HEAD AND NECK ANATOMY

Transcription

1 CHAPTER 3 HEAD AND NECK ANATOMY Basic knowledge of the skull, facial bones, jaws, and muscles of the head and neck region are fundamental for a Dental Technician. It is important to understand the relationship of the bones and muscles as they work together to provide support for the dentition (teeth) and movement for mastication (chewing). STRUCTURE OF BONE The bones of the human skeleton provide rigid support for muscles and skin, and serve to protect the easily injured organ systems of the body. Bone itself is a living, highly vascular tissue, which is made up of both inorganic (minerals) and organic (cells & connective tissue fiber) elements. The inorganic component of bone serves as a warehouse for calcium and phosphorous, two essential minerals for the body. Bone consists of a hard outer shell called cortical or compact bone and an inner spongy, porous portion referred to as cancellous bone (fig. 3-1). Within this cancellous area are the bone marrow spaces responsible for manufacturing blood cells. The majority of blood cells are made by the bone marrow found in the long bones, such as the femur or thigh bone. A thin layer of connective tissue, called periosteum, surrounds each bone and provides nourishment through many vascular vessels. The periosteum also contains many nerve endings that respond to trauma with the sensation of pain. When a bone breaks, it is the periosteum that hurts, not the bone itself. When new bone is required, such as when a break occurs, it is the periosteum which provides the cells that make the new bone. Bone can be classified as to how it develops, its location, and its shape. Membraneous bone forms from the periosteum in successive layers and is usually flat such as those of the skull. The long bones of the arms and legs are cartilaginous bones, which develop from cartilage. BONES OF THE SKULL The skull consists of 28 bones that form the framework of the head and provide protection for the Figure 3-1. Structure of a typical flat bone. brain, eyes, and ears. It can be divided into two parts: the cranium and the bones of the face. The cranium is primarily involved in housing and protecting the brain. The bones of the face are a complex framework that helps to form facial features, the upper jaw (maxilla) and lower jaw (mandible). With the exception of the mandible and the bones of the inner ear, all skull bones are joined together firmly along seams called sutures. An example of sutures is shown in figure 3-2. Sutures are sometimes considered immovable; however, they do permit a small amount of movement and provide mechanical protection for the brain by absorbing much of the force if a blow to the head occurs. The cranium is formed by eight cranial bones, which form the foundation for attachment of many of the muscles necessary for head movements and chewing. Figure 3-3 show the cranial bones, and Table 3-1 lists them as either single or paired bones. Frontal Bone The frontal bone forms the front part of the skull above the eyes, which includes the forehead and part of 3-1

2 Figure 3-2. Sutures of a skull. Table 3-1. Bones of the Cranium Single Bones Occipital Frontal Sphenoid Ethmoid Paired Bones Parietal Temporal Figure 3-3. Cranial bones. The zygomatic process of the temporal bone projects out into the zygomatic bone of the face and forms the lateral part of the zygomatic arch. Both the glenoid fossae and zygomatic process can be seen in figure 3-4. the nasal cavity. In children, the frontal bone develops as two parts. They are usually fused together by age 5 or 6. The two frontal sinuses (air spaces in the bone) are located above each eye socket. Parietal Bones The two parietal bones are located behind the frontal bone. These bones form the greater part of the right and left sides and the roof of the skull. They each have four borders and are shaped like a curved plate. Occipital Bone The occipital bone forms the back part of the skull and the base of the cranium. It joins with the parietal and temporal bones. In the center, underside (inferior) portion of the cranium, there is a large opening called the foramen magnum (fig. 3-5), through which nerve fibers from the brain pass and enter into the spinal cord. Temporal Bones The temporal bones form the sides and part of the base of the skull in the area of the ear. One temporal bone is located on each side of the head. It is readily recognized as fan-shaped. Each encloses the internal ear structures and have depressions called glenoid fossae that forms the articulation with the mandible. Figure 3-4. Temporal bone. 3-2

