SKELETAL SYSTEM OUTLINE 7.1 Skull 175 7.1a Views of the Skull and Landmark Features 176 7.1b Sutures 183 7.1c Bones of the Cranium 185 7 7.1d Bones of the Face 194 7.1e Nasal Complex 198 7.1f Paranasal Sinuses 199 7.1g Orbital Complex 200 Axial 7.1h Bones Associated with the Skull 201 7.2 Sex Differences in the Skull 201 7.3 Aging of the Skull 201 Skeleton 7.4 Vertebral Column 204 7.4a Divisions of the Vertebral Column 204 7.4b Spinal Curvatures 205 7.4c Vertebral Anatomy 206 7.5 Thoracic Cage 212 7.5a Sternum 213 7.5b Ribs 213 7.6 Aging of the Axial Skeleton 214 7.7 Development of the Axial Skeleton 214 MODULE 5: SKELETAL SYSTEM mck78097_ch07_173-219.indd 173 2/14/11 4:58 PM 174 Chapter Seven Axial Skeleton he bones of the skeleton form an internal framework to support The skeletal system is divided into two parts: the axial skele- T soft tissues, protect vital organs, bear the body’s weight, and ton and the appendicular skeleton. The axial skeleton is composed help us move. Without a bony skeleton, we would collapse into a of the bones along the central axis of the body, which we com- formless mass. Typically, there are 206 bones in an adult skeleton, monly divide into three regions—the skull, the vertebral column, although this number varies in some individuals. A larger number of and the thoracic cage (figure 7.1). The appendicular skeleton bones appear to be present at birth, but the total number decreases consists of the bones of the appendages (upper and lower limbs), with growth and maturity as some separate bones fuse. Bones differ as well as the bones that hold the limbs to the trunk of the body in size, shape, weight, and even composition, and this diversity is (the pectoral and pelvic girdles). The axial skeleton is the topic of directly related to the skeleton’s many functions. this chapter; in chapter 8, we discuss the appendicular skeleton. Frontal bone Parietal bone Temporal bone Skull Zygomatic bone Occipital bone Maxilla Mandible Mandible Hyoid bone Sternum Thoracic Ribs Vertebrae cage Costal Vertebral cartilage column Vertebrae Sacrum Sacrum Coccyx Coccyx Bones of the Axial Skeleton (80) Skull (22) Cranial bones (8) Vertebral Vertebrae (24) Frontal bone (1), parietal bones (2), temporal bones (2), column (26) Cervical vertebrae (7), thoracic vertebrae (12), occipital bone (1), sphenoid bone (1), ethmoid bone (1) lumbar vertebrae (5) Facial bones (14) Sacrum (1) Zygomatic bones (2), lacrimal bones (2), Coccyx (1) nasal bones (2), vomer (1), inferior nasal conchae (2), palatine bones (2), maxillae (2), mandible (1) Thoracic Sternum (1) Associated Auditory ossicles (6) cage (25) bones of Malleus (2), incus (2), stapes (2) Ribs (24) the skull (7) Hyoid bone (1) (a) Anterior view (b) Posterior view Figure 7.1 Axial Skeleton. (a) Anterior and (b) posterior views show the axial skeleton, which is composed of the skull, vertebral column, and thoracic cage. A table summarizes the bones of the axial regions. mck78097_ch07_173-219.indd 174 2/14/11 2:43 PM Chapter Seven Axial Skeleton 175 The main function of the axial skeleton is to form a framework The skull is composed of both cranial and facial bones that supports and protects the organs. The axial skeleton also houses (figure 7.2). Cranial bones form the rounded cranium (krā ń ē - special sense organs (the organs for hearing, balance, taste, smell, um; kranion = skull), which completely surrounds and encloses and vision) and provides areas for the attachment of skeletal muscles. the brain.1 Eight bones make up the cranium: the unpaired eth- Additionally, the spongy bone of most of the axial skeleton contains moid, frontal, occipital, and sphenoid bones, and the paired pari- hemopoietic tissue, which is responsible for blood cell formation. etal and temporal bones. These bones also provide attachment We begin our examination of the axial skeleton by discuss- sites for several jaw, head, and neck muscles. Touch the top, sides, ing its most complex structure, the skull. and back of your head; these parts of your skull are cranial bones. Fourteen facial bones form the face: paired maxillae, nasal, lac- rimal, zygomatic, palatine bones, inferior nasal conchae, and the Study Tip! unpaired vomer and mandible. They also protect the entrances to Many bones have the same names as the body regions where they the digestive and respiratory systems as well as providing attach- are found. Before you begin learning about the bones of the axial skel- ment sites for facial muscles. Touch your cheeks, your jaws, and eton, it may help you to review table 1.2 (Anatomic Directional Terms) the bridge of your nose; these bones are facial bones. and table 1.3 (Human Body Regions). We will be using these terms as The skull contains several prominent cavities (figure 7.3). we discuss various features of bones in the next few chapters. The largest cavity is the cranial cavity, which encloses, protects, and supports the brain and has an adult volume of approximately 1300 to 1500 cubic centimeters. The skull also has several smaller cavities, including the orbits (eye sockets), the oral cavity (mouth), 7.1 Skull the nasal (nā zal;́ nasus = nose) cavity, and the paranasal sinuses. Learning Objectives: WHATW DID YOU LEARN? 