Fig. 16.—Front view of the skull.
(After Sobotta.)
The [occipital bone] is at the base of the skull and at birth consists of four pieces. In the lower, anterior part is the foramen magnum, an oval opening through which the spinal cord passes from the skull down into the spinal canal. Half way between the foramen and the top of the bone is the external occipital protuberance for the attachment of the ligamentum nuchæ which holds the head erect. The inner side of the bone is deeply concave and is divided by a cross-shaped grooved ridge into four fossæ, the internal occipital protuberance being situated where the arms of the cross meet. The occipital lobes of the cerebrum lie in the two upper fossæ and the hemispheres of the cerebellum in the two lower ones. In the grooves upon the ridge are the sinuses which collect the blood from the brain.
The occipital and frontal muscles, united by a thin aponeurosis, cover the whole upper cranium and are known as the occipito-frontalis muscle. At the back this is attached to the occipital bone, while in front it interlaces with various face muscles. It is a powerful muscle and raises the brows, wrinkles the forehead, and draws the scalp forward. Long hair grows on the skin over it as a further protection against blows upon the skull and sudden variations in temperature.
The [temporal bones]—said to be so named because the hair over them is the first to turn with age—are situated at the sides and base of the skull and are in three portions: the squamous or scale-like, the mastoid or nipple-like, and the petrous or stony portion. The squamous is the upper portion and has projecting from its lower part the long arched zygomatic process, which articulates with the [malar bone] of the face and from which arises the masseter muscle, one of the chief muscles of mastication, which has its insertion in the ramus and angle of the lower jaw. Just above the zygomatic process the temporal muscle has its origin in part, while below is the glenoid fossa for articulation with the condyle of the lower jaw, the posterior portion of the fossa being occupied by part of the parotid gland.
The rough mastoid portion of the temporal bone is toward the back and affords attachment to various muscles, of which the most important are the occipito-frontalis and the sterno-cleido-mastoid. Within it are the mastoid cells, which communicate with the inner ear and are lined with mucous membrane continuous with that of the tympanum. They probably have something to do with the hearing. In children they often become the seat of inflammation (mastoid abscess) in infectious diseases and the mastoid bone has to be cut to let out pus that has collected. As the lateral sinus is directly behind the mastoid bone, there is very great danger of going through into the sinus and causing a fatal hemorrhage.
Fig. 17.—Side view of the skull.
(After Sobotta.)
The petrous portion, which contains the organ of hearing, is between and somewhat behind the other two portions, at the lower edge of the temporal bone, wedged between the sphenoid and the occipital bones. On its outer surface is the external auditory meatus, and from below projects a long sharp spine called the styloid process, to which several minor muscles are attached. In the same angle between the petrous and squamous portions lies the bony Eustachian tube.
The sphenoid or wedge bone, so called because in the process of development it serves as a wedge, lies at the base of the cranium, forming as it were the anterior part of the floor of the cavity containing the brain. It is a large, bat-shaped bone and articulates with all the cranial and many of the facial bones, binding them all together. It has a body, two large wings, and two lesser wings and, appears on the outside of the skull between the frontal and the temporal bones behind the zygomatic process. In the adult the body of the sphenoid is hollowed out into the sphenoid sinuses, in which pus sometimes forms.