This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The skull is the bony framework of the head. It is divided into the bones of the cranium and those of the face. The hyoid bone is usually classified with the bones of the head.
The cranium consists of the bones forming the brain case. They are the occipital, two parietals, the frontal, two temporals, the sphenoid, and the ethmoid.
The bones of the face are fourteen in number, there being two single bones and six pairs. The single bones are the mandible, or inferior maxilla, and vomer; the pairs are the superior maxilla, malar, nasal, palate, lachrymal, and inferior turbinated bones.
The skull of the infant is markedly different from that of the adult. At birth the face is quite small and undeveloped, while the cranium is relatively large. The frontal and parietal eminences are very marked. The vault of the skull is not entirely ossified and the sutures are not completed. The bones of the base of the skull originate in cartilage, while those of the vault originate in membrane. This membrane has one or more centres of ossification appearing in it for each bone. These centres increase in size and finally meet at the edges of the bone, thus forming the sutures. At the time of birth the sutures are represented by membrane, which joins the adjacent bony edges.
The frontal bone has two centres of ossification; one for each side. These form a suture in the median line of the forehead which becomes obliterated in the course of the first or second year. Traces of it in the shape of a groove or ridge can sometimes be seen or felt in the adult skull. The frontal eminences are far more marked in childhood than later in life and give to children the prominent forehead which is so characteristic. A similar peculiarity is seen in the parietal bones, the parietal eminences being quite prominent. On this account, they are often injured in childbirth, sometimes being compressed by the obstetrical forceps, and are frequently the seat of haematoma neonatorum. The cranial bones not being firmly united allow of a certain amount of play or even overlapping, thus facilitating the delivery of the head at birth. Fontanelles. - At the juncture of the various bones are six spaces called fontan-elles. Two, the anterior and posterior, are in the median line of the cranium, and four, the two anterolateral and two posterolateral, are at the sides. The fontanelles are situated at the four corners of the parietal bones.
The anterior fontanelle is the largest. It is diamond-shaped and formed by the frontal suture in front, the interparietal behind, and the coronal at each side. It is usually closed by the end of the second year, but may be delayed until the fourth. In rickets and malnutrition the fontanelles remain open longer than would otherwise be the case.
The posterior fontanelle is formed by the juncture of the parietal (sagittal) suture with the lambdoidal suture. It is triangular in shape with the apex forward between the two parietal bones, the sides passing down, one to the right and the other to the left of the top of the occipital bone.
Fig. 11. - Infant's skull, showing posterior and anterior fontanelles.
These fontanelles are of the greatest importance in diagnosing the position of the head during labor. If the examining finger encounters first a large diamond-shaped or four-cornered depression with its anterior angle more acute than the posterior, the accoucheur will know that it is the anterior fontanelle which is presenting. By following one of the sutures backward he will come to a triangular or Y-shaped ridge which will be recognized from its shape as being the posterior fontanelle. He will then know that the position of the head is occipitoposterior. If the position is the more usual occipito-anterior one, the finger will first encounter the posterior fontanelle with its three sutures, which are distinctly recognizable. On following the suture which leads backward, the four-cornered anterior fontanelle will be felt. The various sutures constituting the fontanelles can usually be distinctly felt, and, as the presentations are nearly always occipito-anterior, the fontanelle that will usually be first felt will be the posterior, and the sutures forming it can readily be counted.
The antero- and posterolateral fontanelles, located at the anterior and posterior angles of the parietal bones, are of no service in diagnosing the position of the head. They are indistinct, nearly closed, and thickly covered by tissue. In injuries to the skull in young children and infants, we should not mistake the fontanelles and lines of the sutures for fractures. Fissures extending into the occipital bone from the posterolateral fontanelles are normal at birth and not due to injury.