to the positions and salients of these triangles. For this purpose the writer has decided to display the whole group in figure 1. A study of this will show that these triangles can be examined only in mesial sections of the skull. It may be further stated that the Hamann Museum is the only place, so far, where such a research could be carried out, as all the crania in that museum are sectioned mesially. A glance at figure 1 shows that all the triangles possess salients at the pituitary point. The latter craniometric point has been the foundation of many papers in this series of studies, so that it is hardly necessary to mention once more that it is the mesial point on the anterior border of the pituitary fossa. The other salients of the inferior frontal triangle are at the points H and G. H is obtained by finding, by means of a steel millimeter tape, the point in the mesial plane on the outer surface of the frontal bone midway between the extreme limits of that bone, namely, the bregma and the nasion. Professor Todd kindly allowed the writer to make a pencil mark on the mesially sectioned surface of the frontal bone, and the inner end of this mark became the point H. The latter is thus the point on the inner surface of the frontal bone directly internal to the point on the outer surface in the mesial plane midway between the bregma and the nasion. Similarly, G is the point where the line connecting the pitui- tary point to the glabella appears on the inner surface of the skull. As it happened, great care and skill had to be

99 AMERICAN JOURNAL OF PHYSICAL ANTRROPOLOQY, VOL. X P I I , KO. 1

JULY-SEPTEMBER. 1932

100 JOHN CAMERON

exercised in determining the position of the point G, on ac- count of the presence of the frontal crest in this region. The crest would therefore have to be filed level with the inner surface of the skull at this point. It was observed that this crest usually terminated below the level of the point H , but in a few instances due provision had to be made for the re- moval of the crest in that region also. The frontal crest is thus not represented in figure 1.

Fig. 1 Mesial section of a skull, to show the five new cranial triangles devised by the author. All five possess salients at the pituitary point (P). B is the basion, 0 the opisthion. The other pointa are on the inner surface opposite the glabella ( G ) , bregma (Br), lambda ( L ) , and the point halfway between the bregma and the nasion.

The research work on which Craniometric Studies nos. 33, 34, and 35 are founded was carried out in the Hamann Mu- seum, Western Reserve University. The author desires to express once more his deep gratitude to Prof. Wingate Todd for the free use of the cranial material and the splendid

CRANIOMETlUC STUDIES 101

equipment of that museum. The numbers of crania examined were sixty male white, sixty male negro, sixty female white and sixty female negro. As up to that time (in 1929), only sixty female negro crania were available for study, the author decided to limit the other groups to that number. In order to obtain data for the evolutionary section of this paper, it was essential to secure crania of the anthropoids sectioned in the mesial plane. Fortunately, the museum contained crania of the four highest anthropoids sectioned in this man- ner. This was indeed a very happy circumstance, as the crania of these genera are usually preserved intact in mu- seums for exhibition purposes. The anthropoid crania exam- ined were as follows: gorilla B 170, a young adult male; orang B 551, a young adult female; chimpanzee B 240, a young adult female ; gibbon B 549, an adult male.

In figure 1 it will be observed that the parietal and occipital arcs are each represented by one triangle, whereas the frontal arc is represented by two. The writer considered this advis- able, in view of the great evolutionary importance of the frontal cranial arc, the idea being to find out which of these two triangles was most influenced or affected by the evolu- tionary process. It should be noted that the space between the line P-G and the base of the skull is not included in this scheme of triangles. The writer was unable to devise any satisfactory means of doing so in the meantime. This small area will therefore have to be left out of account for the present.

I n this study of the inferior frontal triangle the three sides will be examined first of all and then the three constituent angles. The method by which the angles were measured will require explanation. The points H, G, and P were marked on the mesial section of each skull, and the measurements of the three sides made by means of the Swiss pattern of crani- ometer. Each triangle was then plotted out on paper from these measurements by compasses. Each side was drawn with a fine-pointed pencil, so as to get sharp detail at the angles. Each of the latter was then measured by means of a transparent celluloid goniometer.

