Part 1

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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.

On the right side the division of the parietal may also have begun with a cleft in the anterior border of the bone, but, owing to the advanced state of obliteration of the upper portion of the coronal suture on this side, the existence of the cleft cannot be fully ascertained. Here also the abnormal suture, at first wholly open, runs for the first 26 mm. directly backwards; at this point the suture, still quite patent, takes a turn somewhat sharper than that on the left, and proceeds for 16 mm. backwards and upwards; here it takes a second turn, and proceeds almost directly upwards towards the sagittal suture. This last portion of the abnormal suture is considerably obliterated, and on and beyond the temporal ridge is scarcely traceable. The point at which the division has reached the sagittal suture is situated a little behind the middle of the latter. The abnormal as well as the open part of the coronal suture on this side shows a simpler serration than the corresponding sutures on the left side.

In this specimen there is on neither side any encroachment of the lower portion of the parietal bone upon the frontal, such as Ranke lays stress on in the case of his orangs. A second skull of an adolescent male chimpanzee, in the Museum of Natural History, has a decided bend in the coronal suture, not unlike that which Ranke describes, and which, as he thinks, generally indicates an old parietal division; but in this case the bend is situated between the inferior and superior boundaries of the prominent temporal ridge, and apparently owes its origin to the latter (Figs. 2, 3, 4).

The main interest in the case just described centres in the direction of the abnormal sutures, and in the clearness with which the two divisions appear as equivalent and of the same origin, although one divides the parietal completely, while the other is restricted to one of its angles.

As to the course of the abnormal suture in the parietal bone, in all the cases thus far reported, the division runs in a horizontal direction (cases of Tarin, Soemmering, Gruber, Hyrtl, Welcker, Turner, Putnam, Dorsey, Ranke, and others); or it runs obliquely from or near the middle of the lambdoid suture to some part of the temporo-parietal suture, the sphenoidal angle, or the lower portion of the coronal suture (cases of Curnow, Ekmark, Gruber, Hyrtl, Lucae, Welcker, Putnam, Traquair, Ranke); in a case of Simia silenus described by Gruber and in an Egyptian cranium described by Smith, the divisions run to the lambda and begin respectively slightly above the pterion and at it. In Boyd's and in two of Hyrtl's cases, the abnormal suture begins at or below the bregma on the coronal margin of the parietal bone, and ends at or near its mastoid angle; finally, in Blumenbach's (cited by Welcker), Bianchi's, Fusari's, and Coraini's cases (those of Coraini include two monkeys) the division is vertical, passing between the temporo-parietal and sagittal sutures. The left division in our chimpanzee approaches those in Gruber's Simia silenus and Smith's cases; but it originates much higher anteriorly, and terminates slightly below the lambda on the occipital border of the parietal. The division in the right parietal of the chimpanzee, beginning slightly below the middle of the anterior border of the bone, and ending slightly back of the middle of its sagittal border, has no analogy among the cases previously described.

The difference in extent and terminations of the two abnormal sutures in the chimpanzee is of particular interest in connection with the problem of the significance and origin of those divisions of the parietal bone that involve more or less only one of its angles.

Since the observations of Toldt, and more recently of Ranke, on the development of the parietal bone in the human embryo, it appears, though it cannot as yet be said whether the fact is or is not general, that the bone originates from two centres of ossification. These centres appear in most cases one directly above the other, but, as Ranke himself shows, and as can hardly be otherwise, these primitive components of the parietal do not always show the same relations in size or position. The centres blend together, ordinarily, at the end of the third or during the first half of the fourth month of ftal life. On this account, the typical, complete, horizontal division of the human parietal bone, when met with at any time after the fourth month of ftal life, is generally interpreted to-day as a retardation of the union, or a persistence of separation, of the two original segments of the bone. Opinion, however, is still unsettled as to the significance of the more atypical, oblique divisions of the parietal, particularly of those where the separation is limited to one angle. Up to the recent contribution on the subject by Ranke, the weight of opinion on the point, although rather briefly expressed, seems to have been in favor of attributing to these smaller, oblique divisions, the same significance as was given to the more typical, horizontal ones. Gruber, in reporting a new case of a bilateral oblique suture in the parietal bone, calls the separated mastoid angles "the secondary posterior parietals." Hyrtl and Welcker advance no definite theories on this point, though the latter expresses an opinion that in both the horizontal division and the separation of the mastoid angle of the parietal bone the development of the condition may be identical. In 1883 Prof. F. W. Putnam, in describing one of his Tennessee skulls with an abnormal oblique suture in each parietal, referred the development of the separated mastoid angle on the right side, as well as the larger oblique inferior portion of the parietal on the left side, to a "separate centre" of ossification. Ranke opposes both Gruber's and Putnam's opinion, and presents instead a theory somewhat vague and not satisfactorily demonstrated, by which he accounts for the origin of oblique sutures from partial horizontal sutures in the parietal bone through "half-pathological processes." In his words, "the oblique parietal suture is allied to the half-pathological conditions of the skull; it is wholly unjustifiable to speak, as W. Gruber has done, of a separate Parietale secundarium posterius, severed by the suture, as of a typical, in a certain sense normal, formation. The oblique parietal suture is nothing more than an incomplete (posterior), true, i. e., typical, parietal suture with a sagittal course, modified by certain half-pathological conditions." These half-pathological conditions are produced, the author explains on the preceding page, "durch Einknickung der nach Herrn G. H. Meyer 'plastisch' aufwaerts gebogenen hinteren Scheitelbeinraender."

This opinion of Ranke calls for a few words about the incomplete horizontal parietal sutures. These sutures are apparently very rare in human adults, only five instances being on record (4 Ranke's, 1 Turner's). They are more frequent in orangs (Ranke), and quite common (as Ranke shows, and as I found independently before Ranke's publication of his observations) in the human embryos near term and in new-born or very young infants. In the human family, these partial divisions of the parietal generally begin in the posterior part, and run sagittally to the posterior border of the bone, ending in this border at or near its middle. In orangs the incomplete horizontal divisions seem to begin, as a rule, in the anterior part, and end at or near the middle of the anterior border of the parietal. The length of these divisions varies from a few millimetres to several centimetres, and they even reach up to the centre of the parietal bone. These divisions are, without doubt, the remains of the original anterior and posterior clefts, or, if we go a step further, of the original intervening antero-posterior space between the original inferior and superior segments of the parietal. From the very first contact of the growing centres, the median extremity of these clefts is bounded both below and above by a mass of bone; and when the anterior or posterior border of the parietal comes finally in contact with the frontal or occipital bone, the anterior and posterior sagittal clefts, if they still exist, lie between two well-developed, firm portions of the bone. Under these circumstances it is quite impossible to imagine any disturbance, mechanical or pathological, that could affect solely or mainly the median portion of the cleft, and cause a deflection downward in this portion of the division, or cause its extension to the inferior border or even the anterior-inferior angle of the parietal.

There are only two factors that can possibly affect and modify the course of the incomplete parietal suture, and both of these would show their influence mainly or entirely on the distal portion of the same. These two factors are, first, an abnormal development, either defective or excessive, of one of the original parietal segments; and, secondly, influences that would interfere with the freedom of full growth of the anterior or posterior border of the parietal.