The skull is part of the human skeleton. The skeleton gives firmness to the body. The skull consists of two parts: the brain skull and the face skull. These parts have different functions.
The brain skull is that part of the skull that protects the brain. There are holes in and between the bones on the underside of the skull through which spinal cord, cranial nerves and blood vessels enter and exit.
The facial skull consists of the upper and lower jaw, the bones of the nose and the cheekbones. The main functions of the face skull are:
- Protection of organs such as the eyes;
- Attachment site for muscles.
The entrances to the respiratory tract and digestive tract are located in the facial skull.
In an adult, the various bone pieces that make up the skull have grown together. The only exception to this is the lower jaw. This is attached to the rest of the skull with two joints. Although the bones have grown together, it is quite possible to distinguish seams. These seams, or sutures, are the edges of the skull bones.
The skull bones are indicated in the image below. The tongue bone is also drawn in the side view, because this bone piece is important for the attachment of various muscles.
Development of the skull
The development of the skull starts before birth. At the place where the skull should be, bone nuclei arise in the connective tissue of the (embryonic) brain in various places. These bone cores become larger and, as it were, grow towards each other. For a newborn, these bone pieces have not yet grown together (see image below).
Connective tissue is still present between the bone pieces of a newborn's skull. There are two connective tissue plates on the top of the head, the large and the smallfontanel. The large fontanelle is especially visible in the top view. The two fontanelles and the connective tissue connection between the skull bones have the advantage that the skull can be slightly compressed at birth. The baby's head can thus be easier through the birth path of the mother. The connective tissue seams and the fontanelles only close after birth. With a baby the fontanelle on the head can be felt well. The small fontanelle closes when the baby is two to four months old. The large fontanel grows much slower; this closes when the baby is around eighteen months old.
Growth of the skull
At birth the brain skull is very large compared to the facial skull. With an adult, both are about the same size. The facial skull grows proportionally more than the brain skull.
Apart from the mandibula, the skull bones are flat leg pieces. There is compact bone on the inside and outside. The spongy bone lies between these two hard bone layers; we call that spongy bone. Bone marrow is in the spongy bone. The bone marrow forms red blood cells.
The (brain) skull is spherical and grows in all directions. The skull bones grow because the periost or bone membrane deposits bone on the outside. At the same time bone is broken down on the inside. Because the cavity cavity must remain exactly between the two hard bone layers, bone is created in the cavity cavity on the inside of the skull, while the bone is broken off on the outside. In this way, the merge cavities grow along with the skull bones. Bone growth and bone degradation are well matched so that the skull maintains the correct thickness. The brain must be well protected. The mandible or lower jaw grows mainly through bone deposition on the condylaris process or the jaw head (see image below).
At some places in the skull there are cavities that are not filled with bone marrow, but with air. These cavities have mucous membranes against the wall. We call them the nasal cavities, because they are connected to the nasal cavity. These nasal sinuses are located in different bone pieces. The forehead cavity or sinus frontalis is located in the frontal os. For dentistry, however, the cavity in the maxilla or upper jaw is more important. The cavity in the upper jaw has different names: sinus maxillaris, jaw cavity or antrum, but is usually called "the sinus." Although there are several cavities, a dentist (almost) always means the maxillary sinus.
The maxillary sinus is located above the root points of the elements of the upper jaw, mainly above the root points of the premolars and molars (see image below).
The size of the cavities varies from person to person. For a baby, the cavity is the size of a pea. In an adult, the jaw cavity can be several centimeters in size. When the skull becomes larger, the cavity grows with it. Because of this air cavity, the skull is less heavy than if bone had been here.
Names of the skull bones
The skull bones have a Latin, but also an English name. In the table below, the skull bones are also named by their Latin and English names. Special protrusions or parts of leg pieces are specified with the relevant bone piece.
The skull bones.
|Latin name – special part of the bone||English name – special part of the bone|
|Os Parietal||Wall bone|
|Os occipital||Occipital bone|
|Os temporal||Sleeping bone|
|Processus zygomaticus||Part of the yoke arch|
|Processlo styloid||Part of the yoke arch|
|Mastus processus||Part of the yoke arch|
|Os ethmoidale||Sieve bone|
|Os sphenoidal||Wedge bone|
|Processus temporalis||Part of the yoke arch|
|Os lacrimale||Tear bone|
|Os nasal||Nasal bone|
|Vomer||Plow scissors bone (nasal septum)|
|Processus palatinus||Hard palate|
|Process alveolaris||Tooth-bearing bone|
|Processus frontalis||Walks to the frontal os|
|Concha nasalis media||Middle nose shell|
|Concha nasalis inferior||Lower nose shell|
|Processus condylaris||Jaw cup|
|Coronoid processus||Jaw cup|
|Process alveolaris||Tooth-bearing bone|
Connections between the skull bones
There are two types of connections to the skull:
- Suturen. Most skull bones are connected by sutures. Many sutures or seam connections can be found in the skull. The seam connection does not allow movements.
- (Synovial) joints. The lower jaw is attached to the skull by means of very mobile joints. A synovial joint is a joint whose two bone surfaces are covered with smooth joint cartilage. The synovia or joint lubricant serves as a lubricant between the cartilage layers.
The jaw joint is called Latin articulatio temporomandibularis in Latin. It consists of two joints, one on the right and one on the left. Because the lower jaw is one large bone piece, the two jaw heads are rigidly connected to each other. The two joints form the movable connection between the mandibula and the rest of the skull. Each joint is enclosed by a joint capsule. Each jaw joint has a cartilage disc, the discus articularis, which divides each joint into two. It can clearly be seen that the articular disc is between the jaw head and the skull. The joint is divided into two joint rooms. Joint capsules limit the movement possibilities of a joint. At the jaw joint, this limitation of movement is further enhanced by connective tissue bands that run from the skull to the lower jaw (see image below).
Movement possibilities of the jaw joint
Two movements are possible in the jaw joint:
- Rotation, whereby the jaw head rotates in the joint cup;
- Shift, with the jaw head sliding forward in the direction of the tuberculum articulare.
Rotation and translation can be combined in various ways. While rotation occurs in one joint, translation can occur in the other.
When the mouth is opened slightly, rotation first takes place. As the mouth opens further, the jaw head in the joint cup slides forward. When the lower jaw is moved to the left or right, the movement is limited to almost one joint. For example, when the lower jaw is moved to the left, a small rotation takes place in the left joint. The right-hand jaw head, on the other hand, is moved considerably forward. A shift of the chin point in the direction of the inactive joint occurs (see figure below).
Most movements of the mandible are the result of rotations and translations of the jaw heads in the jaw joints (see image below).