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THE THREE SIDES O F THE IKFERIOR FRONTAL TRIANGLE

I . The side P-G This was found to show the wide ranges of variation that

are exhibited by all cranial measurements. The maximum dimension for the male white crania was 67.5 mm. and the minimum 40.5 mm., the actual range being thus quite exces- sive, hamely, 27 mm. In the case of the male negroes the maximum was 53.2 mm. and the minimum 39.0 mm., thus yielding a range of only 14.2 mm., or almost half that shown by the whites. It was thus to be expected that the standard deviation would be much higher in the white than in the negro crania, and it was so, the respective figures being 4.505 (k 0.2773) and 2.65 (k 0.1631). The coefficients of varia- bility followed these results consistently, being 8.95 ( -L 0.55105) and 5.59 ( 2 0.3441), respectively.

The average dimension of the side Y-G of the inferior frontal triangle, as was to be expected, was found to be defi- nitely greater in the white than in the negro crania. Thus the average for the male whites was 50.4 mm. (k 0.3922), as against only 47.4 mm. (_+ 0.2307) for the male negro crania, thus yielding a racial difference of exactly 3 mm. in favor of the white crania. This result was to be expected, in view of the fact that the author some years ago(1) showed that there was a lengthening of the anterior part of the cranial base in the white as compared with the negro races. This racial difference in the length of the side P-G was consistent for both sexes, the average for the female white crania being 48.5 mm., as contrasted with 47.4 mm. for the female negroes.

The next step in the investigation was to ascertain the dimensions of the side P-G in the four higher anthropoids. I n this group the examination of the gorilla skull provided a surprise, for though its external dimensions were greater than those of the chimpanzee and orang skulls, its P-G length made a much poorer showing. This is due to the enormous extent to which the frontal sinuses are developed in the skull of the male gorilla, the resuIt of which is to create a marked

CRANIOMETEIC STUDIES 103

degree of encroachment upon the cranial cavity in this region. Thus the measurements provided by the chimpanzee and orang skulls were, respectively, 42 mm. and 35.5 mm., as com- pared with 33.5 mm. for the gorilla skull. It was still more surprising to observe further that the dimensions of the P-G length in the latter cranium were only 4 mm. greater than those in the gibbon, by f a r the most lowly evolved member of the Simiidae. It was of interest to record the fact that the P-G length in this chimpanzee skull represented a reduction of 16.6 per cent and in this gibbon skull a reduction of as much as 41.4 per cent from the average of this dimension in the male white skull.

This comparative study of the side P-G of the inferior frontal triangle yielded one fact of some evolutionary sig- nificance. Thus it was observed that its minimum dimensions in both the white and negro crania all fell below the level of the dimension found in the chimpanzee skull. For exam- ple, the minima of 40.5 mm. and 39 mm. found in the male white and male negro compared unfavorably with the meas- urement of 42 mm. for the chimpanzee skull. It was certainly rather remarkable to note that the evolutionary gap between the Hominidae and the Simiidae was in the case of this im- portant cranial dimension not only obliterated, but actually showed a result slightly in favor of one of the highest anthropoids.

2. The side P-H This side of the inferior frontal triangle also exhibited wide

ranges of variation. The maximum dimension for the male white crania was 83.1 mm. and the minimum 63.2 mm., thus yielding the wide range of 19.9 111111. The range was rather greater for the male negro crania, the maximum in this cranial group being 84.5 mm. and the minimum 62.1 mm., the actual range being 22.4 mm. The standard deviation for the male white crania was 4.39 (f 0.2702) and for the male negroes 4.05 ( f 0.2493). The coefficients of variability fol- lowed these results consistently, being 6.01 (k 0.37003) and 5.704 (t 0.3511).

