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

by Aleš Hrdlička

In case the upper segment was not vertically above the lower one, but in a position a little more forward or backward of it; and, furthermore, if the relative growth of the two segments differed, and their separation remained permanent,—the separation of any portion of the parietal bone in almost any form and to almost any extent might result. Such coincidence of anomalous conditions, although necessarily rare, cannot, from what we know on the subject in parietal and other bones, be declared improbable. All cases where oblique suture on one side co-exists with more or less horizontal suture on the other side in the parietal bone, as in the second of Putnam's cases, would of course point directly to a similar origin of the anomaly on both sides of the cranium. That such cases have not been more frequently observed is largely due, I think, to the rarity of bilateral parietal divisions.

A third mode of development of the oblique suture in the parietal bone suggests itself where the severed portion of the bone is small, and that is the possible existence of a supernumerary, third centre of ossification. I am by no means ready to defend this theory, yet there are cases in which it would afford the easiest explanation. I have a Peruvian skull at hand, in which there is a bilateral, quite symmetrical quadrangular separate piece of bone, encroaching on the mastoid process of the parietal. The surface of the left parietal bone in this skull measures across its middle in antero-posterior direction 120 mm., in infero-superior direction 130 mm.; similar measures of the right parietal are respectively 117 and 130 mm. The separate bone on the left measures across its middle in antero-posterior direction 20 mm., in infero-superior direction 12 to 21 mm.; the same portion on the right measures respectively 25 and 11 to 15 mm. Both pieces are joined to the parietal bone by a squamous suture ([Fig. 6]).

Fig. 6 (99/3550). Quadrilateral Fontanel Bones in a Peruvian Male Skull, encroaching upon the Mastoid Angle of the Parietals.

It is apparent that the separate pieces of bone in this case are too small to be easily taken for representatives of one of the regular centres of ossification of the parietal bone; but the same pieces are somewhat too large, and especially too singularly outlined and joined to the parietal, to be without difficulty diagnosed as simple Wormian or fontanel bones. One of Ranke's cases, [17] though the separation of the mastoid angle is oblong instead of quadrangular, as in the Peruvian skull, seems to me to present a similar difficulty in properly diagnosing the nature of the severed portion. This group of cases needs further observation, particularly on the bones of infants and embryos. I have two monkey skulls at hand which actually show a multiplicity of the original segments of the parietal. These specimens will be described in a future publication.

So much as to the formation of the oblique sutures in the parietal. It should not be forgotten that such sutures can be simulated by those which divide true Wormian or fontanel bones from the parietal. The distinction between the real oblique parietal and these extra-parietal sutures must depend largely on the extent of the division and form of the separate piece of bone.

We may now return to the skull of our chimpanzee. In considering the nature of the divisions in the parietal bones of this skull, we can at once and absolutely discard the idea of the divisions being due to fractures, or being boundaries of Wormian or fontanel bones, and thus really extra-parietal in their nature. There is nothing about the sutures, or the divided pieces, or the neighboring bones, that would even suggest such an explanation; and in our records on Wormian and fontanel bones we find no analogies either in man, or apes, or lower animals, to the conditions here observed. The necessary conclusion from this can only be that we have before us two examples of real parietal division.

The division on the left side, had it existed alone, would be readily acceptable as an instance of the "parietal suture." The anterior extremity and more than the anterior third of the course of the division correspond exactly to the same features of a typical, horizontal "parietal suture;" while the elevation of the posterior extremity of the division, though unusual, can readily be explained as due to an excess in growth of the inferior original centre of the bone, which may, in addition, have been situated slightly posterior to the upper centre.

The division in the right parietal of the chimpanzee begins at its anterior end, and runs for the first third of its course in the same way as that on the left side; its posterior end, however, does not reach the lambdoid, but turns up and ends in the sagittal border. Should this formation have existed alone, I should be inclined to consider it either as the result of an accessory centre of the parietal, or, possibly, as a persistence of the anterior portion of the divided superior centre of the bone, the posterior portion of the same being united with the lower segment of the parietal in the usual way. With the division of the left parietal in the same skull before me, everything points to a similar origin of the division on both sides, and to the right as well as the left division being a true "parietal suture," deflected less on the left and more on the right side by a disproportion in growth of the two original, regular segments of each of the bones.