1. Identify the cranial and facial bones of the skull. ●1 What are the two groups of skull bones? 2. Locate and name the sutures between cranial bones. 3. Describe the structure of the nasal complex and the paranasal sinuses. 1 Osteologists (scientists who study bones) define the cranium as the entire 4. Identify the three auditory ossicles. skull minus the mandible. In this text, we use the term cranium to denote 5. Discuss the structure of the hyoid bone. the bones of the braincase only. Figure 7.2 Cranial Cranial and Facial Divisions of the Skull. The bones of the bones cranium protect the brain. The bones of both the cranium and the face form the orbits, nasal cavity, and mouth. Facial bones Parietal bone Frontal bone Sphenoid bone Sutural bone Occipital bone Ethmoid bone Temporal bone Nasal bone Zygomatic bone Lacrimal bone Vomer Maxilla Mandible mck78097_ch07_173-219.indd 175 2/14/11 2:43 PM 176 Chapter Seven Axial Skeleton Coronal sectional plane Cranial cavity Frontal bone Figure 7.3 Major Cavities of the Skull. A coronal Frontal sinus section diagram identifies the cavities Ethmoid bone within the skull. Orbit Zygomatic bone Paranasal Ethmoidal sinuses Perpendicular plate sinuses of ethmoid bone Superior Maxillary sinus Middle Nasal Inferior conchae Vomer Nasal Maxilla cavity Oral cavity Mandible 7.1a Views of the Skull and Landmark Features maxilla, is an infraorbital foramen, which conducts blood vessels and nerves to the face. The lower jaw is formed by the mandible. The The skull is composed of multiple bones that exhibit complex shapes, prominent “chin” of the mandible is called the mental protuberance. and each bone articulates with at least one other. To best understand An anterior view also shows the nasal cavity. Its inferior bor- the complex nature of the skull, we first examine the skull as a whole der is marked by a prominent anterior nasal spine. The thin ridge of and learn which bones are best seen from a particular view. Note that bone that subdivides the nasal cavity into left and right halves helps only some major features will be mentioned in this section. Later in form the nasal septum. Along the lateral walls of the nasal cavity are the chapter, we examine the individual skull bones in detail. two scroll-shaped bones called the inferior nasal conchae. A cursory glance at the skull reveals numerous bone markings, such as canals, fissures, and foramina, which are passageways for blood vessels and nerves. The major foramina and canals of the cranial Superior View and facial bones are listed in table 7.1. Refer to this table as we exam- The superior view of the skull in figure 7.5a primarily shows four ine the skull from various directions. (This table also will be important of the cranial bones: the frontal bone, both parietal bones, and the when we study individual blood vessels and nerves in later chapters.) occipital bone. The articulation between the frontal and parietal bones is the coronal suture, so named because it runs along a coro- Anterior View nal plane. The sagittal suture connects the left and right parietal bones almost exactly in the midline of the skull. An anterior view best shows several major bones of the skull Along the posterior one-third of the sagittal suture are either (figure 7.4). The frontal bone forms the forehead. Put your hand a single parietal foramen or paired parietal foramina. This foramen on your forehead; you are feeling your frontal bone. The left and conducts tiny emissary veins from the veins of the brain to the right orbits (eye sockets) are formed from a complex articulation veins of the scalp. The number of parietal foramina can vary in of multiple skull bones. Within the orbits are two large openings, individuals and between left and right sides of the same skull. The called the superior orbital fissure and the inferior orbital fissure. superior part of the lambdoid suture represents the articulation of Superior to the orbits are the superciliary arches, otherwise known the occipital bone with both parietal bones. as the brow ridges. The left and right nasal bones form the bony “bridge” of the nose. Superior to the nasal bones and between the orbits is a landmark area called the glabella.