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The average dimension of the side P-H of the inferior frontal triangle, like that of the side P-G, was also found to be greater in the male white than in the male negro crania, although the racial difference, namely, 1.9 mm., was not nearly so great as one would have expected. The respective dimen- sions were 73 mm. (t 0.3822) and 71.1 mm. ( * 0.3526). We were, however, thereby enabled to find in this portion of the cranial wall definite evidence of the expansion that was tak- ing place in order to provide accommodation for the evolu- tionary growth of the frontal lobes of the brain. In fact, this expansion in the higher races of mankind was made all the more impressive by comparison with the measurement found in a Melanesian skull(2) which yielded a P-H length of only 65 mm. The racial difference in the Hamann Museum female crania was only 1.9 mm., the average P-H length for the female white crania being 70.2 mm. and for the female negro crania 68.3 mm.

The next step was to ascertain the dimensions of the P-23 length in the four highest anthropoids. In these the chim- panzee repeated its supremacy in the case of the side P-G by again presenting the greatest dimension, namely, 52 mm. The much more massively developed gorilla skull showed a measurement of only 46 mm. Moreover, the comparatively small orang skull yielded a dimension exactly equal to that of the gorilla cranium. This fact may probably suggest that the cranial measurements of the anthropoids display the usual wide ranges as in man. The calculation of the aver- ages of these will, of course, have to be postponed until a sufficient number of anthropoid crania becomes available. The smaller skull of the gibbon naturally lagged f a r behind the others, with a P-H dimension of only 33.5 mm., thus once more placing this genus at the bottom of the higher anthro- poid group. Note in passing that the P-H length in this chimpanzee skull represented a reduction of 17.4 per cent and in this gibbon skull a reduction of 46.8 per cent from the average of this dimension in the male white skull. That is to say, the P-H length in the gibbon skull was only a little

CRANIOMETRIO STUDIES 105

more than one-half of the average dimension for the male white skull.

The minimum measurements for the P-H dimension were, as was to be expected, found in the negro crania. These naturally indicated that the nearest approximations to the corresponding dimensions found in the Simiidae existed in the negro crania. It was indeed interesting to record the fact that a gap of only 9 mm. separated the minimum meas- urement (found in a female negro skull) from the measure- ment of 52 mm. yielded by the chimpanzee skull. There was, however, still this definite evolutionary gap existing between the Hominidae and the Simiidae.

3. The side H-G This might with advantage also be termed the chord H-G

(fig. 1) . It also exhibited the usual wide ranges of variation. For instance, the, maximum dimension in the male white crania was 56.1 mm. and the minimum 40.2 mm., the actual range being 15.9 mm. In the case of the male negro crania the maximum was 54.5 mm. and the minimum 36.5 mm., thus yielding the extensive range of 18 mm. At this point it should be noted that, just as in the cases of the P-G and P-H lengths, the minimum dimensions of the H-G length were found in the negro crania. The standard deviation for the male white crania was 3.45 (k 0.2124) and for the male negro crania 3.801 (k 0.23402). The coefficients of variability fol- lowed these results consistently, being, respectively, 7.18 (k 0.44207) and 8.48 (+ 0.5221).

The average length of the side H-G of the inferior frontal triangle, like that of the other two sides, was ascertained to be slightly greater in the male white crania than in the male negro crania, the respective measurements being 45.5 mm. (k 0.3004) and 44.1 mm. (k 0.3309). This racial difference, though small, appeared a t first sight to possess some degree of evolutionary importance. On further investigation, how- ever, it was found that dolichocephaly was a factor which could not be excluded. This fact was brought very forcibly

106 J O H N CAMERON

to the mind of the writer after he had examined an extremely dolichocephalic Melanesian skull( 2) . This possessed an H-G dimension of as much as 48 mm., that is to say, well above the average for the male white crania. It was therefore clear that this dimension would probably be found to be relatively long in the dolichocephalic type of skull, even of the lowest races. This observation indicated of course that the H-G dimension could be counted out as possessing no real evolu- tionary significance.

