Human Anatomy - H. I. Koliadenko 2009

Musculoskeletal System
Skull Skeleton
Development of the Skull

The Development of the cranial base and the cranial vault occurs differently. The base and the facial Skeleton undergo three Selection/3.html">Stages of development: membranous, cartilaginous, and bony, whereas the cranial vault undergoes only two stages: membranous and bony. From the middle of the second month of embryonic development, ossification centers appear in various PARTS OF THE cartilaginous cranial base and the membranous cranial vault, from which Bone tissue begins to develop. Bone-forming Cells, or osteoblasts, play the primary role in Bone Formation. Osteoblasts function in conjunction with osteoclasts—Cartilage-destroying cells. Two stages of development are characteristic of the following bones or their parts: the two halves of the Frontal bone (which later fuse into one), the parietal bones, the squamous part of the Temporal bone, and the squamous part of the Occipital bone. All other BONES OF THE neurocranium go through all three stages of development.

Ossification of the cranial bones is not completed during the prenatal period.

The Development of the bones of the facial skeleton is associated with the presence of a special branchial apparatus in the human embryo during early stages, consisting of branchial arches formed from mesenchyme. The upper and lower jaws, the styloid process of the temporal bone, the Middle ear ossicles (malleus, incus, and stapes), the Hyoid bone, and the laryngeal cartilages develop from the branchial arches. The pouch-like spaces of the branchial arches, located between the visceral arches, give rise to the external acoustic meatus, the tympanic cavity of the middle ear, and the auditory tube connecting the nasopharynx to the middle ear.

The bones of the facial skeleton develop from Connective Tissue and are therefore primary bones.

The human Skull changes throughout life. After the age of thirty, the sutures gradually ossify, and the skull becomes a single monolithic Structure. In elderly individuals, Osteoporosis of the bones occurs, resulting in a decrease in their thickness. The upper and lower jaws lose their Teeth, the alveoli resorb, and the height of the jaws decreases by approximately 1/3. The face of an elderly person comes to resemble that of a child.

The SKULL OF A newborn (Fig. 62) accounts for one-quarter of the infant's body length, whereas in an adult it constitutes only one-eighth of the body length. The facial region of the skull is still insufficiently developed and in size does not exceed one-quarter of the neurocranium. The jaw bones are underdeveloped. Since the child is born without teeth, the cavities in the upper jaws are underdeveloped, and alveolar processes and sockets are absent. Certain bones of the newborn's skull consist of separate parts. For instance, the frontal bone has two halves, between which a perpendicular suture forms that sometimes persists into adulthood. The temporal bone consists of three parts—squamous, tympanic, and petrous—while the mastoid part of this bone develops only after birth. In the tympanic part of the temporal bone, the external acoustic meatus and tympanic membrane are underdeveloped. The occipital bone consists of four separate bones that subsequently fuse. The body of the Sphenoid bone is connected to the greater wings by cartilage, which slowly ossifies after the child's birth.

The cranial vault of the newborn features prominent parietal eminences and frontal tuberosities. Viewed from above, it has a quadrilateral shape. There are yet no sutures on the cranial vault, and the edges of the bones lack serrations. Between the vault bones lie layers of connective tissue and cartilage known as fontanelles. There are six fontanelles in total—two unpaired superior and two paired lateral. The largest is the unpaired anterior fontanelle, located between the frontal and parietal bones superiorly. It has an irregular diamond shape and closes in children 1.5–2 years after birth. The second unpaired posterior fontanelle lies between the posterior angles of the parietal bones and the squamous part of the occipital bone. It closes During the first months of the child's development. Sometimes it is absent in newborns, as it may close during the final month of fetal development. The lateral fontanelles are located between the anterior and posterior lateral angles of the Parietal bone. They are termed the sphenoid and mastoid fontanelles. The paired sphenoid fontanelle lies at the junction of the lateral angle of the parietal bone with the greater wing of the sphenoid bone and the squamous part of the temporal bone. The mastoid fontanelle lies at the junction of the inferior angle of the parietal bone with the mastoid process of the temporal bone and the occipital squama.

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Fig. 62. Skull of a newborn infant (lateral view):

1 — anterior fontanelle; 2 — sphenoid fontanelle; 3 — greater wing of sphenoid bone; 4 — frontal squama; 5 — Nasal bone; 6 — Lacrimal bone; 7 — Zygomatic bone; 8 — Maxilla; 9 — Mandible; 10 — tympanic ring; 11 — squamous part of temporal bone; 12 — mastoid process; 13 — lateral part of occipital bone; 14 — mastoid fontanelle; 15 — occipital squama; 16 — posterior fontanelle; 17 — parietal eminence

Both fontanelles close immediately after birth or during the final period of intrauterine development. In newborns, pneumatic bones lack sinuses, with the exception of the rudimentary maxillary sinus of the upper jaw. Their development occurs gradually after the child's birth. All fontanelles are remnants of the membranous skull.

It is not always possible to determine the sex from an adult skull, and even when sexual characteristics are evident, it is noted that the male skull is somewhat larger than the female. Muscle attachment sites, tuberosities, and crests are less pronounced in females, making the skull surface smoother. The frontal and parietal eminences are significantly smaller in females, and the profile of the forehead is more vertical.

The shape of the skull also varies according to individual and racial characteristics.

To determine the shape of the skull, longitudinal and transverse measurements and their ratios are used. Spreading calipers are used for cranial measurements. The longitudinal dimension of the skull is the length from the nasion to the external occipital protuberance. The transverse dimension is measured between the most prominent parts of the zygomatic bones. From these measurements, the so-called breadth-length (cephalic) index is derived using the formula: width $a$ is to length $b$ as $X$ (the unknown dimension) is to 100 ($a : b = x : 100$), from which

for example, length $b = 18$ cm, and width $a = 13$ cm. Thus, $13 : 18 = x : 100$, from which

If the width is greater, for example $a = 15$, then the index will be

The shape of the skull depends on the cephalic index. If the cephalic index is less than 75, such a skull is termed dolichocephalic. Such skulls are characteristic of Representatives of the Negroid race. A skull with an index exceeding 80.0 is termed brachycephalic. This skull shape is typical of the Mongoloid race. Skulls with an index ranging from 75 to 80 are called mesocephalic and are found in Caucasoids. This Classification of skulls among different human races gave rise to racist theories asserting that skull shape is allegedly a sign of human intellectual development and natural inequality. Skull shape was linked to Skin color, and racists claimed that people with dark skin and wide or long skulls were mentally retarded and of a low cultural level. However, anthropologists have proven that racial characteristics of skull structure and skin color are caused by the climatic conditions of their habitat, whereas the mental and physical development of humans is determined by social living conditions and the state of society's scientific and technological progress.

When viewing skulls from above, one can observe various shapes: pentagonal, or five-sided, when the frontal tuberosities are well developed (more frequently in females) along with the parietal eminences, and the occipital bone protrudes as a fifth eminence; ovoid, or egg-shaped, when eminences are absent and the greatest width of the skull falls within its posterior third; ellipsoid, or oval, also smoothed, with the greatest width in the middle part of the skull.