|Anatomical terms of bone|
Evolutionarily, the human neurocranium has turned from being (just) the back part to (also) being the upper part, because during the evolutionary expansion of the brain, the neurocranium has overgrown the viscerocranium. The upper-frontmost part of the cranium also houses the evolutionarily newest part of the human brain, the frontal lobes.
The term "cranium" can be ambiguous in that it can refer to the neurocranium, or the whole skull, meaning the neurocranium and the facial skeleton.
The size of the braincase is variable among mammals. The roof may contain ridges such as the temporal crests. Below the braincase is a complex of foramina (openings) and bones, including the foramen magnum which houses the neural spine. The auditory bullae, located in the same region, aid in hearing.
Human neurocranial bones
In humans, the neurocranium is usually considered to include the following eight bones:
The neurocranium arises from paraxial mesoderm. There is also some contribution of ectomesenchyme. In Chondrichthyes and other cartilaginous vertebrates this portion of the cranium does not ossify; it is not replaced via endochondral ossification.
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- In small children, the frontal bone is still separated into two parts, by the frontal suture, which normally closes during postnatal development.
- but if they are included, the neurocranium will then have to be said to consist of fourteen bones