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To complete this topic, we now have to discuss the bones of the neck. Most of these bones are vertebrae but the one that is not is both unique and important to the oral cavity.
The hyoid bone is unlike any other bone in the human body as it has no articulation with another bone. It is a u-shaped bone that mirrors the mandibular body and is positioned deep to that larger bone. It functions as the attachment point for the muscles that control the tongue. The tongue, which is mainly a muscle covered with epithelium, is anchored mainly to the hyoid bone. Movement of the tongue in the superior/inferior and the anterior/posterior directions is related to the movement of the hyoid in those directions. The muscles attached to the hyoid are divided into the suprahyoid and infrahyoid groups depending on the placement of their origins. These muscles attached to the hyoid are all important to the proper movement of the tongue and epiglottis during swallowing and speaking.
Anatomically the main central part of the hyoid is known as the body (above) and projecting superiorly are two superiorly placed projections. The smaller more anterior one is known as the lesser cornu and the one at the posterior extent of the bone is the greater cornu. The cornu, like many structures, are also referred to by another name. In this case they are the greater and lesser horns of the hyoid.
The Cervical Vertebrae
The cervical vertebrae act to support the skull, allow for movement of the head on the neck and some movement of the neck itself, in addition to protecting the spinal cord. Humans have seven cervical vertebrae which is also true of virtually all mammal species, except the manatee and two species of sloths, which only have 6. We will discuss the first two vertebra in detail but the next five are very similar in appearance and will be lumped together under the heading of typical cervical vertebra. All cervical vertebrae have a large central foramen known as the vertebral foramen to allow passage of the spinal cord and two laterally known as the transverse foramen to allow passage of the vertebral blood vessels. The two vertebral arteries are especially important as at the base of the skull they merge to form the single basilar artery which supplies the brain stem and cerebellum with blood and then splits into the two posterior cerebral arteries that join with branches of the internal carotid artery to supply the cerebrum. In addition, between adjacent vertebrae there is a passageway to allow spinal nerves to pass from the spinal cord to the body.
The uppermost cervical vertebra atlas (C1) articulates with the occipital bone superiorly and the second cervical vertebra, the axis, inferiorly. The bone is unique among all vertebrae in that it is the only one without a body to support the weight placed on it. The body of a vertebra is located in the sagittal plane but is placed anteriorly in the bone. Rather than the centrally placed mass of bone it has two lateral support points that articulate with the occipital condyles. These lateral masses as they a called have a concave articular surface to accept the convex condyle. As condylar joints are biaxial the head is allowed to rotate in the superior/inferior direction which produces the yes movement and in the medial/lateral direction which results in tilting the head towards the shoulder. In addition to the atlas being unique by lacking a body it is also the only vertebra that lacks a spinous process. Furthermore, it has a unique articulation point with the second cervical vertebra, the axis. The articular surface for the dens faces the vertebral foramen and is bounded from it by ligaments which keep the axis in place. We will discuss this further in the following section.
The axis is a more conventional vertebra in that it has a body centered in the anterior part of the bone and a like other cervical vertebrae it has a bifid spinous process posteriorly. However, the body is only conventional on the inferior surface where it articulates with C3. The superior surface has a circular projection called the dens that articulates with the atlas. Held in place by ligaments this forms a type of joint known as a pivot joint which allows rotation in one axis. This translates to lateral and medial rotation of the head which is the rotational motion used to indicate no.
Typical Cervical Vertebrae
While they become larger and the spinous processes become more longer as we proceed from C3-C7 all of these vertebrae have very similar features and can be considered together. They all have a body with the transverse foramina flanking them. Posterior to the foramina are articular surfaces or facets which are shaped to allow some flexibility in moving the vertebrae to change the curvature of the spine at that level. The articular surfaces are joined by the posterior arch composed of bony processes called the lamina which join at the location of the bifid spinous process though C7 does not bifurcate like C2-C6 as it is a transitional form having that attribute in common with the thoracic vertebra which have a more bulbous spinous process without a bifurcation.