GEN ANA

• The head and neck region of the body contains many important structures compressed into a relatively small area.
• The head is formed mainly by the skull with the brain and its covering meninges enclosed in the cranial cavity.
• The cranial nerves, their component parts, their function, and the openings through which they exit from the skull are summarized in Table 11.6.
• The olfactory, optic, and vestibulocochlear nerves are entirely sensory; the oculomotor, trochlear, abducent, accessory, and hypoglossal nerves are entirely motor; and the remaining nerves are mixed.
• The origins and courses of the cranial nerves are described on page 605.
• The special senses, the eye and the ear, lie within the skull bones or in the cavities bounded by them.
• The brain gives rise to 12 pairs of cranial nerves, which leave the brain and pass through foramina and fissures in the skull.
• All the cranial nerves are distributed to structures in the head and neck, except the 10th, which also supplies structures in the chest and abdomen.
• The skull is composed of several separate bones united at immobile joints called sutures.
• The connective tissue between the bones is called a sutural ligament.
• The mandible is an exception to this rule, for it is united to the skull by the mobile temporomandibular joint.
• The bones of the skull can be divided into those of the cranium and those of the face.
• The vault is the upper part of the cranium, and the base of the skull is the lowest part of the cranium.
• The skull bones are made up of external and internal tables of compact bone separated by a layer of spongy bone called the diploe.
• The internal table is thinner and more brittle than the external table.
• The bones are covered on the outer and inner surfaces with periosteum.
• The cranium consists of the following bones, two of which are paired: Frontal bone, Parietal bones, Occipital bone, Temporal bones, Sphenoid bone, and Ethmoid bone.
• The facial bones consist of the following, two of which are single: Zygomatic bones, Maxillae, Nasal bones, Lacrimal bones, Vomer, Palatine bones, Inferior conchae, and Mandible.
• At birth, the mastoid antrum lies about 3 mm deep to the floor of the suprameatal triangle.
• The mandible has right and left halves at birth, united in the midline with fibrous tissue.
• As growth of the skull continues, the lateral bony wall thickens so that at puberty the antrum may lie as much as 15 mm from the surface.
• The anterior fontanelle is diamond shaped and lies between the two halves of the frontal bone in front and the two parietal bones behind.
• In old age, the size of the mandible is reduced when the teeth are lost.
• During childhood, the tympanic plate grows laterally, forming the bony part of the meatus, and the tympanic membrane comes to face more directly laterally.
• Although the tympanic membrane is nearly as large as in the adult, it faces more inferiorly.
• The tympanic part of the temporal bone is merely a C-shaped ring at birth, compared with a C-shaped curved plate in the adult.
• Most of the skull bones are ossified at birth, but the process is incomplete, and the bones are mobile on each other, being connected by fibrous tissue or cartilage.
• The angle of the mandible at birth is obtuse, the head being placed level with the upper margin of the body and the coronoid process lying at a superior level to the head.
• The two halves fuse at the symphysis menti by the end of the Ist year.
• The bones of the vault are ossified in membrane; the bones of the base are ossified in cartilage.
• The fibrous membrane forming the floor of the anterior fontanelle is replaced by bone and is closed by 18 months of age.
• This means that the external auditory meatus is almost entirely cartilaginous in the newborn, and the tympanic membrane is nearer the surface.
• The posterior fontanelle is triangular and lies between the two parietal bones in front and the occipital bone behind.
• By the end of the Ist year, the fontanelle is usually closed and can no longer be palpated.
• The mastoid process is not present at birth and develops later in response to the pull of the sternocleidomastoid muscle when the child moves his or her head.
• It is only after eruption of the permanent teeth that the angle of the mandible assumes the adult shape and the head and neck grow so that the head comes to lie higher than the coronoid process.
• Anterior View of the Skull: The frontal bone, or forehead bone, curves downward to make the upper margins of the orbits.
• As the alveolar part of the bone becomes smaller, the ramus becomes oblique in position so that the head is bent posteriorly.
• The bones of the vault are not closely knit at sutures, as in the adult, but are separated by unossified membranous intervals called fontanelles.
• The superciliary arches can be seen on either side, and the supraorbital notch, or foramen, can be recognized.