The human skull is the most complex part of the skeleton as it is a unique set of bone structures housing a variety of organs located in the head. Skull bones hold and protect the human brain and the sensory organs of vision, hearing, smell, and taste. They also include all the necessary sinuses and foramina through which nerves and vessels travel. Bones of the skull form the upper airways and oral cavity, serving as attachment sites for the masticatory and facial muscles. Understanding the human skull anatomy is necessary for a wide range of professionals from doctors (dentists, oral surgeons, neurosurgeons, etc.) to anthropologists, neuroscientists, and even artists or sculptors.
There are 22 bones in the human skull and some of them are paired. Some authors also include one more bone in the skull — the hyoid, which is located under the mandible near the larynx and not directly attached to the other bones of the skull. The skull is anatomically divided into two parts — the neurocranium and the facial (visceral) skull (1). During embryonic development, bones of the neurocranium (ethmoid, sphenoid, frontal bone, two parietal bones, two temporal bones and the occipital bone) are formed from the mesoderm — a primary germ layer that from which connective tissue and muscles usually originate. Facial bones are formed from the neural crest — an ectoderm-derived embryonic layer found only in vertebrates. The ectodermal layer is responsible for the formation of the nervous system, skin epidermis and tooth enamel (2).
Flat skull bones (parietal, frontal, occipital, nasal, lacrimal, and the vomer) — are among a number of small bones in the human body that are formed by intramembranous ossification. In this case, the formation of bone tissue is not preceded by the formation of cartilage (2). The ossification is completed only after 20 years when the sutures of the skull that connect the cranial bones are ossified. Before this,the flexible joints between cranial bones are very important for the brain development and avoiding trauma in childbirth (3).Skull bones differ in density and elasticity. The densest bone of the skull is the temporal bone, and the masseter attachment site on the zygomatic process of the temporal bone is the stiffest part of the skull (4).
The bones of the cranial vault consist of outer and inner layers of cortical bone, between which the spongy bone tissue is located. This may contribute to redistribution of the outer traumatic influence energy, so that compact bone tissue would stay undamaged even in the event of the spongy tissue damage (5).
The structure of the skull bones is to a large extent determined by and interconnected with the anatomy of the sensory organs, situated in the head, as well as nerves, blood vessels and brain structures. Sphenoid, temporal and ethmoid bones probably have the most complex structure. For example, the temporal bones harbor cavities for the organs of hearing and balance, as well as 10 channels for branches of the cranial nerves and blood vessels (1). As many as 15 to 20branches of the olfactory nerves penetrate the cribriform plate of the ethmoid bone to connect olfactory receptors in the nasal cavity with the olfactory bulb in the brain. Apart from its role in olfaction, this region may serve as a route for several pathogens to enter the brain (6). The sphenoid bone contains a depression called the sella turcica, where the pituitary gland (hypophysis) is located.
Surprisingly not all the functions of skull bone elements have been fully described to date. In particular, there is no clarity about the evolutionary role of paranasal sinuses in the ethmoid, frontal, and maxillary bones (7). According to some accounts, they can improve the sense of smell and the acoustic properties of the skull, maintain immunity in the nasal cavity, or participate in thermoregulation (8, 7). The latter hypothesis, however, remains debatable (9).
The first people to gain systemic knowledge about the skull structure were the ancientEgyptians. As they practiced embalming of dead bodies, most of their methods involved removing the brain without damaging the face. Consequently, Egyptians had to learn the anatomy of the skull to access the brain through nasal cavities or the foramen magnum. Later, a major contribution to the study of skull anatomy was made by theGreek physicians Herophilus and Erasistratus from Alexandria. We know about their lost works because they were much quoted by Galen, the most well-known anatomist, surgeon and physician of ancient times (10).
The works of Galen remained the main source of anatomical knowledge in Europe throughout the Middle Ages. The nomenclature of skull bones was updated by the anatomists of the Renaissance and modern times, based on the Greek and Latin terms. Renewed interest in anatomical studies emerged not only due to the removal of a formal ban on the dissection of human corpses for research purposes by Pope Sixtus IV in 1472, but also because of changes in the artist’s view of the image of the human body that had shifted towards greater realism (11, 12).
Artists and anatomists of the Renaissance began to cooperate, creating anatomical illustrations. According to some sources, the anatomist Marcantonio della Torre planned to create one of the first anatomical atlases together with Leonardo da Vinci, whom he had invited as an illustrator, but the scientist died from the plague, not having time to complete this work (13). There is some evidence indicating that the first anatomical atlas of the modern type “De humani corporis fabrica”, published by Andreas Vesalius in 1543, was created in collaboration with Jan van Calcar, a famous artist, a pupil of Titian (14).
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