A Bilateral Division of the Parietal Bone in a Chimpanzee

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A Bilateral Division of the Parietal Bone in a Chimpanzee; with a Special Reference to the Oblique Sutures in the Parietal.

By ALE HRDLIKA

AUTHOR'S EDITION, extracted from BULLETIN OF THE American Museum of Natural History, VOL. XIII, ARTICLE XXI, pp. 281-295.

New York, Dec. 31, 1900.

The Knickerbocker Press, New York

=Article XXI.=--A BILATERAL DIVISION OF THE PARIETAL BONE IN A CHIMPANZEE; WITH SPECIAL REFERENCE TO THE OBLIQUE SUTURES IN THE PARIETAL.

By ALE HRDLIKA.

The first to describe a case of division of the parietal bone in apes was Johannes Ranke, in 1899. The skull in question is that of an adolescent female orang, one of 245 orang crania in the Selenka collection in the Munich Anthropological Institute. The abnormal suture divides the right parietal into an upper larger and a lower smaller portion. "The suture runs nearly parallel with the sagittal suture," but, as the illustration shows (Fig. 1), it descends in its posterior extremity towards the temporo-parietal suture, and terminates in this a few millimetres in front of the lambdoid suture. The abnormal suture shows but little serration, and the articulation of the two divisions of the parietal bone is squamous in character, the lower portion overlapping the upper. Below the junction of the abnormal with the coronal suture, the latter takes a pronounced bend forward. A similar bend in the coronal suture is present in the same specimen on the left side. This is common among the other orang skulls in the collection. The portions of the coronal suture below and above the bend differ somewhat in character.

Besides the above-mentioned complete division, Ranke found among the 245 orang skulls 13 with incomplete division of the parietal bone. The division consisted invariably of a longer or shorter remnant of a horizontal "parietal suture," ending in the coronal suture at the top of the bend above referred to. A similar anterior remnant of an abnormal parietal suture was found by Ranke in a young chimpanzee skull; but the author questions the word "chimpanzee," which evidently means that the identity of the skull is somewhat doubtful.

In consequence of his finds, Ranke believes both complete and incomplete divisions in the parietal bone to be much more frequent in the orang than in man. He also thinks that the bend usually present in the coronal suture in the orang signifies that, "even where there are no traces of a parietal suture, such a suture has actually existed in an earlier stage of development." This implies the development of the adult parietal bone in the orang from two original segments, one above the other.

The divisions which I am about to describe occur, one in each parietal, in the skull of a nine-year-old male chimpanzee, which was captured, when young, in West Africa. Later on he was one of the attractions of the Barnum and Bailey Circus, and was familiarly known as Chico. The chimpanzee died in 1894, since when his skin and bones have been preserved in the American Museum of Natural History, New York City. Prof. J. A. Allen, the curator of the Zoological Department of the Museum, has kindly given me permission to describe the skeletal parts for publication.

The most interesting part of Chico is unquestionably the skull. The divisions of the parietal bones which the specimen presents are not only the first complete divisions of the parietal observed in a chimpanzee, but are also unique in character, no divisions of the same nature having been observed before, either in man, in apes, or in monkeys. The position and extent of the divisions in this skull will throw considerable light on the question of the aberrant, complete divisions of the parietal bone, by which term may be designated divisions differing from the typical horizontal ones.

The skull under consideration shows in general a good development and an almost perfect symmetry. The capacity of the brain cavity, measured according to Flower's method, is 390 c.c.

The masculine features of this skull, and particularly the temporal ridges, are not quite as marked as those of another skull of an adolescent male chimpanzee in the Museum. The temporal ridges are slightly prominent, and in their middle third, over part of the frontal and the parietal bones, not more pronounced than in some human crania. They are, however, situated very high. Their upper lines or boundaries touch each other over a part of the sagittal suture, a little back of the bregma; while the lower lines approach to within 6 mm. of the sagittal suture. The supraorbital ridges are not very massive, although prominent to such a degree that, when the skull rests on the occipital condyles and on the teeth, the plane of the orbits is almost vertical. The sagittal crest is insignificant; the occipital crest is high, but not very massive. The zygomatic arches are less strong than they are in an average white male; and the mastoids are small, even smaller than in an average adult white female.

The second dentition is incomplete; the third molars have not reached the level of the opening of their sockets. The condition of the sutures, so far as their patency is concerned, does not bear the same relation to the stage of dentition as it does in man: all the sutures of this skull are more or less obliterated. There are no signs on any part of the skull that point to the closure of any of the sutures as premature. In detail, the condition of the sutures is as follows: The spheno-maxillary articulation is completely closed, but still plainly traceable. Of the various facial sutures, only remnants are open; the suture in the zygomatic arch, however, is almost fully patent on both sides. The spheno-frontal articulation is completely obliterated on the left, but traces of it remain on the right side. The left temporo-sphenoidal and squamo-frontal sutures (the squama of the temporal articulates with the frontal bone) are, with the exception of the basal part of the former, which remains open, quite obliterated, but on the right side both are open. The temporo-parietal sutures, with the exception of 8 mm. of the anterior end of the suture on the right side, are both entirely closed and hardly traceable. The coronal suture is partly open on the left, and wholly open on the right, up to a point a little below the middle of the anterior border of the parietal bone. At this point on each side, the lower portion of the coronal suture bends backward and continues as the anomalous suture; the upper portion of the coronal, particularly on the right, is completely obliterated, though still traceable. There are no signs left of the sagittal and lambdoid sutures, and only the basal portions of the temporo-occipital articulation remain. The palatine sutures, also, are entirely obliterated.

The skull shows no important anomalies besides the division of the parietals.

The divisions of the parietal bones begin on the left 32 mm., on the right 28 mm. (measured with a tape), above the point of junction of the coronal and temporo-parietal sutures. From the point where the anomalous sutures leave the coronal suture, to the bregma, the distance on the left is 44 mm., on the right 42 mm. The excess of size of the left over the right parietal bone along the coronal suture (6 mm.) compensates the greater height of that portion of the right temporal squama which articulates with the frontal bone. Measured across their middle from the temporo-parietal suture, the two parietals appear to be almost of equal size (left 82 mm., right 80 mm.). In an antero-posterior direction, from the beginning of the division to the middle of the parietal portion of the occipital crest, both bones measure the same, namely 75 mm.

The division in the left parietal begins at a V-shaped cleft, which is filled with a process of the frontal bone. There are slightly distinct markings on the bone and a number of insular ossicles, which make it probable that the cleft had been originally much greater and was largely filled by a Wormian or, rather, a fontanel bone, the lower border of which has subsequently united with the parietal.

For 30 mm. from its beginning the abnormal suture proceeds directly backward, and to this extent shows but little obliteration. The original cleft has, it seems, extended up to this point. From here the suture takes a slight bend upwards, and proceeds almost directly upwards and backwards, becoming gradually obliterated, until it disappears at the temporal ridge, 16 mm. from the median line. Originally the suture must have terminated on the posterior border of the parietal bone, not far from the lambda. The whole suture shows fairly good serration. The coronal suture on this side, below the division, shows serration about equal to that of the abnormal suture; the obliterated portion above this was, so far as can be seen, more simple.

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