Similarly, in the leg, the shafts of the tibia and fibula are also united by an interosseous membrane. In the forearm, the wide gap between the shaft portions of the radius and ulna bones are strongly united by an interosseous membrane (see Figure 9.2.1 b). The gap between the bones may be narrow, with the bones joined by ligaments, or the gap may be wide and filled in by a broad sheet of connective tissue called an interosseous membrane. SyndesmosisĪ syndesmosis (“fastened with a band”, plural = syndesmoses) is a type of fibrous joint in which two parallel bones are united to each other by fibrous connective tissue. Figure 9.2.2 – The Newborn Skull: The fontanelles of a newborn’s skull are broad areas of fibrous connective tissue that form fibrous joints between the bones of the skull. Late in life, the sagittal, coronal, and lambdoid sutures of the skull will begin to ossify and fuse, causing the suture line to gradually disappear. At the time of birth, the frontal and maxillary bones consist of right and left halves joined together by sutures, which disappear by the eighth year as the halves fuse together to form a single bone. Examples of synostosis fusions between cranial bones are found both early and late in life. This fusion between bones is called a synostosis (“joined by bone”). At some sutures, the connective tissue will ossify and be converted into bone, causing the adjacent bones to fuse to each other. When the connective tissue between the adjacent bones is reduced to a narrow layer, these fibrous joints are now called sutures. The fontanelles greatly decrease in width during the first year after birth as the skull bones enlarge. After birth, these expanded regions of connective tissue allow for rapid growth of the skull and enlargement of the brain. During birth, the fontanelles provide flexibility to the skull, allowing the bones to push closer together or to overlap slightly, thus aiding movement of the infant’s head through the birth canal. These broad areas of connective tissue are called fontanelles ( Figure 9.2.2). In newborns and infants, the areas of connective tissue between the bones are much wider, especially in those areas on the top and sides of the skull that will become the sagittal, coronal, squamous, and lambdoid sutures. (See Figure 9.2.1 a) Thus, skull sutures in the adult are functionally classified as a synarthrosis. The suture is frequently convoluted, forming a tight union that prevents most movement between the bones. In adults, the skull bones articulate closely and fibrous connective tissue fills the narrow gap between the bones. The fibrous connective tissue found at a suture (“to bind or sew”) strongly unites the adjacent skull bones and thus helps to protect the brain and form the face. SutureĪll the bones of the skull, except for the mandible, are joined to each other by fibrous joints called sutures. (c) A gomphosis is a specialized fibrous joint that anchors a tooth to its socket in the jaw. (b) An interosseous membrane forms a syndesmosis between the radius and ulna bones of the forearm. (a) Sutures join most bones of the skull. Figure 9.2.1 – Fibrous Joints: Fibrous joints form strong connections between bones. Lastly, a gomphosis is the narrow fibrous joint between the roots of a tooth and the bony socket in the jaw into which the tooth fits. This type of fibrous joint is found between the shaft regions of the long bones in the forearm and in the leg. At a syndesmosis, the bones are more widely separated but are held together by a strap of fibrous connective tissue called a ligament or a wide sheet of connective tissue called an interosseous membrane. A suture is the narrow fibrous joint found between most bones of the skull. The fibers joining the bones may be short or long, thus the gap between bones at fibrous joints vary from narrow to wide.
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