The bones of the cranium articulate with each other by way of sutures, which are fibrous joints between two interlocking denticulate edges which allow very little movement. This type of joint is very different from the more familiar synovial joints, highly mobile joints encased by a sac and lubricated with fluids. Most familiar joints such as the knee, elbow, and hip are synovial joints. The only synovial joints in the skull are the jaw joint and the articulations of the tiny auditory ossicles. The sutures between bones of the cranium are most evident in young individuals. Over time the bones fuse together such that some sutures in extremely old individuals are virtually invisible.
See the Digital Teaching Collection for more specimens of the skull.
The maxilla possesses a body at its center which surrounds the large maxillary sinus, opening towards the medial side. Emanating from the body are several processes which are named for the bones with which they articulate. From an anterior view, the frontal process is visible as it projects superiorly and medially to articulate with the frontal bone. The infraorbital foramen, which transmits the infraorbital nerve to the face, is visible at the base of the frontal process just inferior to the orbital margin. Just inferior to the infraorbital foramen is a depression of variable depth, the canine fossa. The anterior nasal spine is a superiorly projecting, thin process near the midline of the inferior border of the nasal aperture. The zygomatic process is the superiorly positioned lateral projection forming much of the bony cheek as it articulates with the zygomatic bone. The alveolar process is the inferior projection from the body which contains numerous alveoli, the openings which hold the roots of the teeth. The canine jugum is a bony ridge running superiorly from the alveolus of the maxillary canine.
In an inferior view, the maxillary teeth are visible in their alveoli. The palatine process projects medially to form most of the bony palate and to articulate with the palatine bones. Near the midline where the palatine processes articulate with each other, the palate is pierced by the incisive foramen, just posterior to the incisors.
In superior view, the orbital process is visible as it projects posteriorly from the orbital margin to form the floor of the orbit. The posterior margin of the orbital plate is marked by the infraorbital groove, which connects with the infraorbital foramen by means of the infraorbital canal.
Siding the Maxillae
Intact maxillae are readily sided. The frontal process is superior, the zygomatic process is lateral, the nasal aperture is medial, and the occlusal surfaces of the teeth are inferior. More fragmentary specimens may be more difficult to side, although the presence of alveoli often offers a useful siding tool.
Siding the Zygomatic
The superiorly projecting frontal process is the longest and narrowest of the processes. The orbital margin is on the anterior surface of the frontal process. The roughened origin for the masseter is on the inferior surface of the body. The zygomatic process is the only process which does not contribute to the orbit; this process projects posteriorly.
Siding the Nasals
The superior border is marked by the denticulate suture with the frontal. The medial surface where the nasals articulate with each other is straight and smooth. The lateral (exterior) surface bears the nasal foramen.
Siding the Lacrimals
The lacrimal crest sweeps toward anterior, and the lacrimal groove opens superiorly. The groove and crest are on the medial (orbital) side of the bone.