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Correlation of Types of Cortical Grain Structure with Architectural Features of the Human Skull ’ WILFRID T. DEMPSTER Department of Anatomy, University of Michigan, Ann Arbor, Michigan ABSTRACT Seven grain-form relationships, as indicated by the split-line pat- terns, are recognized in the cortical bone of the adult human skull: (1) random pat- tern of braincase, (2) planes and (3) ridges with elongated grain, (4) troughs with transverse grain, (5) concavities with circular grain, (6) edges, and (7) spines. Con- cavities may show superimposed trough or ridge structure, and troughs may be marked by localized ridges and planes. That is, trough patterns are dominant over concavity patterns, and ridge patterns are dominant over both trough and concaviby patterns. Finally, there are a few small cranial areas that are random distributions in some skulls and planes in others; the skull vault proper, however, except for the forehead region and internal sagittal markings, has a random pattern throughout. The mechanical significance of the various patterns and the areas on which they are found are discussed and explained on the basis of principles of mechanics and architecture. The form-texture relationships are discussed as architectural features of the skull, and their adequacy and limitations are analyzed in terms of their reaction to force systems and their proneness to fracture. Does the grain structure of cortical ink; instead of round holes, the punctures bone make the human skull stronger or usually produced elongate, ink-marked weaker? An attempt to answer this ques- “split-lines’’ that indicated a grain direc- tion brings to light several unrecognized tion in the organic matrix. They were sim- relationships. Distinctive patterns of grain ilar in appearance to Langer’s (1861) texture are found on the eminences, fos- split-lines of the skin, Hultzkranz’s (1898) sae, processes, and certain other markings split-lines in joint cartilage, Benninghoffs of the adult skull, and similar forms all (’34) split pattern in mucous membrane, over the skull have similar grain patterns. Buhler’s (’34) puncture patterns in laryn- These form-texture relationships at first geal cartilages, and Ilberg’s (’35) punc- sight suggest that some mechanical ad- ture patterns in nasal cartilages. vantage might accrue from the relation- Although Tappen (’53) published pho- ship. This paper, then, (1) examines the tographs showing split-line patterns in an several form-texture relationships to clar- adult human skull, Benninghoff’s original ify the various grain patterns that are re- study, including illustrations of the facial, peatedly found, (2) lists nearly 150 skull lateral, and interior basal regions, is the forms with their characteristic textures to most complete demonstration to date of attest to the generality of the finding, and the grain of the adult skull. Dowgjallo (3) examines the mechanical and archi- (’32) and Seipel (’48) reported more tectural features of these form-texture re- highly detailed split-line studies of the lationships with regard to the weak and mandible. (A study of vascular canals in strong features of skull architecture and the cortex of the mandible - Dempster to fracture mechanics. Finally, the cur- and Enlow, ’59-by an ink injection- rent interpretations of force “trajectories” clearing technique, revealed still more of the skull are criticized. elaborate patterns of 11 tracts.) Bruhnke Forty years ago, Benninghoff (’25) (’29) used the split-line technique on the demonstrated an architectural pattern or horse skull, Henckel (’31) on several “grain” in the fibromatrix of the cortex of mammalian skulls, Tappen (’53, ’54, ’64) several flat bones, including those of the on primate skulls, and Ahrens (’36) on skull. Bones were decalcified in acid and 1 This investigation was entirely supported by Na- punctured with a needle dipped in India tional Science Foundation grant GB-356. AM. J. ANAT., 120: 7-32. 7 8 WILFRID T. DEMPSTER various child skulls. None of these studies, ately suggested itself. This recognition of however, covers the entire skull or cor- a few recurring patterns was ineffectual relates grain with structural features. until a fresh approach was used. The present study reports a more ex- Three skulls that had been sawed to tensive investigation than others have show the interior of the braincase were made into the grain patterns of the bones first painted neutral grey. Exterior and of the whole skull, excluding the mand- interior surface forms were then marked ible. The split-line technique was used on by painting them different colors, each six complete adult skulls (new commer- color corresponding to a different shape or cial osteological material) and on miscel- form. Initially, the colors were used mere- laneous parts of some damaged discards. ly to distinguish planar regions, elevated The skulls were soaked overnight in 10% (convex) areas, and