This provides strength and stability to the leg and ankle, which are important during weight bearing. In the leg, the syndesmosis between the tibia and fibula strongly unites the bones, allows for little movement, and firmly locks the talus bone in place between the tibia and fibula at the ankle joint. However, a syndesmosis does not prevent all movement between the bones, and thus this type of fibrous joint is functionally classified as an amphiarthrosis. The syndesmoses found in the forearm and leg serve to unite parallel bones and prevent their separation. Together, the interosseous membrane and these ligaments form the tibiofibular syndesmosis. In addition, at the distal tibiofibular joint, the articulating surfaces of the bones lack cartilage and the narrow gap between the bones is anchored by fibrous connective tissue and ligaments on both the anterior and posterior aspects of the joint. 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 1b). 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”) is a type of fibrous joint in which two parallel bones are united to each other by fibrous connective tissue. 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. These broad areas of connective tissue are called fontanelles (Figure 2).ĭuring 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. 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. The fontanelles of a newborn’s skull are broad areas of fibrous connective tissue that form fibrous joints between the bones of the skull. The skull features numerous foramina through which pass the cranial nerves, the spinal cord, arteries, and veins.Figure 2. The Newborn Skull. The temporomandibular joint connects the lower jaw to the temporal bone and enables movement for speech and mastication. The viscerocranium consists of the unpaired mandible and vomer and the paired zygomatic bones, inferior nasal conchae, lacrimal bones, nasal bones, maxillary bones, and palatine bones. The fontanelles, which comprise the anterior, posterior, and the paired mastoid and sphenoid fontanelles, close between the age of 2 months and 18 months. The bones of the infant neurocranium are not fused but are instead connected by membranous gaps between the bone plates called fontanelles, which enable flexion of the skull to facilitate passage through the birth canal and accommodate the rapidly growing brain. The cranial bones are connected by fibrous joints called sutures. The neurocranium consists of the frontal, the ethmoid, the sphenoid, the occipital, and the paired temporal and parietal bones. The human skull consists of approximately 30 bones, which can be anatomically divided into the cranial bones ( neurocranium) and the facial bones ( viscerocranium).
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