Mechanical effect of vocalization on human brain and meninges

Vibrations of human skull, as produced by loud vocalisation, exert a massaging effect on the brain and facilitate elution of metabolic products from the brain into the cerebrospinal fluid (CSF). In addition, these vibrations, through their effect on arachnoid villi, speed up the flow of CSF from the subarachnoid space into the blood within the superior sagittal sinus and lacunae lateralis. In this way, the speed of renewal of CSF is increased, which again contributes to a faster cleaning process of the brain. The most important feature of human evolution is enlargement of the brain. This by itself would not be enough. The Neandertals had a brain 15% larger than we have, yet they did not survive in competition with modern humans. Their brains were more polluted, because their massive skulls did not vibrate and therefore the brains were not sufficiently cleaned. In the evolution of modern humans the thinning of cranial bones was important. In addition, the chin remained jutting out of the face as in no other hominids, in order to maintain the distance from the chin to the hyoid bone equal to the distance from the latter to the styloid process. This situation facilitates transmission of laryngeal vibrations onto the skull base via the mandible.