3 Ethmoid Bone The ethmoid bone is situated in front of the sphenoid bone in the front part of the cranium (fig. 3-7). Through small openings in this bone pass nerves to the roof of the mouth that are responsible for sense of smell. BONES OF THE FACE The facial skeleton consists of 14 stationary bones and a mobile lower jawbone (mandible). These 14 bones (table 3-2) form the basic shape of the face, and are responsible for providing attachments for muscles that make the jaw move and control facial expressions. Figures 3-8 and 3-9 show the bones of the face. Maxillae Bones Figure 3-5. Foramen magnum of cranial cavity viewed from above. The occipital bone is an irregular, four-sided bone that is somewhat curved upon itself. Sphenoid Bone The sphenoid bone has a wing-like shape and is internally wedged between several other bones in the front part of the cranium (fig. 3-6). This bone assists with the formation of the base of the cranium, the sides of the skull, and the floors and sides of the orbits. The maxillae bones are the largest bones of the face and together form the upper jaw. The maxilla (singular) consists of a body and. four processes: zygomatic, frontal, alveolar and palatine. The maxilla forms the hard palate, floor of the nose, part of the orbits (eye sockets), and the tooth sockets of the upper teeth. Above the roots of the upper teeth and below the Single bones Vomer Mandible Table 3-2. Bones of the Face Paired bones Maxillary Palatine Zygomatic Lacrimal Nasal Inferior nasal conchae Figure 3-6. Sphenoid bone viewed from above. Figure 3-7. Ethmoid bone viewed from above. 3-3

4 Figure 3-8. Anterior view of facial skeleton. Figure 3-9. Posterior view of facial skeleton. 3-4

5 floor of the orbits are the maxillary sinuses; the largest of the sinuses. Palatine Bones The palatine bones are located behind the maxillae (fig. 3-10). The bones are somewhat L-shaped and form the posterior portion of the hard palate and the floor of the nose. Anteriorly, they join with the maxillary bone. Zygomatic Bones (Zygoma, Malar Bone) The zygomatic bones make up the prominence of the cheeks and extend from the zygomatic process of the temporal bone to the zygomatic process of the maxilla. The zygomatic bones form the cheek bones and help to form the sides and floor of the orbits. Lacrimal Bones The lacrimal bones are the smallest and most fragile of the cranial bones. These thin, scalelike structures are located in back of the frontal process of the maxilla. Nasal Bones The nasal bones are small oblong bones somewhat rectangular in shape. They lie side by side and are fused at the midline to form the bridge of the nose (nasal septum). These bones are responsible for the shape of the nose. Inferior Nasal Conchae The inferior nasal conchae are curved, fragile, scroll-shaped bones that lie in the lateral walls of the nasal cavity. They provide support for mucous membranes within the nasal cavity. Vomer Bone The vomer bone is a thin, flat, single bone almost trapezoid in shape. It connects with the ethmoid bone and together they form the nasal septum. Mandible Figure Anterior view of palatine bones. The mandible (lower jaw-bone) is the longest, strongest, and the only movable bone in the skull. Figure 3-11 illustrates the anatomy of the mandible. Figure Anatomy of the mandible; lateral view (left), inferior view (right). 3-5

6 The mandible is horseshoe-shaped, with an upward sloping portion at each end called the ramus. The rami are divided into two different processes: Condyloid process Also called mandibular condyle, located posterior on the ramus and forms the head of the mandible. It is knuckle-shaped, and articulates in the glenoid fossa of the temporal bone to form the temporal mandibular joint. Coronoid process Located anterior of the condyle, and provides attachment for the temporal s muscle, which helps lift the mandible to close the mouth. Other important anatomical landmarks of the mandible you should be able to recognize are as follows: Alveolar process Supports the teeth of the mandibular arch. Mental protuberance Also referred to as the chin and is located at the midline of the mandible. Mental foramen Located on the facial surfaces of the mandible on both the right and left sides, just below the second premolars. This opening contains the mental nerve and blood vessels. Body The heavy, horizontal portion of the mandible below the mental foramen extending from the angle to the parasyplysis region. Angle Juncture where the body of the mandible meets with the ramus. Mandibular foramen Located near the center of each ramus on the medial side (inside), through this opening passes blood vessels and the interior alveolus nerve, which supply the roots of the mandibular teeth. This is a common area where the dental officer will inject anesthetic to block the nerve impulses and make the teeth on that side insensitive (numb). BONES OF THE EAR In each middle ear and located in the auditory ossicles are three small bones named the malleus, incus, and staples (fig. 3-12). Their function is to transmit and amplify vibrations to the ear drum and inner ear. TEMPORAL MANDIBULAR JOINT The right and left temporal mandibular joints (TMJs) are formed by the articulation of the temporal bone and the mandible. This is where TMJs connect with the rest of the skull. Figure 3-13 illustrates the TMJ. The mandible is joined to the cranium by ligaments of the temporal mandibularjoint (fig. 3-14). Figure Anatomy of the middle ear. 3-6

