UPPEK ASPECT OF THE SKULL. 171 



Posterior Aspect of the Skull (Norma Occipitalis). 



The view of the cranium as seen from behind includes the posterior halves of 

 the two parietal bones above, the squamous part of the occipital bone below, and the 

 mastoid portions of the temporal bones on either side, inferiorly. The shape of this 

 aspect of the skull varies much, but ordinarily the greatest width corresponds to the 

 level of the parietal tuberosities. The sutures on this view of the calvaria display a 

 triradiate arrangement, one limb of which is vertical, and corresponds to the posterior 

 part of the interparietal or sagittal suture. The other two limbs pass laterally 

 and downwards in the direction of the mastoid processes, uniting the two parietal 

 bones in front with the occipital bone behind ; these constitute the A- g haped 

 lambdoid suture. The point of confluence of the sagittal and lambdoid sutures is 

 called the lambda. This can generally be felt in the living, owing to the tendency 

 of the squamous part of the occipital to project slightly, immediately below this spot. 

 About one inch and a quarter above the lambda the two small parietal foramina are 

 seen, through which pass the small emissary veins of Santorini, which connect the 

 intra-cranial venous system with the superficial veins of the scalp. These small 

 holes lie about T V of an inch apart on either side of the sagittal suture, which 

 here, for the space of about an inch, displays a simplicity of outline in striking 

 contrast with its serrated arrangement elsewhere. The term obelion is applied to 

 a point on the sagittal suture in line with the two parietal foramina. The lambdoid 

 suture is characterised by great irregularity of outline, and not infrequently chains 

 of separated ossicles are met with in it, the ossa suturarum (sutural bones). The 

 squamous part of the occipital bone is divided into two parts by the superior nuchal 

 or curved line, the central part of which forms the external occipital protuberance or 

 inion. The part above, called the planum occipitale or occipital surface comes within 

 our present consideration ; the part below, called the planum nuchale or the nuchal 

 surface, though seen in . perspective, had best be considered when the base is 

 examined. A little above the level of the superior curved line the occipital surface 

 is crossed on either side, by a faint lunated line, the linea nuchse suprema (highest 

 nuchal or curved line), to which are attached the occipitales muscles and the galea 

 aponeurotica. The projection of the occipital surface varies much in individual 

 skulls ; most frequently it overhangs the external occipital protuberance, forming a 

 distinct boss ; exceptionally, however, the latter may be the most projecting part of 

 the bone. The extremity of the superior nuchal line on either side corresponds to 

 the position of the asterion (p. 285). Lateral to these points the outline of the 

 skull is determined by the downward projection of the mastoid processes, the medial 

 surfaces of which are deeply grooved by the mastoid notches for the attachment of 

 the posterior bellies of the digastric muscles, thus causing these processes to appear 

 more pointed when viewed from this aspect. 



Upper Aspect of Skull (Norma Verticalis). 



This is the view of the calvaria as seen from above. It is liable to great 

 diversities of form. Thus, its shape may vary from an elongated oval to an outline 

 more nearly circular. These differences have been classified, and form important 

 distinctions from a craniometrical standpoint, the rounder varieties being 

 termed the brachycephalic, whilst the elongated belong to the dolichocephalic group. 

 Another noteworthy point in this view is the fact that in some instances the 

 zygomatic arches are seen, whilst in others they are concealed by the overhang 

 and bulge of the sides of the anterior part of the cranium. The former condition is 

 described as phaenozygous, the latter as cryptozygous, and each is more or less closely 

 associated with the long or round varieties of head-form respectively. 



The sutures displayed have a T-shaped arrangement. Placed medially between 

 the two parietal bones is the sagittal suture. This is finely denticulated, except in 

 the region of the obelion, though, of course, this will not be apparent if obliteration 

 of the suture has taken place through fusion of the two parietal bones. Posteriorly 

 the sagittal suture unites with the lambdoid suture at the lambda, which marks 

 in the adult the position of the posterior fontanelle of the foetus. Anteriorly it 

 terminates by joining the transverse suture which separates the frontal bone 