The study of this cranial length in the higher anthropoids yielded some points of interest. It proved to be longest in the chimpanzee skull, which gave an H-G measurement of 34.5 mm. This genus thus made the best showing among the Simiidae for all three sides of the inferior frontal triangle. The orang skull took second place, with 31 mm. The very massive gorilla skull yielded a measurement of only 26 mm., while the much smaller gibbon skull waa of course at the bottom of the list, with an H-G length of 17 mm. It was sig- nificant to note that the H-G length in this particular chim- panzee skull represented a reduction of 24.1 per cent and in the gibbon skull a reduction of as much as 62.6 per cent from the average of this dimension in the male white crania. That is to say, the H-G length in the gibbon was only a little more than one-third of the average of this dimension in the male white crania.

It was interesting to be able once more to record the fact that an evolutionary gap of only 2 mm. separated the mini- mum measurement, namely, 36.5 m., found in the negro crania, from the measurement yielded by the chimpanzee skull. In this connection, it is important to mention at this point that throughout the whole of his craniometric work the author has been constantly impressed by the high stand- ing of the chimpanzee in the anthropoid group.

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THE THREE ANGLE8 OF THE INFERIOR FRONTAL TRIANGLE

1. The angle H-G-P This angle was found to vary from a maximum of 107.5'

to a minimum of 91.2' in the case of the male white crania, thus yielding a range of 16.3', which would be regarded as not excessive. The range was not quite so much in the male negro crania, the maximum for this group being 108.2" and the minimum 92.5", the actual range being thus 15.7". The higher maximum found in the negro crania was, as it proved, a fact of considerable significance, as it W ~ S observed that this angle tended to increase in dimensions as one descended the evolutionary scale. For the same reason it was noted that the lowest minimum Tas to be found in the white crania. The standard deviation of this angle was 4.806 (2 0.2959) in the male whites and slightly less in the male negroes, namely, 4.61 (k 0.2838). The coefficients of variability followed these results consistently, being 4.85 (c 0.4223) and 4.59 (c 0.2826), respectively.

The average size of the H-G-P angle proved to be 98.7" (-I- 0.4185) in the male white crania. The male negro crania, on the other hand, yielded the slightly higher average of 100.3' (c 0.3974). As this racial difference was more than three times the amount of the probable error of the mean for the white crania, it evidently possessed some degree of significance. This result therefore suggested that this angle tended to exhibit a greater average size in the lower racial type. This point, however, will have to be confirmed by an examination of other types representative of the black races, whenever these become available.

A study of the H-G-P angle in the four highest anthropoids yielded rather equivocal results. The chimpanzee skull fur- nished an angle of 84.3" and the orang skull an angle of 88.3", both of which were decidedly below the average for the white and negro crania. It is, of course, probable that in these anthropoids the angles will be found to display the usual wide ranges of variation as found in man. This impression was

108 JOHN .CAMERON

confirmed by an examination of the gorilla skull, which pre- sented an E-G-P angle of 102.5". The gibbon skull pos- sessed an angle of 93".

2. The angle G-H-P In the male white crania this angle varied from a maximum

of 57.5" to a minimum.of 33.5", thus yielding an actual range of 24", which was extensive for such a small angle. The range of variation in the case of the male negro crania was not quite so extensive, the maximum for this group being 51" and the minimum 34", thus giving an actual range of 17". The standard deviation of this angle in the male white crania was 4.209 (k 0.2591) and much less in the male negroes, namely, 3.43 ( f 0.2111). The coefficients of variability followed these results consistently, being 10.45 ( I+ 0.6434) and 8.28 ( k 0.5097).

The average size of the G-H-P angle in the male white crania was ascertained to be 41.9" (t 0.3665). The average for the male negroes was practically the same, namely 41.4" (k 0.2986). It was therefore apparent that the dimensions of this angle showed very little difference in these two racial groups.