7 Figure Temporal mandibular joint. Figure Ligaments of a temporal mandibular joint. 3-7

8 The TMJ consists of three bony parts: Glenoid fossa Oval depression in the temporal bone that articulates with the mandibular condyle. Articular eminence Ramp-shaped segment of the temporal bone located anterior to the glenoid fossa. Condyle The knuckle-shaped portion of the mandibular ramus found on the end of the condyloid process. It is positioned underneath the glenoid fossa and makes up the hinge joint of the TMJ. MUSCLES OF THE HEAD The muscles of the head can be classified into two groups, muscles of facial expression and muscles of mastication. How muscles work and function depends on the action of each muscle (movement), the type of joint it is associated with, and the way the muscle is attached on either side of the joint. Muscles are usually attached to two places: one end being joined to an immovable or fixed portion, and the other end being joined to a movable portion on the other side of a joint. The immovable portion is called the origin of the muscle, and the movable portion is called the insertion. When muscles of the head contract, the insertion end is pulled toward the origin. MUSCLES OF FACIAL EXPRESSION The muscles that are underneath the skin of the face are responsible for helping communicate our feelings through facial expression. The muscles of the mouth help us express surprise, sadness, anger, fear, and pain. Table 3-3 lists the muscles of facial expression and figure 3-15 illustrates these muscles. MUSCLES OF MASTICATION Mastication is defined as the process of chewing food in preparation for swallowing and digestion. Four pairs of muscles in the mandible make chewing movements possible. These muscles can be grouped into two different functions. The first group includes three pairs of muscles that elevate the mandible to close the mouth as in biting down. The last group includes one pair that can depress the mandible (open the mouth), make grinding actions side to side, and can make the mandible go forward in a protruding motion. Table 3-4 lists the muscles of mastication and figure 3-16 illustrates these muscles. ORAL STRUCTURES OF THE MOUTH The oral cavity (mouth) contains various structures that aid in the digestion process of food and also serves as an organ of speech and sensory Figure Anatomy of muscles of facial expression. 3-8

9 Muscle Orbicularis oris Buccinator Figure Muscle anatomy of mastication. Origin Encircles the mouth (no attachement to bone) Alveolar process of maxilla and mandible Table 3-3. Muscles of Facial Expression Insertion Corners of the mouth Orbicularis oris at the corner of the mouth Description Located between the skin and mucous membranes of the lips. Makes lips close and pucker. Located in the walls of the cheeks, holds food in contact with teeth when chewing, and assists in blowing air out of the mouth. Mentalis Mandible Skin of chin Raises and wrinkles the skin of the chin and decreases and protrudes the lower lip. Zygomaticus Major Zygomatic bone Orbicularis oris (angle of the Raises the corner of the mouth) mouth when smiling. 3-9

10 Table 3-4. Muscles of Mastication Muscle Masseter Origin Zygomatic arch Insertion Mandible (external surface) Description Closes jaw; flat, thick muscle Temporalis Temporal bone Coronoid process at the anterior border of the ramus Closes jaw; fan-shaped Medial pterygoid Sphenoid, palatine, and maxillary bones Inner (medial) surface of the ramus Closes jaw; parallels masseter muscle Lateral pterygoid Sphenoid bone Anterior surface of mandibular condyle Opens jaw; allows grinding action side to side, and protrudes the mandible reception. We receive food in the mouth, reducing it in size, and mixing it with saliva for the digestion process. CHEEKS The cheeks are the side walls of the mouth. They are made up of layers of skin, a moist inner lining called mucosa, fat tissue, and certain muscles. The buccinator muscle of the cheeks prevents food from escaping the chewing action of the teeth. LIPS The lips are covered externally by skin and internally by the same mucous membranes that line the oral cavity. They form the anterior border of the mouth. The area of the external lips where the red mucous membrane ends and normal outside skin of the face begins is known as the vermilion border. Figure 3-17 illustrates the anatomy of the lips. The lips are very sensitive and act as sensory receptors, allowing food and liquids to be placed in the Figure Anatomy of the lips. 3-10

