For the purpose of operating on the brain it is essential to know the bony landmarks of the skull, the lower level of the brain, and the relation which the various fissures and convolutions bear to the surface.

The most important fissures are the longitudinal, Sylvian, Rolandic, and parietooccipital. If these can be properly located, the convolutions and subsidiary fissures can be readily filled in.

Bony Landmarks

Nasion

The nasofrontal suture in the median line.

Glabella

The smooth spot in the median line on the frontal bone between the superciliary ridges. It is about on a level with the upper edge of the orbit.

Fig. 49.   Landmarks of the skull.

Fig. 49. - Landmarks of the skull.

Bregma

The point in the midline where the sagittal and coronal sutures cross. It corresponds with the anterior fontanelle in the infant.

Lambda

The point of meeting of the sagittal and lambdoid sutures. It is about 6 cm. (2 1/4 in.) above the occipital protuberance.

Inion

The external occipital protuberance.

Pterion

This name was given by P. Broca to the point where the frontal, parietal, and sphenoid bones meet in the region of the temple. It is about 2.5 cm. (1 in.) behind the angular process and should not be confounded with the Sylvian point, which is 1.5 cm. (5/8 in.) farther posterior, where the temporal, parietal, and sphenoid bones meet. Horsley called this latter point the pterion. The region of the pterion is the seat of the anterolateral fontanelle in the foetus.

Asterion

This lies 2 cm. (4/5 in.) behind the base of the mastoid process, where the parietal, occipital, and temporal bones meet. It is on the superior curved line and in fetal life forms the posterolateral fontanelle.

Temporal Ridge

This marks the upper attachment of the temporal fascia and muscle. It begins at the external angular process of the frontal bone and ends at the asterion. Its anterior third is well marked, but as it crosses the coronal suture it fades away and gradually broadens out, its upper margin being called the superior and its lower the inferior temporal ridge. The superior ridge marks the attachment of the superficial layer of the temporal fascia, the inferior, the deep layer.

External Angular Process

This is the outer extremity of the frontal bone, where it articulates with the malar. The line of the suture can be distinctly felt in the living.

Malar Tubercle

This is the small bony projection on the posterior edge of the malar bone, 1.25 to 2 cm. (1/2 to 3/4 in.) below the frontomalar suture.

Stephanion

The point where the temporal ridge crosses the coronal suture.

Topographical Points

Pre-Auricular Point

The depression in front of the ear and just behind the condyle of the lower jaw.

Sylvian Point

Where the anterior ascending and anterior horizontal limbs come off from the posterior horizontal limb of the fissure of Sylvius. It lies 4 cm. (1 5/8 in.) posterior and a little above the external angular process, at the junction of the parietal, sphenoid, and temporal bones.

Superior Rolandic Point

Where the upper end of the line marking the Rolandic fissure crosses the median line.

Inferior Rolandic Point

Where the lower end of the line marking the Rolandic fissure crosses the line of the Sylvian fissure.

The Lower Level Of The Brain

The lower level of the brain is marked by a line beginning in the median line 1 cm. (2/5 in.) above the nasion, thence above the orbit 1 cm. from its edge to the external angular process; from here it goes to the middle of the zygoma, thence backward along its upper border, above the auditory meatus and along the superior curved line to the inion (occipital protuberance).