The dimensions of this angle in the four anthropoid crania were well above the averages given above for the human crania. A glance at figure 1 will show that this angle would tend to increase with a recession of the forehead and con- sequent shortening of the P-H length in lower mammalian types. The chimpanzee skull yielded an angle of 53", the orang skull an angle of 49.5", the gorilla skull an angle of 44.6", and the gibbon skull an angle of 57.3". It will be noticed that the gorilla skull most nearly approached the human average, while the gibbon skull, as one would expect, was the most remote.

3. The angle H-P-G This angle was ascertained to vary in the case of the male

white crania from a maximum of 45.6" to a minimum of 34",

CRANIOMETRIC STUDIES 109

thus providing an actual range of 11.6", which was less than half that shown by the previous angle. I n the male negro crania the dimensions varied from a maximum of 48" to a minimum of 33.5", the actual range in this case being thus 14.5". The standard deviation for the male white crania was 3.406 ( k 0.2097), and was slightly less for the male negroes, namely, 3.28 (k 0.2019). The coefficients of varia- bility followed these results consistently, namely, 8.601 ( f 0.5295) and 8.54 (-t 0.5258).

The average size of the H-P-G angle in the male white crania proved to be 39.6" (2 0.2965). The male negro crania yielded a slightly smaller result, namely, 38.4" ( 2 0.2856). As this racial difference was more than three times the amount of the probable error of the mean for the white crania, it is possible that it may possess some degree of significance. This observation, however, will require con- firmation by an examination of other types representative of the black races, whenever these become available. It may be noted a t this point that the H-P-G angle was found to be the smallest constituent angle of the inferior frontal tri- angle in the human crania.

The size of this angle in the chimpanzee and orang crania was slightly above the averages for the white and negro crania, the dimensions being 41.6" and 42.6", respectively. The size yielded by the gorilla skull was much less, namely, 33.3", and still less in the gibbon skull, namely, 29.5". Here the question of wide ranges of variation would, of course, have to be considered.

Now that all three constituent angles of the inferior frontal triangle have been studied, one is enabled to note that the racial differences, as between white and black, were small and indeed insignificant. One would thus gather that the general shape and the configuration of this triangle were not very much different in the Hamann Museum white from what they were in the Hamann Museum negro. It will be shown, however, in Craniometric Study no. 35, that the average size of the triangle is definitely greater in the Hamann Museum

110 JOHN CAMERON

white than in the Hamann Museum negro. This is, of course, a very important observation in relation to the evolution of the frontal cranial arc. The alterations in the dimensions of the three angles in the anthropoid apes were due in great measure to marked reductions in the dimensions of the three sides of the triangle, when contrasted with the Hominidae. It was of interest to note that these reductions affected the H-G side most of all, then the P-H side, the P-G side being thus affected least of all. Even in the case of it, the reduc- tion was of course considerable.

CONCLUSIONS

1. This paper deals with the sides and angles of a new cranial triangle that has been devised by the author.

2. The three sides of the inferior frontal triangle exhibit small thought definite and consistent increases of their aver- age lengths in the Hamann Nuseum male white skull, as con- trasted with the Hamann Museum male negro skull. The evolutionary advantage in favor of the whites is, however, not so great as one would have expected.

3. The minimum dimensions for all three sides were found to reach their lowest ebb in the Hamann Riiuseum negro crania.

4. The three constituent angles of the inferior frontal tri- angle showed very small racial differences in the negro cra- nia, as compared with the white crania. It thus followed that the general shape and configuration of this triangle were not very much different in these two cranial groups.

5. The alterations in the dimensions of the three constituent angles of the inferior frontal triangle in the Simiidae were due to an unequal degree of reduction of the dimensions of the three sides of the triangle.

LITERATURE CITED

1 CAMERON, JOHN 1926 The lengthening of the anterior portion of the cra-

2 1918 Two remarkable skulls from the New Hebrides. Trans. nial base in the white races. Am. J. Phys. Anthrop., IX.

N. 5. Instit. Sci., XIV.