Human Anatomy - Kotsan I. Y. 2009

Skeleton of the Head
Age, sex, and individual characteristics of the skull

The Skull exhibits well-defined age-related structural features. In newborns, the neurocranium is relatively larger than the viscerocranium, which is due to the somewhat delayed Development of the masticatory apparatus compared to the Brain and Sense Organs. The Proportions of the neurocranium and viscerocranium differ significantly between adults and newborns. The newborn's face is short and wide. In lateral view, The ratio of the facial to the cranial surface area is 1:8 in newborns, 1:6 in two-year-olds, 1:4 in five-year-olds, 1:3 in ten-year-olds, 1:2.5 in adult females, and 1:2 in adult males. The frontal and parietal eminences are very prominent in the newborn skull, and when viewed from above, the skull appears pentagonal in shape. Paranasal Sinuses are absent in the newborn skull, with the exception of the maxillary sinus, which is also barely developed. All rough surfaces and bony projections serving for Muscle attachment are weakly expressed. Cranial sutures are absent in newborns. The most characteristic feature of the newborn skull is the fontanelles.

Fontanelles (fonticuli) are unossified Connective Tissue (membranous) areas of the cranial vault. There are a total of six fontanelles: two located along the midline of the cranial vault (frontal and occipital) and four lateral ones (sphenoid and mastoid) situated on the lateral surfaces of the skull (Fig. 50).

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Fig. 50. Newborn skull (after R. D. Sinelnikov)

A — lateral view, B — superior view. 1 — anterior fontanelle, 2 — sphenoid fontanelle, 3 — greater wing of Sphenoid bone, 4 — frontal eminence, 5 — Nasal bone, 6 — Lacrimal bone, 7 — Zygomatic bone, 8 — Maxilla, 9 — Mandible, 10 — tympanic ring of Temporal bone, 11 — squamous part of temporal bone, 12 — lateral part of Occipital bone, 13 — mastoid fontanelle, 14 — occipital squama, 15 — posterior fontanelle, 16 — parietal eminence, 17 — frontal suture

The largest of these, the anterior (frontal) fontanelle (fonticulus anterior), is diamond-shaped and located between the two halves of the Frontal bone and both parietal bones (at the junction of the coronal and sagittal sutures); it closes during the 2nd year of life. The posterior (occipital) fontanelle (fonticulus posterior) is triangular in shape and situated between the occipital bone and the two parietal bones (at the junction of the lambdoid and sagittal sutures); it closes by the second month of life. The lateral fontanelles are paired. The anterior of these, the sphenoid fontanelle (fonticulus sphenoidalis), is located at the junction of the greater wing of the sphenoid bone with the frontal and parietal bones and the squamous part of the temporal bone; it closes by the 2nd–3rd month of life. The posterior, the mastoid fontanelle (fonticulus mastoideus), is situated at the junction of the parietal and occipital bones with the mastoid process of the temporal bone; it closes by the 2nd–3rd month of life. The dimensions and timing of fontanelle closure exhibit considerable individual variation, depending on The rate of bone ossification. Additional fontanelles are sometimes observed. In the region of the fontanelles, the brain is covered only by a thin connective tissue membrane, beneath which the pulsation of cerebral Arteries can be easily palpated through the Skin.

The presence of fontanelles and the absence of formed sutures and synostoses between certain parts of bones such as the frontal, occipital, sphenoid, and temporal bones render the newborn skull pliable. This is of great functional importance as it allows the cranial bones to shift significantly, enabling the skull to adapt to the shape and size of the birth canal during delivery.

After birth, skull growth occurs unevenly. Three periods of skull GROWTH AND DEVELOPMENT are distinguished in postnatal ontogeny.

The first period — from birth to 7 years — is characterized by intensive growth of the skull, particularly in its occipital region.

The second period—spanning from age 7 to the onset of Puberty (12–13 years)—is characterized by slow yet steady growth of the skull, particularly in the region of its base.

The third period—from 13 to 20–23 years of age—is once again marked by intensive growth, predominantly of the facial Skeleton, along with The Emergence of Sexual Dimorphism.

In adulthood, the cranial sutures gradually obliterate as the syndesmoses between the BONES OF THE cranial vault transform into synostoses.

In old age, the cranial bones become somewhat thinner and more fragile, and additional cavities develop. Due to tooth loss and Atrophy of the alveolar process of the jaws, the height of the facial skeleton decreases with age.

The human skull also exhibits certain sex-related features. Owing to larger overall body size, the male skull is generally larger than the female skull. The cranial capacity averages 1559 cm³ in males and 1347 cm³ in females; however, the relative cranial capacity per centimeter of body length is actually greater in females than in males. Males tend to have a more developed facial skeleton, whereas females exhibit a more prominent neurocranium. The external surface of the female skull is smoother; various prominences and roughnesses for muscle and ligament attachment are less pronounced than in males. Bone wall thickness is greater in males than in females. Male eye sockets are relatively larger, and the paranasal sinuses are more pronounced. The superciliary arches are less developed in the female skull. The forehead is more vertical in females, whereas in males it is more sloping. These sexual differences in the skull are relative and in most cases require confirmation by examining other PARTS OF THE skeleton, particularly the pelvis.

The shape of the skull in modern humans varies within a fairly wide range.

Skull size and shape are assessed using craniometry—a specialized system of measurements. The most frequently determined parameters include the longitudinal, transverse, and vertical diameters of the neurocranium. The longitudinal diameter represents the maximum fronto-occipital length of the skull, the transverse diameter corresponds to the maximum width, and the vertical diameter measures the maximum height from the skull base to the vault. Skull shape is classified based on the cranial index.

With an index up to 74.9, the skull is classified as dolichocranic; from 75 to 79.9, as mesocranic; and 80 and above, as brachycranic.

Among the various shapes of the facial skeleton, distinctions are made between narrow, medium, and wide types, as well as those with pronounced forward projection (prognathic), moderate projection (mesognathic), and slight projection (orthognathic). When viewing the HEAD in profile, it can be observed that in some individuals the upper part of the skull projects more, while in others the lower part does. By drawing a straight line connecting the glabella to the most anterior point of the upper jaw, one can determine the angle between this line and the horizontal plane passing through the external auditory meatus and the inferior orbital margin—known as the facial angle. The value of this angle typically ranges from 80⁰ to 90⁰. A decrease in this angle indicates cranial prognathism, whereas an increase signifies orthognathism. The skull is more orthognathic in newborns and children than in adults, and more prognathic in males than in females.