Brain and braincase

Bruner et al  - JAnat 2015Parietal lobes are a main source of morphological variation within humans and within hominids. This month we have published a study on the relationships between bones and lobes, to evaluate how and how much such variation in the cerebral areas can influence the variation of the corresponding cranial points. There is a size correlation  between parietal bones and parietal lobes, but it is small. There is a lot of individual variation, most of all in the precuneus. Changes of the brain proportions do not seem to influence the spatial relationships of the bones. Therefore, when the boundaries of the parietal lobes change, the boundaries of the parietal bone do not. It is like the brain “slides” under the bones, without strict constraints due for example to the connective meningeal interface. So, the larger the parietal lobe, the more it approaches the frontal bone. Such lack of marked correspondence between bones and lobes suggest cautions when using cranial landmarks to estimate brain boundaries, like in neurosurgery or in paleoneurology. Now, two hypotheses can be put forward, taking into consideration that the growth of the parietal area in our species is characterized by a very early post-natal stage. First hypothesis: such lack of correspondence can be present since the beginning, and hence that early parietal bulging will separate the limits of the parietal lobe and bone. Second hypothesis: during that early stage parietal lobe and bone grow together (the latter in response to the former), but later stages of spatial changes in the anterior districts (frontal bone and lobe) alter their original correspondence. This study deals with modern humans, and it will be interesting to consider the same spatial relationships in other primates. Nonetheless, at least for Homo sapiens, we can say that between parietal lobes and parietal bones there is a good geometrical correspondence (overall curvature), a modest size correlation (length), and a variable spatial relationship (boundaries).

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