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THE PARIETAL BONES. 103 
sinus is so large, and extends so far back, that the optic nerve is carried through it in a bony 
tube. Another point of some practical importance is that the sinuses are hardly ever sym- 
metrical. It is rare to meet with cases of their complete absence, although sometimes the sinus 
on one or other side may be wanting. 
The external angular process, from the arrangement of its surfaces and the density of its 
structure, is particularly well adapted to resist the pressure to which it is subjected when the 
jaws are firmly closed. 
Variations.—That most frequently met with is a persistence of the suture which unites the 
two halves of the bone in the infantile condition: skulls displaying this peculiarity are termed 
metopic. The researches of various observers—Broca, Ranke, Gruber, Manouvrier, Anoutchine, 
and Papillault (Rev. mens. de Vecole d’Anthropol. de Paris, année 6, n. 3)—point to the more 
frequent occurrence of this metopic suture in the higher than in the lower races of man; and 
Calmette asserts its greater frequence in the brachycephalic than the dolichocephalic type. 
Separate ossicles (Wormian bones) may occur in the region of the anterior fontanelle. The fusion 
of these with one or other half of the frontal explains how the metopic suture is not always in 
line with the sagittal suture (Stieda, Anat. Anz. 1897, p. 227); they occasionally persist, however, 
and form by their coalescence a bregmatic bone (G. Zoja, Bull. Scientifico, xvi. p. 76, Pavia). 
Turner (Challenger Reports, part xxix.) records an instance of direct articulation of the frontal 
with the orbital plate of the superior maxilla in a Bush skull, and other examples of the same 
anomaly, which obtains normally in the skulls of the chimpanzee and gorilla, have been observed 
(Journ. Anat. and Physiol. vol. xxiv. p. 349). 
Ossification.—Ossification begins in membrane from two centres, which appear about 
the sixth or seventh week, one on either side immediately above the orbital margin. By 
extension inwards and backwards from these the 1 
orbital plates are formed. Serres, Rambaud, and 
Renault and v. Ihering describe the occurrence of 
three pairs of secondary centres somewhat later : 
one pair for the nasal spine on either side of the 
foramen cecum; a centre on either side in corre- 
spondence with the position of each trochlear fossa ; 
and a centre for each external angular process. 
Fusion between these secondary and the primary 
centres is usually complete about the sixth or 
seventh month of foetal life. At birth the two = 
symmetrical halves of the bone are separated by the 
metopic suture, obliteration of which gradually oe 2 
takes place, so that about the fifth or sixth year it 
is more or less completely closed, traces only of the ¢ ; 
suture being left above and below. In about eight 
per cent of Europeans, however, the suture persists 16. 79.—OssIFICATION OF FRONTAL BONE. 
in the adult (see ante). At birth the supraorbital ¢, Metopic suture still open. , Position of 
notches lie near the middle of the supraorbital arches. PY aac 2 mle ie fede 
Traces of the frontal sinuses may be met with pea, e, Centres for nasal spine. 
about the second year, but it is only about the age 
of seven that they can be definitely recognised. From that time they increase in size till 
the age of puberty, subsequent to which time they attain their maximum development. 
THE PARIETAL BONES. 
The parietal bones (ossa parietalia), two in number, are placed on either side 
of the vault of the cranium, articulating with the frontal anteriorly, the occipital 
posteriorly, and the temporals and sphenoid inferiorly. Each bone possesses an 
external and internal surface, four borders, and four angles. 
The external surface, convex from above downwards and from before backwards, 
displays towards its centre a more or less pronounced elevation, the parietal eminence 
(tuber parietale). This marks the position of the primitive ossific centre, and not 
unfrequently corresponds to the point of maximum width of the head. At a 
variable distance from the lower border of the bone, and more or less parallel to it, two 
curved lines can usually be distinguished ; these together constitute the temporal crest. 
The superior temporal line (linea temporalis superior) serves for the attachment of 
the temporal fascia; the inferior temporal line (linea temporalis inferior) defines the 
attachment of the temporal muscle, the extent and development of which necessarily 
determines the position of the crest. The surface below the crest enters into the 
formation of the floor of the temporal fossa, and is called the planum temporale ; it 
