The majority of the joints in the skull are known as sutures. Many are interlocking at their edges, like teeth in a zipper or a dovetail joint in carpentry. These are considered fibrous synarthrotic joints—which are immovable and are held together by connective tissue that is dense with fibers. What role do these play in forensic science?
The Cranial Region
The human skull is composed of 22 bones, united by joints. It is typically divided into a cranial region, often called the braincase, and a facial region that surrounds most of our sensory openings. The cranial region includes eight bones that have the most direct relationship with the brain and its coverings.
There are a pair of parietal bones in the cranial region that make up most of the dome or vault of the skull, a pair of temporal bones on either side of the skull, a single frontal bone in the forehead region, as well as a single occipital bone at the posterior skull.
The other two bones of the cranial region are a bit more obscure from an external view. One is the sphenoid bone, which is really like an architectural buttress for the skull, since it supports and joins with each of the other seven cranial bones. The final braincase bone is the ethmoid, and while it has the majority of its anatomy in the facial region, a projection of the ethmoid reaches up to the floor of the braincase, so that’s where it is classified.
The Facial Skeleton
The facial skeleton is composed of 14 bones. There are a pair of zygomatic bones that make the prominences at the cheeks and a tiny pair of lacrimal bones in the eye sockets. A pair of nasal bones make up the bridge of the nose, and a singular bone called the vomer helps make up the nasal septum that divides the nasal cavity in two. The pair of maxillary bones of the mid-face surround the nasal opening called the nasal aperture; these maxillae also form the upper jaw. The scientific name given to the unpaired lower jaw is the mandible. The remaining four bones are not readily seen externally. Two are the paired inferior nasal conchae on the sides of the nasal cavity, and a pair of L-shaped palatine bones likewise contribute to the lateral nasal wall, but also help make up the roof of the mouth.
The majority of the joints in the skull are known as sutures.
Sutures and Forensic Science
Some practitioners claim the bones of the skull move in adults, but that is controversial. However, the sutures between skull bones in the fetus do move: In fact, the fetal and newborn skull has fontanels—so-called soft spots—between some of the sutures that allow the actual overlap of cranial bones during the birth process, facilitating the journey through the birth canal.
Through forensic work it has been made clear that the sutures of the skull are somewhat protective in that they relieve some of the stress of a blow. A skull fracture often propagates through a cranial bone, but frequently stops when the fracture reaches a suture, and there the stress is relieved. An exception would be a high velocity impact, such as in a high-caliber gunshot wound—those will cross a suture and can shatter a skull.
Another forensic application relates to the fact that the sutures of the skull may completely close up with age; they will literally obliterate. This is definitely more common in the elderly, but it has been seen in individuals as early as in their 20s. Decades ago, cranial suture closure used to be more heavily relied on when assessing the age of an unknown person, and while it is somewhat correlated with age, the pattern is not definitive enough to use as a reliable forensic method.
This article comes directly from content in the video series How We Move: The Gross Anatomy of Motion. Watch it now, on Wondrium.
Naming the Major Sutures
There are names for all sutures of the skull, but let’s only cover the larger sutures. The coronal suture is between the frontal and paired parietal bones; it sits more or less across the crown of the head—which makes sense, since coronal means ‘crown’. The sagittal suture runs between the two parietal bones, along the midsagittal plane. The lambdoid suture is posteriorly between the parietal bones and the occipital bone. Where the parietal bones intersect with the occipital bone, the result resembles an upside-down V, like the Greek letter lambda, that’s why it’s called the lambdoid suture.
The final major suture is the squamous suture found laterally between the temporal bone and the sphenoid and parietal bones. Squamous means ‘flat’, and not only is this suture less squiggly or jagged than most other sutures, it is also not an interlocking or serrate suture like the others. The squamous suture is what we call a beveled or lap suture, where a pair of flat surfaces simply overlap. For this reason, in forensic cases, especially if the skeletal remains are very dried out, or in archaeological samples, the squamous sutures tend to flare open, and separate rather easily, because they are more like miter joints than the other sutures.
One other interesting feature of cranial sutures is that they can have what are known as sutural bones or Wormian bones within them—some books call these ossicles, little islands of bone within the suture. They are called Wormian bones, as it is an eponym; these islands of bone within a suture are named after the Danish physician Ole Worm.
Another set of ossicles associated with the skull are the three tiny bones in the middle ear cavity within each temporal bone. They relate to hearing and are named the malleus, incus, and stapes or as the hammer, anvil, and stirrup.
Common Questions about the Sutures of the Skull
The human skull is composed of 22 bones, united by joints. It is typically divided into a cranial region, often called the braincase, and a facial region that surrounds most of our sensory openings.
The pair of maxillary bones of the mid-face surround the nasal opening are called the nasal aperture; these maxillae also form the upper jaw. The scientific name given to the unpaired lower jaw is the mandible.
A forensic application relates to the fact that the sutures of the skull may completely close up with age. Decades ago, cranial suture closure used to be more heavily relied on when assessing the age of an unknown person.