11 mouth but guarding the oral cavity against the ingestion of excessively hot or cold substances. They also provide a seal for the mouth to keep food and saliva from escaping. The lips help to maintain the position of the teeth and are very important in speech. TONGUE The tongue (fig. 3-18) is a vascular, thick solid mass of voluntary muscle surrounded by a mucous membrane (epithelium tissue). Located on the underneath side of the tongue is the lingual frenulum, which anchors the tongue in the midline to the floor of the mouth. The tip of the tongue is free moving and can readily change size, shape, and position. Surface (Dorsal Aspect) On the surface of the tongue are rough projections called papillae. They provide the tongue with friction in handling food and also act as taste buds. Taste Buds The four types of taste sensations are sweet, sour, bitter, and salty-all resulting from stimulation of the taste buds. Most are located on the tongue and the roof of the mouth. Figure 3-19 illustrates taste buds of the tongue. Figure Taste buds of the tongue. Tongue and Digestion The tongue is an important muscle in the chewing process. It crushes food against the palate; it deposits food between the chewing surfaces of the teeth for Figure Dorsal aspect of tongue (left), anatomy floor of mouth (right). 3-11

12 mastication; it transfers food from one area of the mouth to another; it mixes food with saliva, which assists in the digestive process; assists in swallowing; and cleans the mouth of residue. MYLOHYOID AND GENIOHYOID The mylohyoid muscles anatomically and functionally form the floor of the mouth (fig. 3-20). They elevate the tongue and depress the mandible. Their origin is the mandible and insertion is the upper border of the hyoid bone. The geniohyoid muscles are found next to each other, on each side of the midline, directly on top of the mylohyoid muscle. They have the same origin and function as the mylohyoid muscle. PALATE The palate (fig. 3-21) forms the roof of the mouth and is divided into two sections: Hard palate This section is formed by the palatine process of the maxillary bones and is located in the anterior portion of the roof of the mouth. It has irregular ridges or folds behind the central incisors called rugae. Soft palate This section forms a soft muscular arch in the posterior part of the palate. The uvula is located on the back portion of the soft palate. When you swallow, the uvula is drawn upward and backward by the muscles of the soft palate. This process blocks the opening between the nasal cavity and pharynx, not allowing food to enter the nasal cavity. The soft palate must function properly to allow good speech quality. Located in the posterior part of the mouth, on both sides of the tongue, are two masses of lymphatic tissue called the palatine tonsils. They assist the body to protect against infections. TEETH The teeth are located in the alveolar process of the maxillae and the mandible. They serve important functions of tearing and masticating food, assisting in swallowing, speaking, and in appearance. The health of the teeth affects the health of the entire body. SALIVARY GLANDS The functions of the three major salivary glands are to keep the lining of our mouths moist, and to bond with food particles creating a lubricant effect that assists in the swallowing process of food. It acts as a cleaning agent to wash away food particles that accumulate in the mouth and on the teeth. Figure 3-22 illustrates the salivary glands. The salivary glands produce two to three pints of saliva daily, which greatly aids in the digestion process. Enzymes are present in saliva, they act on food, and start the breakdown process. In dentistry, knowing exactly where the saliva glands and ducts (openings) are located is important in keeping the mouth dry Figure Mylohyoid and geniohyoid muscles. 3-12

13 Figure Anatomy of the palate. Figure Salivary glands. 3-13

14 during certain dental procedures. Table 3-5 lists the three major salivary glands. MASTICATION AND DEGLUTITION The mastication process includes the biting and tearing of food into manageable pieces. This usually involves using the incisors and cuspid teeth. The grinding of food is usually performed by the molars and premolars. During the mastication process, food is moistened and mixed with saliva. Deglutition is the swallowing of food and involves a complex and coordinated process. It is divided into three phases; the first phase of swallowing is voluntarily; phases two and three are involuntary. Phase one: the collection and swallowing of masticated food. Phase two: passage of food through the pharynx into the beginning of the esophagus. Phase three: the passage of food into the stomach. Gland Sublingual Table 3-5. List of the Three Major Salivary Glands Location Duct Description On each side underneath Multiple separate ducts Smallest of salivary glands, the tongue, in the floor of secretes, thick stringy mucus. the mouth Submandibular Posterior portion of mandible, lingual to mandibular incisors Parotid Inside cheek, opposite maxillary second molar Opens under the tongue, close to the frenulum Parotid ducts go through the buccinator muscles and enter the mouth opposite maxillary second molars Walnut sized. Secretes watery fluid with some mucus. More viscous (thick) than parotid secretion. Largest of salivary glands. Secretes clear watery fluid. 3-14