depressed (concave) formalin to fix the protein constituents; surfaces. Later, further distinctions were then the skulls were decalcified for short made : elongate ridges were considered periods in 5% or 10% HC1. The material apart from domes and saddle-shaped in acid was tested with a needle at inter- areas; troughs were distinguished from vals until the thinnest regions could be circular basins, and so on. Some of the easily punctured. If left too long, exces- finer distinctions proved unnecessary sive decalcification resulted in blurring when the corresponding grain patterns and smudging of the split-lines. A sew- were compared; others, however, were ing needle, mounted in a jeweler’s pin vise consistently correlated with differences in and dipped in India ink, was used to punc- grain patterns. ture the decalcified bone; after each punc- As the split-line skulls were examined, ture, excess ink was wiped away with a seven patterns of grain, corresponding bit of wet cotton wool to avoid indiscrim- with seven skull forms, emerged as appar- inate staining. After the thin areas had ently the most basic. These seven pat- been treated, the skulls were returned to terns included almost all the surface of the acid for further decalcification of the the skull. Many of the forms in the seven thicker areas; these areas were then punc- categories of the new grouping have stan- tured as above, with little deterioration of dard anatomical names, but there are the split-lines in thin areas. others that have no specific names. Best results were obtained when the Since the new grouping of seven forms, specimens were decalcified in a buffer each with a specific grain pattern, applies mixture of formic acid (8N) and sodium to so many forms, both named and un- formate (1s).Most of the material was named, it will be important to develop a stored in 80% alcohol and could be pho- new terminology in which both form and tographed after draining. Other material texture are indicated, a terminology that was allowed to dry slowly and thoroughly cannot be confused with standard ana- to produce hard, and somewhat shrunken, t omical nomenclature. dry specimens. The latter could be han- The seven form-texture types are dia- dled and demonstrated more easily than grammed in figure 3; the illustration also the specimens stored in alcohol. includes an eighth type to be treated separ- ately. Each has a name and a code letter FORMS AND TEXTURE that will identify patterns in the photo- The three drawings of figure 1 show graphic illustrations. In the following dis- most of the formations of the skull. Figure cussion of these types, conventional ana- 2 shows the same views with a superim- tomical terms such as “eminence,” “crest,” posed grain pattern based on the foregoing and “fossa” are avoided, since they are material. However, before these summary generalized descriptive terms that apply drawings could be made, a number of equally to bones, viscera, and surface skull specimens marked with split-lines anatomy. The present need, rather, is for had to be examined. Although careful terms relating specifically to bone struc- study showed that certain form and tex- ture and denoting both the physical forms ture relationships recurred in different and the grain architecture associated with regions, no clue to interpretation immedi- each. Thus, “ridge” and “trough” will be CORTICAL GRAIN AND SKULL ARCHITECTURE 9 Fig. 1 Three views of the adult skull showing by shading the principal forms, apart from those of the nasal cavity. A, intact skull; B, right zygomatic arch cut away; C, much of left side of cranial vault removed to show interior. used to designate both the specific forms tions, or saddle-shaped elevations. These they describe and the underlying micro- forms may lie on either plane or moderate- organization of the forms. ly curved surfaces. In each instance, the B, Braincase cortex (fig. 3) shows a dis- grain direction shown by split-lines runs persed pattern of random lines. This pat- parallel to the less convex axis of an eleva- tern is found on both convex and concave tion; that is, parallel to the crest of the surfaces that have relatively large radii of curvature; for example, in most of the ridge. The grain runs parallel to the ridge exterior and interior of the skull vault. whether the top of the ridge is straight, P, Plane surfaces show either a par- convex, concave, or sigmoid - but the allel grain or, in triangular areas, a radi- grain is always perpendicular to that cross ating, fanlike pattern of grain. section of the ridge having the smallest R, Ridges and crests show the most radius of curvature. (This is true even for clearly expressed pattern. They may be the occipital condyle, as shown in figs. 2, plano-convex elevations (like the “barrel 5, where the ridge lies obliquely across arches” of architecture), elliptical eleva- the length of the condyle.) 10 WILFRID T.

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