Correlation of Types of Cortical Grain Structure with Architectural Features

Seven grain-form relationships, as indicated by the split-line patterns, are recognized in the cortical bone of the adult human skull: (1) random pattern 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. Concavities 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 bone make the human skull stronger or weaker? An attempt to answer this question brings to light several unrecognized relationships. Distinctive patterns of grain texture are found on the eminences, fossae, processes, and certain other markings of the adult skull, and similar forms all over the skull have similar grain patterns. These form-texture relationships at first sight suggest that some mechanical advantage might accrue from the relationship. This paper, then, (1) examines the several form-texture relationships to clarify the various grain patterns that are repeatedly found, (2) lists nearly 150 skull forms with their characteristic textures to attest to the generality of the finding, and (3) examines the mechanical and architectural features of these form-texture relationships with regard to the weak and strong features of skull architecture and to fracture mechanics. Finally, the current interpretations of force “trajectories” of the skull are criticized. Forty years ago, Benninghoff (’25) demonstrated an architectural pattern or “grain” in the fibromatrix of the cortex of several flat bones, including those of the skull. Bones were decalcified in acid and punctured with a needle dipped in India AM. J. ANAT., 120: 7-32. ink; instead of round holes, the punctures usually produced elongate, ink-marked “split-lines’’ that indicated a grain direction in the organic matrix. They were similar in appearance to Langer’s (1861) split-lines of the skin, Hultzkranz’s (1898) split-lines in joint cartilage, Benninghoffs (’34) split pattern in mucous membrane, Buhler’s (’34) puncture patterns in laryngeal cartilages, and Ilberg’s (’35) puncture patterns in nasal cartilages. Although Tappen ( ’53) published photographs showing split-line patterns in an adult human skull, Benninghoff’s original study, including illustrations of the facial, lateral, and interior basal regions, is the most complete demonstration to date of the grain of the adult skull. Dowgjallo (’32) and Seipel (’48) reported more highly detailed split-line studies of the mandible. (A study of vascular canals in the cortex of the mandible Dempster and Enlow, ’59-by an ink injectionclearing technique, revealed still more elaborate patterns of 11 tracts.) Bruhnke (’29) used the split-line technique on the horse skull, Henckel (’31) on several mammalian skulls, Tappen (’53, ’54, ’64) on primate skulls, and Ahrens (’36) on 1 This investigation was entirely supported by National Science Foundation grant GB-356.